Chapter 10: Muscle tissue

Two primary factors must be considered when describing the consequences of consciously initiating skeletal muscle contraction:

(1) force generated by the muscle and (2) resistance (load) that must be overcome.

energy for muscle contraction

*** ATP is the only direct source of energy for muscle fiber contraction

alternating bands form striations

A band H zone I band M line

sliding filament theory.

A description of the repetitive movement of thin filaments sliding past thick filaments is called the

Neuromuscular junction

Each skeletal muscle fiber is typically described as having one of these. It is the specific location, usually in the mid-region of the skeletal muscle fiber where it is innervated by a motor neuron. The neuromuscular junction has the following parts: synaptic knob, motor end plate, and synaptic cleft.

what disappears in contraction?

H band disappears and A band is left

Actin protein

In each strand of actin, many (about 300 to 400) small, spherical molecules (G, or globular, actin) are connected to form a fibrous strand (F, or filamentous, actin). F-actin resembles two beaded necklaces that are twisted and intertwined together, with G-actin as the individual beads. Each G-actin molecule has a significant feature called a myosin binding site. The myosin head attaches to the myosin binding site of actin during muscle contraction.

Striations

The repeating light and dark bands of the overlapping myofilaments form unique striped patterns within a skeletal muscle fiber called -Are visible when viewing a longitudinal section of skeletal muscle tissue in an image produced by either a light microscope or an electron microscope. This striated appearance is due to both the size and density differences between thin filaments and thick filaments. -Shows cross sections through various regions in a sarcomere. It presents the relative sizes, arrangements, and organization of thick and thin filaments at different locations within the sarcomere. Notice that in a cross section of an A band the arrangement of thick filaments relative to thin filaments is the following: Each thin filament has three thick filaments around it that form a triangle at its periphery, and each thick filament is sandwiched by six thin filaments.

twitch contraction

the response of a motor unit to a single nerve impulse in its motor neuron

incomplete tetany

the tension tracing continues to increase and the distance between waves decreases

location of smooth muscle

Walls of hollow viscera, airways, blood vessels, iris and ciliary body of eye, arrector pili muscles of hair follicles

Muscular Dystrophy

a collective term for several hereditary diseases in which the skeletal muscles degenerate, lose strength, and are gradually replaced by adipose and fibrous connective tissue. In a viscous cycle, the new connective tissues impede blood circulation, which further accelerates muscle degeneration.

aponeurosis

a thin, flattened sheet of dense regular connective tissue broad, flat sheet extension of three connective tissue layers attaches muscle to another muscle or bone attach a muscle either to a skeletal component (bone or ligament) or to fascia example: epicranial aponeurosis between frontal and occipital bellies of occipitofrontalis muscle

elasticity

ability to return to original length after contraction or extension

contractility

ability to shorten forcefully when stimulated generating tension *take resting muscle and contracting it

extensibility

ability to stretch within lights without being damaged *going to the gym (lifting weights) trying to recognize limits

amount of sarcoplasmic reticulum- skeletal muscle

abundant

contraction regulation by......?- smooth muscle

acetylcholine and norepinephrine released by autonomic motor neurons several hormones local chemical changes stretching

contraction regulation by......?- cardiac muscle

acetylcholine and norepinephrine released by autonomic motor neurons several neurons

frequency of stimulation is caused by:

additional release of Ca2+ ions, adding to Ca2+ ions still in sarcoplasm from previous stimulus

muscle fibers vary in:

amount of red myoglobin

myofilaments

are contractile proteins that are bundled within myofibrils. It is not as long as a myofibril; rather, it takes many successive units of myofilaments to extend the entire length of the myofibril. Myofibril bundles contain two types of myofilaments: thick filaments and thin filaments

Somatic motor neurons

are nerve cells that transmit electrical signals (nerve signals) from the brain or spinal cord to control skeletal muscle activity. The axon of each motor neuron divides into many individual branches to innervate numerous skeletal muscle fibers. A single motor neuron and the skeletal muscle fibers it controls is called a motor unit

location of skeletal muscle

attached primarily to bones by tendons

What happens when ACh receptors are activated?

bind to receptors (Ach) to open Na+ ion channels

relaxation period

cell events allowing muscle to resume original length

latent period

cell events leading up to contraction brief delay between stimulus and contraction

no muscle tone=

cells are fully relaxed

what occurs during relaxation stage?

cessation of motor neuron impulses stops ACh release AChE breaks down any ACh in synaptic cleft Ion channels close, as action potential stops

what happens when muscle generates into action potential?

change in membrane potential triggers contraction

Functional categories of smooth muscle

classified into two broad groups based upon whether the smooth muscle fibers are stimulated to contract either independently or as one unit. Multiunit smooth muscle cells receive stimulation to contract individually, whereas single-unit smooth muscle cells are stimulated to contract in unison

capacity for regeneration?- smooth muscle

considerable (compared with other muscle tissues, but limited compared with epithelium), via pericytes

Muscle proteins

contractile regulatory structural

what is the first source of energy when contraction begins?

creatine phosphate

the ways for muscles to produce ATP

creatine phosphate anaerobic cellular respiration aerobic cellular respiration

what does fibrosis replace?

damaged muscle fibers with scar tissue instead

red muscle fibers

dark red with high myoglobin

A band

darker middle with thick and think overlapping

epimysium

dense connective tissue, outermost layer encircles entire muscle organ

many muscle fibers are bundled within a

fascicle; and many fascicles are bundled within the whole skeletal muscle.

speed of contraction- skeletal muscle

fast

fast glycolytic (FG)

fastest strong contraction use glycolysis fatigue rapidly *bursts of movement **fight or flight- 1st instinct

fast glycolytic- capillaries

few

multiunit

found in walls of large arteries, lung airways, arrector pili of hair follicles and internal eye muscles individual fibers with own motor neuron few gap junctions and lacks autorhythmicity **on its own**

2 types of muscle growth after birth

hypertrophy hyperplasia

connective tissues of organ

hypodermis fascia 3 layers of connective tissue in organ tendon or aponeurosis

tetany

the tension tracing is a smooth line; continuous contraction If stimulation continues, the muscle reaches fatigue—a decrease in muscle tension occurs from repetitive stimulation . Changes in muscle stimulation frequency by the nervous system primarily allow skeletal muscle contraction to exert a coordinated action that gradually increases in force. Nervous stimulation of skeletal muscles in the human body usually does not exceed 25 stimuli per second. Therefore, muscle tetany is seen only in laboratory experiments. Sustained contractions in the body occur when you are holding something you do not want to drop because the nervous system stimulates different motor units within the same muscle in an overlapping pattern, so the muscle tension can be maintained for a longer period.

isometric contraction

muscle does not change length the load equals or exceeds the muscle tension created

without calcium, you will get ____?

muscle fatigue

sarcoplasm

muscle jelly-like liquid cytoplasm of muscle fiber contains organelles contains glycogen (glucose storage molecule) and myoglobin (oxygen-binding red protein) -> hemoglobin in muscles have a huge affinity for O2

contractile muscle proteins (2)

myosin actin

H zone

narrow center with only thick filaments

I band

near Z discs with only thin filaments

during contraction, transfer phosphate group to ADP quickly form ______?

new ATP

Autorythmicity?- skeletal muscle

no

transverse tubules present?- smooth muscle

no

sliding filament mechanism

thin actin filaments at both ends of the sarcomere "slide" as they are pulled to the center of the sarcomere by myosin head activity Z discs come closer together, shortening the sarcomere

actin

thin myofilament component has myosin-binding site for myosin head

endomysium

thin sheath of areolar connective tissue separates each muscle fiber within fascicle

structural muscle proteins (2)

titin dystrophin

primary function of muscle:

to change chemical energy into mechanical energy to produce movement

after Ca2+ pumps move back, what happens?

troponin-tropomyosin complex slides back to cover myosin-binding sites actin filaments slide, returning back to relaxed position

fiber diameter of skeletal muscle

very large

what happens when acetylcholine is released?

vessicles undergo exocytosis in nervous tissue in response to Ca2+ ions Ca2+ rushes in

smooth muscle tissue types

visceral multinunit

where are cardiac muscle fibers found?

wall of the heart

frequency of stimulation (3)

wave summation unfused tetanus fused tetanus

eccentric

when muscle lengthens

concentric

when muscle shortens

fast glycolytic- color

white (pale)

Autorythmicity?- cardiac muscle

yes

atrophy

Lack of exercise (and, thus, lack of muscle use) results primarily in decreasing the skeletal muscle fiber size, a process called this. This causes a decrease in muscle fiber size, tone, and power, and the muscle becomes flaccid. Even a temporary reduction in muscle use can lead to muscular atrophy. Comparing limb muscles before and after a cast that has been worn for a fracture reveals the loss of muscle tone and size for the casted limb. Individuals who suffer damage to the nervous system or are paralyzed by spinal injuries gradually lose skeletal muscle tone and size in the areas affected. Although skeletal muscle atrophy is initially reversible, dead or dying skeletal muscle fibers are not replaced. When extreme atrophy occurs, the loss of gross skeletal muscle function is permanent because muscle is replaced with connective tissue, including adipose connective tissue. For these reasons, physical therapy is required for patients who suffer temporary loss of mobility.

Sarcomere- functional unit of contraction within myofibrils

Myofilaments within myofibrils are arranged in repeating, microscopic, cylindrical units (2 micrometers in length) called this myofibrils extend length of muscle fiber The number of sarcomeres varies with the length of the myofibril within the skeletal muscle fiber. Each sarcomere is composed of overlapping thick filaments and thin filaments. thick and thin myofilaments in myofibril do not extend entire length of muscle fiber each sarcomere is delineated at both ends by Z discs.

junctions between fibers- skeletal muscle

none

frequency of stimulation

number of impulses per second governs total tension produced by a single muscle fiber when stimulated

cardiac muscle fibers compared to skeletal muscle fibers

same sarcomere contractions of actin and myosin long, sustained contractions supported by inflow of Ca2+ ions from both SR and interstitial fluid Autorhythmicity of specialized cardiac muscle fibers repeatedly generate spontaneous action potentials; autonomic impulses regulate their rate (goes at its own rate, automatic) can use lactic acid from skeletal muscle anaeroic respiration to make ATP during exercise (body doesnt like to do it)

Source of Ca2+ for contraction- skeletal muscle

sarcoplasmic reticulum

Source of Ca2+ for contraction- smooth muscle

sarcoplasmic reticulum and interstitial flui

Some myoblasts do not fuse with muscle fibers during development and instead remain in adult skeletal muscle tissue as

satellite cells, which are adult stem cells. If a skeletal muscle is injured, some satellite cells may be stimulated to differentiate and then fuse with a damaged skeletal muscle fiber to assist to a limited extent in its repair and regeneration.

amount of sarcoplasmic reticulum- smooth muscle

scanty

anaerobic cellular respiration

series of ATP-producing reactions in sarcoplasm that do not require oxygen

Each skeletal muscle is composed of

skeletal muscle fibers, connective tissue layers, blood vessels, and nerves.

muscle tone

small amount of tension due to involuntary, alternating contractions of a small number of motor units of a skeletal muscle, even at rest do not produce movement flaccid

smooth muscle cells

small and fusiform shaped (widest in the middle and tapered on the ends) with a centrally located nucleus. They typically have a diameter of 5 to 10 micrometers and a length of 50 to 200 micrometers. Thus, their diameter is up to 10 times smaller and their length thousands of times shorter than a skeletal muscle fiber. An endomysium wraps around each smooth muscle cell. The small, tapered ends of the cells overlap the larger, middle area of adjacent cells to provide for close packing of cells.

amount of sarcoplasmic reticulum- cardiac muscle

some

excitation-contraction coupling- Ca2+

steps that connect excitation (action potential propagation) to contraction (sliding of filaments)

cardiac muscle tissue

striated, under involuntary control, and pumps blood with audtorhythmicity in heart only

fused tetanus

sustained contraction caused by rapid frequency of stimulation *no relaxation at all

what consists of the neuromuscular junction (NMJ)? (3)

synapse between a motor neuron and a skeletal muscle fiber synaptic end bulb motor end plate

Anabolic steroids

synthetic substances that mimic the actions of natural testosterone. Recall from section 3.2b that the term anabolism refers to the synthesis of complex molecules (e.g., protein) from simple molecules (e.g., amino acids). To date, over 100 compounds have been developed with anabolic properties, but they all require a prescription for legal use in the United States. Anabolic steroids have only a few accepted medical uses—among them, the treatment of delayed puberty, certain types of impotence, and the wasting condition associated with HIV infection and other diseases. Because anabolic steroids stimulate the manufacture of muscle proteins, these compounds have been used by some athletes as performance enhancers.

individual fibers of tissue

tapered ends, single nucleus, non-striated and involuntary

tendon or aponeurosis

tendon= adds stronger base to muscle rope-like or broad flat extension of connective tissue beyond muscle fibers for attachment of muscle organ

myoblasts

A skeletal muscle fiber is typically between 10 and 500 micrometers (µm) in diameter and may extend the length of the entire muscle, ranging from about 100 µm to 30 centimeters. To reach this length, groups of embryonic muscle cells termed this fuse to form single skeletal muscle fibers during development. During this fusion process, each myoblast nucleus contributes to the eventual total number of nuclei in the fiber. Consequently, skeletal muscle fibers are multinucleated (i.e., they have numerous nuclei).

Binding of ACh at Motor End Plate

ACh diffuses across the fluid-filled synaptic cleft to bind with ACh receptors within the motor end plate. This causes excitation of a skeletal muscle fiber. Note that nerve signals are repeatedly propagated along the motor axon (at about 10 to 40 times per second). Thus, these events (steps 1a-1c) will continue until stimulation of the skeletal muscle fiber by the neuron ceases (stops) and acetylcholinesterase catalyzes breakdown of ACh that is within the synaptic cleft

Ca2+ active transport pumps use:

ATP to move Ca2+ ions back into SR cisterns *pumps ATP constantly

calmodulin and calsequestrin

Also embedded within the membrane of the sarcoplasmic reticulum are Ca2+ pumps, which move Ca2+ from the cytosol into the sarcoplasmic reticulum, where it is stored bound to specialized proteins called Calcium pumps function through primary active transport mto maintain low cytosol levels of calcium. These pumps return Ca2+ to the terminal cisternae of the sarcoplasmic reticulum following its release to initiate muscle contraction.

skeletal muscle cells

Because of their potentially extraordinary length, they are often referred to as muscle fibers (or myofibers).

Skeletal muscle fibers have a great demand for energy and contain several components that facilitate the production of ATP

Skeletal muscle fibers have abundant mitochondria for aerobic cellular respiration; a typical skeletal muscle fiber contains approximately 300 mitochondria. The fibers also contain glycogen stores (granules called glycosomes) for use as an immediate fuel molecule. Myoglobin is a molecule unique to muscle tissue. Myoglobin is a reddish, globular protein that is somewhat similar to hemoglobin. It binds oxygen when the muscle is at rest and releases it for use during muscular contraction. This additional source of oxygen provides the means to enhance aerobic cellular respiration and the production of ATP. Skeletal muscle fibers also contain another type of molecule called creatine phosphate. Creatine phosphate provides muscle fibers with a very rapid means of supplying ATP.

what is typically low in sarcoplasm? where is it stored?

Ca2+ stored in SR cisterns

what happens when Ca2+ release channels in SR open when muscle action potential propagates into T-tubule?

Ca2+ ions are released into the sarcoplasm bind to troponin, moving tropomyosin, and exposing myosin-binding sites on actin

connective tissue components of cardiac tissue

endomysium

connective tissue components of smooth muscle

endomysium

hypertrophy

enlargement of existing muscle fibers *build up muscle

Classification of muscle fiber types

Physiologists use both the type of contraction generated and the primary means to supply ATP to differentiate skeletal muscle fibers into three subtypes (table 10.1): -Slow oxidative (SO) fibers, also called type I fibers, typically have half the diameter of other skeletal muscle fibers and contain slow myosin ATPase. These fibers produce contractions that are slower and less powerful. However, they can contract over long periods of time without fatigue because ATP is supplied primarily through aerobic cellular respiration. These fibers appear dark red because of the presence of large amounts of both myoglobin molecules and mitochondria. -Fast oxidative (FO) fibers, also called intermediate fibers or type IIa, are the least numerous of the skeletal muscle fiber types. They are intermediate in size and contain fast myosin ATPase. They produce a fast, powerful contraction with ATP provided primarily through aerobic cellular respiration. However, the vascular supply to fast oxidative fibers is less extensive than the network of capillaries serving SO fibers—thus, the delivery rate of nutrients and oxygen is lower. These fibers also contain myoglobin, but less than the amount found in SO fibers. Consequently, these fibers can be distinguished from SO fibers on a microscopic image because they appear a lighter red than SO fibers. -Fast glycolytic (FG) fibers, also called fast anaerobic fibers or type IIb, are the most prevalent skeletal muscle fiber type. They are largest in diameter, contain fast myosin ATPase, and provide both power and speed. However, they can contract for only short bursts because ATP is provided primarily through glycolysis. These fibers appear white because of the relative lack of myoglobin and mitochondria.

Muscle fiber structure

Sarcolemma Transverse tubules (T-tubule) Sarcoplasm Myofibrils Sarcoplasmic reticulum

length-tension relationship.

The amount of tension a skeletal muscle can generate when stimulated is influenced significantly by the amount of overlap of thick and thin filaments within its muscle fibers when the muscle begins its contraction. \ A skeletal muscle generates different amounts of tension, depending upon its length at the time of stimulation. The graphical presentation of the length-tension relationship is called the length-tension curve

troponin

can bind Ca2+ and holds tropomyosin **important for muscle = muscle breaks down and troponin breaks down

tropomyosin

can cover myosin-binding sites on actin

oxygen sources in aerobic cellular respiration

diffuses into muscle fibers from blood hemoglobin released by myoglobin in sarcoplasm

what does the Ca2+ ion concentration in sarcoplasm?

increase starts muscle contraction decrease stops it

slow oxidative (SO)- color

red

fascia

*above muscle* *clear, shiny* *protect, separate, and reduce friction* dense connective tissue sheet unites muscles with similar functions, carries nerves and vessels, and fills spaces between muscles

hypodermis

*below integumentary system* areolar and adipose tissue separates muscle from skin, insulates and protects

myosin

*bring muscle in* molecule shaped like two twisted fold clubs with tail and heads converts chemical energy in ATP into mechanical energy of motion *use ATP-> use it ->contracts

smooth muscle fibers compared to skeletal muscle fibers

*lacks sarcomere* contraction starts slowly and lasts an extended time, twisting in a helix to lengthwise shorten muscle fiber lacks transverse tubules, has little sarcoplasmic reticulum and has **calmodulin instead of troponin** responds to autonomic nervous system impulses, hormones and other local factors

regulatory muscle proteins (2)

*no random movements tropomyosin troponin

synapse between a motor neuron and a skeletal muscle fiber

*space between synaptic cleft is between the two cells communication by release of neurotransmitter chemical

The thick filaments and thin filaments overlap within a sarcomere, forming the following regions:

-I bands extend from both directions of a Z disc and are bisected by the Z disc. These end regions contain only thin filaments; this region appears light when viewed with a microscope. At maximal muscle shortening, the thin filaments are pulled parallel along the thick filaments, causing the I bands to disappear. -The A band is the central region of a sarcomere that contains the entire thick filament. Thin filaments partially overlap the thick filament on each end of an A band. The A band appears dark when viewed with a microscope. The A band does not change in length during muscle contraction. -The H zone (also called the H band) is the most central portion of the A band in a resting sarcomere. This region does not have thin filament overlap; only thick filaments are present. During maximal muscle shortening, this zone disappears when the thin filaments are pulled past thick filaments. -The M line is a thin transverse protein meshwork structure in the center of the H zone. It serves as an attachment site for the thick filaments and keeps the thick filaments aligned during contraction and relaxation events.

functions of muscle tissue

-produces body movements (#1) integrated function of skeletal muscles with bones and joints -stabilizes body positions skeletal muscle contractions without movement -moves substances within the body all 3 kinds of muscles as part of different organ systems -generates heat involuntary shivering of skeletal muscle helps maintain temperature homeostasis *lose energy when cold because you use up sugars; you then use body muscles for energy

steps in the contraction cycle (4)

1. ATP splits (reorients and energizes myosin head) 2. Myosin attaches to actin (head attaches to myosin-binding site) 3.Power stroke occurs (releases phosphate group, triggering ADP release, and myosin rotation slides actin toward M line) 4. Myosin detaches from actin (ATP molecule binds; ATPase splits ATP and reorients head)

Overview of Events in Skeletal Muscle Contraction.

1. neuromuscular junction: Release of neurotransmitter acetylcholine (ACh) from synaptic vesicles and subsequent binding of ACh to ACh receptors 2. Sarcolemma, T-tubules, and sarcoplasmic reticulum: excitation-contraction coupling: ACh binding triggers propagation of an action potential along the sarcolemma and T-tubules, to the sarcoplasmic reticulum, which is stimulated to release CA2+ 3. Sarcomere: cross bridge cycling: CA2+ binding to troponin triggers sliding of thin filaments past thick filaments of sarcomeres; as sarcomeres shorten, the muscle contracts

nerve impulse (action potential)- steps

1. release of acetylcholine 2. activation of ACh receptors 3. generation of muscle action potential 4. termination of ACh activity

Motor unit

A single motor neuron and the skeletal muscle fibers it controls is called this. The number of skeletal muscle fibers a single motor neuron innervates—and thus the size of the motor unit—varies and can range from small motor units that have less than five muscle fibers to large motor units that have several thousand muscle fibers. The size of the motor unit determines the degree of control. There is an inverse relationship between the size of a motor unit and the degree of control. For example, motor neurons innervating extrinsic eye muscles are small because greater control is essential in the muscles that move the eye. In contrast, a single motor neuron controls several thousand individual skeletal muscle fibers in the power-generating muscles in our lower limbs, where less precise control is required. The skeletal muscle fibers of a motor unit are not clustered within one area of a muscle, but rather are dispersed throughout most of a muscle. Normally, the stimulation of a motor unit does not produce a strong contraction in a localized area within the muscle, but a weak contraction over a wide area.

myofibrils

Approximately 80% of the volume of a skeletal muscle fiber is composed of long, cylindrical structures termed this A skeletal muscle fiber contains hundreds to thousands of these long contractile organelles inside sarcoplasm extend entire length of muscle fiber arrangement of filaments gives striated appearance Each myofibril extends the entire length of the skeletal muscle fiber (and is about 1 to 2 micrometers in diameter). Note that each myofibril is composed of bundles of contractile proteins called myofilaments and is enclosed in portions of the sarcoplasmic reticulum

Calcium Binding

Calcium released from the sarcoplasmic reticulum binds to a subunit of globular troponin, a component of thin filaments. This induces a conformational change in troponin. Recall that troponin is attached to tropomyosin, forming the troponin-tropomyosin complex. When troponin changes shape, the entire troponin-tropomyosin complex is moved and the myosin binding sites of actin are exposed.

transverse tubules (T-tubules)

Deep invaginations of the sarcolemma are called this *activates inside of the cell many invaginated (folded), tunnel-like extensions of sarcolemma extend into the skeletal muscle fiber as a network of narrow, membranous tubules to the sarcoplasmic reticulum, which is the endoplasmic reticulum (ER) of the muscle (described shortly). Located within the membrane of both the sarcolemma (along its length) and the T-tubules are voltage-gated channels. These channels include both voltage-gated Na+ channels and voltage-gated K+ channels, which participate in conducting an electrical signal (an action potential) open to the cell's exterior, thus filled with interstitial fluid

Supplying Energy for Skeletal Muscle Metabolism

Most of the ATP required by skeletal muscle fibers is used to reset the mysosin heads of the thick filaments during muscle contraction,, which demands very large amounts of ATP (approximately 2500 ATP molecules per thick filament per second). ATP is also required by the calcium pump within the sarcoplasmic reticulum membrane to return Ca2+ to the terminal cisternae for storage.. A very limited amount of ATP is already present within skeletal muscle fibers, and additional small amounts can be rapidly produced as phosphate (Pi) is transferred from one ADP to another ADP, yielding ATP and adenosine monophosphate (AMP), an enzymatic reaction catalyzed by myokinase. This usually provides only enough energy for about 5 to 6 seconds of maximal exertion. Thus, meeting these high energy demands requires forming ATP from other sources. These include ATP formed from creatine phosphate, by glycolysis, and through aerobic cellular respiration.

Arrangement of Anchoring Proteins and Contractile Proteins of Smooth Muscle

Smooth muscle contains a unique arrangement of anchoring protein structures that includes the cytoskeleton, dense bodies, and dense plaques. The cytoskeletal network is composed of an extensive array of intermediate filaments. The intermediate filaments are linked with dense bodies at points where they interact within the sarcoplasm of the smooth muscle cell, whereas intermediate filaments are linked by dense plaques at points where they attach on the inner surface of the sarcolemma. Thus, intermediate filaments extend across the cell, with dense bodies as "spot welds" that anchor intermediate filaments to each other and dense plaques that anchor the intermediate filaments to the plasma membrane. The contractile proteins in smooth muscle are arranged between dense bodies and dense plaques, rather than in sarcomeres as they are in both skeletal and cardiac muscle. The Z discs that anchor the sarcomere on either end in skeletal muscle fibers are also absent. Lack of sarcomeres and Z discs contributes to the absence of striations, giving these muscle cells their "smooth" appearance. The contractile proteins are oriented at oblique angles to the longitudinal axis of the smooth muscle cell and appear to spiral. Consequently, contraction results in a twisting of the smooth muscle, which is similar to a corkscrew

Characteristics of Smooth Muscle Contraction: Prolonged Duration of Contraction

Smooth muscle contraction is usually slow to develop with maximum tension at about 500 milliseconds after stimulation. The relatively long latent period is due primarily to both the requirement for phosphorylating the myosin head by MLCK enzymes and variations in the speed of myosin ATPase activity. The duration of contraction typically extends over a 1- to 2-second period due to the slowness of Ca2+ pumps in removing Ca2+ from the cytosol, the requirement for dephosphorylation of the myosin head by phosphatase, and the possibility of myosin locking to actin (latchbridge mechanism). Contraction of smooth muscle does not generally require a rapid onset, but it does require the ability to remain in the contracted state for extended periods of time. This characteristic is important because smooth muscle must maintain continuous tone (tonic contraction) in visceral walls, such as the gastrointestinal tract and blood vessels.

Characteristics of Smooth Muscle Contraction: Broader Length-Tension Curve

Smooth muscle exhibits a broader length-tension curve than skeletal muscle. Recall that the force of muscle contraction generated by skeletal muscle is dependent upon its muscle length at the time of stimulation. It shows maximum force at its optimal resting length, but a decreased force of contraction if it is either shortened or lengthened (see figure 10.25). These limitations are due to the Z discs that prevent additional shortening and lack of myosin heads in the center of thick filaments, respectively. Smooth muscle has neither of these limitations—thus, it can contract forcefully when compressed to approximately half its resting length or stretched to twice its resting length. Consider, for example, that the storage of urine in the urinary bladder results in stretching of the urinary bladder wall. The greater the amount of urine, the greater the amount of stretch of the smooth muscle in the urinary bladder wall. The ability of smooth muscle to contract forcefully at varying degrees of stretch allows us to easily empty our bladder regardless of the amount of urine it is holding.

Development of an End-Plate Potential at the Motor End Plate

The ACh receptors, which are chemically gated ion channels, are stimulated to open temporarily when ACh binds to them. The opening of these channels allows relatively small amounts of both Na+ to rapidly diffuse into the skeletal muscle fiber and K+ to slowly diffuse out of the skeletal muscle fiber. More Na+ diffuses in than K+ diffuses out, and there is a net gain of positive charge on the inside of the skeletal muscle fiber. The flow of both Na+ and K+ ions quickly slows and then ceases as the ions meet with resistance. Thus, these changes in membrane potential in the motor end plate are both transient (short-lived) and local. However, if there is sufficient gain of positive charge to change the RMP of about −90 mV to −65 mV, an end-plate potential is produced. An end-plate potential (EPP) is the minimum voltage change (or threshold) in the motor end plate that can trigger opening of voltage-gated channels in the sarcolemma to initiate an action potential.

Initiation and Propagation of Action Potential Along the Sarcolemma and T-tubules

The EPP triggers an action potential that is propagated along the sarcolemma and T-tubules of the skeletal muscle fiber. An action potential involves two events: depolarization, which causes the inside of the sarcolemma of the skeletal muscle fiber to become positive due to the influx of Na+, and repolarization, which is the returning of the inside of the sarcolemma to its relatively negative resting membrane potential due to the outward flow of K+. The electrical change of the EPP in the motor end plate stimulates the opening of voltage-gated Na+ channels in the adjacent area of the sarcolemma. The opening of voltage-gated Na+ channels allows Na+ to move rapidly across the sarcolemma down its concentration gradient into the skeletal muscle fiber. Sufficient Na+ enters to cause a reversal of the membrane potential of the sarcolemma. The inside, which was relatively negative, becomes relatively positive with a change in the membrane potential from the threshold value of −65 mV to +30 mV. This reversal in polarity at the sarcolemma is referred to as depolarization. The propagation of depolarization along the length of the sarcolemma and T-tubules involves the sequential opening of voltage-gated Na+ channels. The inflow of Na+ at the initial portion of the sarcolemma causes adjacent regions of the sarcolemma to experience electrical changes that initiate voltage-gated Na+ channels in these areas to open. Sodium flows in to cause depolarization in this region of the sarcolemma. Adjacent depolarization is repeated rapidly down the sarcolemma and T-tubules. The propagation of an action potential along the sarcolemma and T-tubule is similar to the falling of a series of stacked dominoes—once started, it does not stop until it reaches the end. Voltage-gated K+ channels located along the sarcolemma and T-tubules open immediately following the opening of the voltage-gated Na+ channels. The opening of voltage-gated K+ channels allows K+ to move across the sarcolemma down its concentration gradient and out of the skeletal muscle fiber. Sufficient K+ exits so that the membrane potential at the sarcolemma and T-tubules reverses and the negative resting membrane potential (−90 mV) is reestablished. This process, which changes the membrane potential from +30 mV to reestablish the RMP of -90 mV, is referred to as repolarization. The opening of voltage-gated K+ channels also occurs sequentially, and repolarization is propagated along the sarcolemma and T-tubules. Repolarization allows the skeletal muscle fiber to propagate a new action potential when stimulated again by a motor neuron. Note that an action potential is a self-sustaining electrical change in the membrane potential that is propagated along the sarcolemma and is caused by the sequential opening of voltage-gated channels. Action potential propagation at the sarcolemma is similar to action potential propagation that occurs in neurons . These electrical changes include reaching the threshold, depolarization, and repolarization. The period of time that includes depolarization and repolarization is called the refractory period. The refractory period is significant because during this brief period of time the muscle cannot be restimulated. A new action potential can occur only when the resting membrane potential at the sarcolemma has been reestablished.

Release of ACh from Synaptic Knob

The binding of Ca2+ to synaptic vesicles triggers the merging of synaptic vesicles with the synaptic knob plasma membrane, resulting in exocytosis of ACh into the synaptic cleft. Acetylcholine is released from approximately 300 vesicles per nerve signal with each vesicle releasing thousands of molecules of ACh

Characteristics of Smooth Muscle Contraction: Fatigue-Resistant

The energy requirements for smooth muscle contraction are relatively low in comparison to skeletal muscle, and ATP is generally supplied through aerobic cellular respiration. The latchbridge mechanism provides the means of maintaining muscle contraction without use of additional ATP. Consequently, smooth muscle is fatigue-resistant (i.e., it may contract for extended periods of time without becoming fatigued). This characteristic is obviously a requirement for maintaining the tonic contractions just described.

Skeletal muscle relaxation

The first step in skeletal muscle relaxation is the termination of the rapid nerve signals propagated along the motor neuron. When the nerve signals stop, there is no additional release of acetylcholine, and the acetylcholine remaining in the synaptic cleft is hydrolyzed by acetylcholinesterase. The ACh receptors close, and both the end-plate potentials at the motor end plate and the action potentials along the sarcolemma and T-tubules cease. Calcium channels in the triads (both those in T-tubules and those in terminal cisternae of the sarcoplasmic reticulum) return to their normal (unstimulated) position with no further release of Ca2+. The Ca2+ already released from the sarcoplasmic reticulum is continuously returned into the terminal cisternae by Ca2+ pumps. After cessation of skeletal muscle fiber stimulation, the remaining Ca2+ in the sarcoplasm is transported back into storage within the sarcoplasmic reticulum, where it is bound by both calmodulin and calsequestrin proteins. Troponin returns to its original shape when Ca2+ is removed, and simultaneously the tropomyosin moves over the myosin binding sites on actin. This prevents myosin-actin crossbridge formation. Through the natural elasticity of the skeletal muscle fiber, the muscle may return to its original relaxed position, a process facilitated by the release of passive tension that developed in connectin proteins that were compressed during shortening. It is interesting to note that a significant amount of ATP is used by the Ca2+ pumps of the sarcoplasmic reticulum. Calcium levels within the cytosol of muscle fibers must be kept low to prevent Ca2+ from binding with phosphate ions (which are released from ATP) to form hydroxyapatite, which would calcify and harden muscles in a process similar to that in bone tissue. Thus, ATP is required for both contraction (by myosin ATPase) and relaxation. In fact, if sufficient ATP is not available (as occurs following death), muscle relaxation cannot occur and the muscle remains in a contracted state

recruitment, or multiple motor unit summation

The increase in muscle tension that occurs with an increase in stimulus intensity

Sarcolemma

The plasma membrane of a skeletal muscle fiber (around entire cell) muscle action potentials travel along the sarcolemma ability for muscle to send an impulse all the way down itself

Means for Supplying ATP

The second criterion to differentiate skeletal muscle fibers is whether the primary means the fiber uses to supply ATP is either aerobic cellular respiration or glycolysis. Oxidative fibers specialize in providing ATP through aerobic cellular respiration and have several features that support these processes, including an extensive capillary network, large numbers of mitochondria, and a large supply of the red pigment myoglobin. (The presence of both myoglobin and mitochondria gives these skeletal muscle fibers a red appearance, and they are sometimes called red fibers.) The higher levels of ATP generated provide energy for oxidative fibers to continue contracting for extended periods of time without tiring, or fatiguing—thus, these fibers are also called fatigue-resistant. In contrast, glycolytic fibers specialize in providing ATP more rapidly through glycolysis. Generally, they have fewer structures needed for aerobic cellular respiration—thus, they have less extensive capillary networks, fewer mitochondria, and smaller amounts of myoglobin. (The relatively small amount of myoglobin and mitochondria is why these skeletal muscle fibers have a white appearance and are sometimes called white fibers.) However, they do have large glycogen reserves for supplying glucose for glycolysis, which is useful when oxygen stores are low. Glycolytic fibers generally tire easily after a short time of sustained muscular activity—thus, these fibers are also called fatigable.

Sarcolemma, T-tubules, and Sarcoplasmic Reticulum: Excitation-Contraction Coupling

The second physiologic event of muscle contraction is excitation-contraction coupling—an event that involves the sarcolemma, T-tubules, and sarcoplasmic reticulum. This event "couples," or links, the events of skeletal muscle stimulation at the neuromuscular junction (first step) to the events of contraction caused by sliding myofilaments within the sarcomeres of skeletal muscle fiber. Three events occur during excitation-contraction coupling: development of an end-plate potential at the motor end plate, initiation and propagation of an action potential along the sarcolemma and T-tubules, and release of Ca2+ from the sarcoplasmic reticulum.

concentric contraction

The shortening of muscle length; It occurs because the muscle tension is greater than the resistance. It may occur in the biceps brachii (muscle of the anterior arm) when lifting a baby.

single-unit smooth muscle

The smooth muscle cells of single-unit smooth muscle typically form two or three sheets. These sheets of smooth muscle are within the walls of the digestive, urinary, and reproductive tracts, as well as smaller portions of the respiratory tract and most blood vessels. These large sheets of smooth muscle are functionally linked by gap junctions between cells. Nerve stimulation of single-unit smooth muscle occurs through numerous swellings of the autonomic motor neurons that pass in close proximity to several smooth muscle cells; these swellings are called varicosities. Synaptic vesicles within the varicosities contain one type of neurotransmitter (e.g., ACh, norepinephrine). Receptors in smooth muscle cells are scattered diffusely across the sarcolemma of these cells. This contrasts to their distribution in skeletal muscle cells, where receptors are clustered in a motor end plate. This scattered and loose arrangement of receptors in single-unit smooth muscle is called a diffuse junction. Neurotransmitter released from varicosities stimulates numerous smooth muscle cells simultaneously. This occurs much like the extensions of a sprayer releasing water onto a lawn or garden. The stimulation may subsequently be spread from cell to cell via gap junctions, and smooth muscle cells contract synchronously as one unit.

Energy Supply and Varying Intensity of Exercise

The use of creatine phosphate, glycolysis, and aerobic cellular respiration as the primary means for supplying ATP during physical activity is dependent upon both the intensity and the duration of the activity. At rest, skeletal muscle obtains the needed ATP almost exclusively through aerobic cellular respiration involving oxidation of fatty acids. To illustrate the use of energy during exercise, we describe the primary means of supplying ATP for runners at a track meet in which individuals run different distances When an individual participates in a 50-meter sprint, an event that may take 5 to 6 seconds, ATP is supplied primarily by available ATP and Pi transfer between two ADP molecules and between creatine phosphate and ADP. In a longer sprint of 400 meters, an event that may take 50 to 60 seconds, ATP is supplied initially by ATP and Pi transfer and then primarily by glycolysis. Finally, in a 1500-meter run, an event that may take 5 to 6 minutes, ATP is supplied by all three means, but primarily by aerobic processes after about the first minute. Keep in mind, however, that there is overlap between the three different energy sources. Intense exercise that is sustained longer than approximately 1 minute is dependent upon the body's ability to deliver sufficient oxygen through the cardiovascular and respiratory systems. One consequence of participating in regular aerobic exercise (defined as a sustained exercise of moderate intensity that involves raising the heart rate above the baseline) is that it produces changes within both the respiratory system and the heart and blood vessels of the cardiovascular system that enhance oxygen delivery. This allows an individual to more effectively provide ATP through aerobic cellular respiration and, thus, to be able to exercise both at greater levels of intensity and for longer periods of time

Comparison of Myofilaments of Smooth Muscle and Skeletal Muscle

Thick filaments in smooth muscle have myosin heads along their entire length, rather than only at the ends as in skeletal muscle. The more numerous heads can form additional crossbridges with actin to produce a powerful muscle contraction. Additionally, these myosin heads have modifications that allow them to "latch on" to the actin of thin filaments and remain attached without using additional ATP. This mechanism is called the latchbridge mechanism. Smooth muscle thin filaments are composed of actin and tropomyosin, but they do not contain troponin molecules as in skeletal muscle fibers and cardiac muscle cells. Instead, two other proteins are required for initiation of smooth muscle contraction: (1) calmodulin, a protein that binds Ca2+ to form a Ca2+-calmodulin complex, and (2) myosin light-chain kinase (MLCK), an enzyme that is activated by the Ca2+-calmodulin complex to phosphorylate the smooth muscle myosin head. The phosphorylation of the smooth muscle myosin head causes activation of its ATPase activity. A third protein called myosin light-chain phosphatase is an enzyme that dephosphorylates the myosin head, resulting in the inactivation of the ATPase activity. This inactivation is required for relaxation of smooth muscle.

resting muscle tone

This random contraction of small numbers of motor units causes the skeletal muscle to develop tension These random contractions do not generate enough tension to cause Page 360movement. The resting muscle tone establishes constant tension on the muscle's tendon, thus stabilizing the position of the bones and joints. Another function of muscle tone is to "prime" a muscle for contraction, so that it can respond more readily to stimulation requiring muscle movement. Note that muscle tone decreases during deep sleep (sleep associated with rapid eye movement, or REM, sleep). Consider the difference in muscle tone when carrying an awake child (who has muscle tone) compared to carrying a sleeping child (who temporarily lacks muscle tone). You may have noticed that it is more difficult to carry a sleeping child because the child's body is less rigid.

Distribution of muscle fiber types

Variations are also present between individuals, and this is seen most dramatically in high-caliber athletes. Elite distance runners have higher proportions of SO fibers in their lower limb muscles, and top athletes who participate in brief periods of intense activity, such as sprinting or weight lifting, have a higher percentage of FG fibers. These variations in the proportion of the skeletal muscle fiber types are determined primarily by a person's genes and less so by the type of training. A proportion of FG fibers may develop the appearance and functional capabilities of FO fibers with physical conditioning if the muscle is used repeatedly for endurance events. Whether this shift actually represents a change of skeletal muscle fiber type—or is simply a temporary alteration to the muscle, which reverts back when the training ceases—remains controversial.

Controlling Smooth Muscle

We cannot voluntarily control the contraction of the smooth muscle in the wall of the digestive tract, as we find when our stomach "growls" at an inappropriate time. Smooth muscle, like cardiac muscle, is controlled by the autonomic nervous system. The response of smooth muscle to stimulation by the nervous system—that is, whether it contracts or relaxes—is dependent upon the specific neurotransmitter that is released and the receptors to which the neurotransmitter binds. Smooth muscle within the walls of bronchioles, for example, contracts in response to the release of ACh, and relaxes in response to norepinephrine. Smooth muscle also contracts in response to being stretched. This physiologic response is called the myogenic (mī′o-jen′ik; genesis = origin) response. The myogenic response occurs, for example, in smooth muscle in the walls of blood vessels, the stomach, and the urinary bladder. Its response, however, is not continuous if the stretch is prolonged. Instead, the smooth muscle exhibits what is called the stress-relaxation response. This occurs when smooth muscle is "stressed" by being stretched. It responds by contracting, but after a given period of time, it relaxes. For example, swallowed materials entering the stomach cause its wall to stretch, and the smooth muscle in the wall initially contracts. After a period of time it relaxes, allowing additional food to more easily enter the stomach. Smooth muscle is also stimulated to contract by various hormones, a decrease in pH, low oxygen concentration, increased carbon dioxide levels, certain drugs, and pacemaker cells. For example, the hormone oxytocin causes contraction of smooth muscle in the uterus to expel the baby at childbirth.. A pacemaker (similar to the pacemaker in the heart) stimulates smooth muscle in the walls of the stomach and small intestine to contract rhythmically to mix and propel the contents through the lumen of these organs

Isometric contraction

When skeletal muscle tension is insufficient to overcome the resistance (i.e., force generated is less than the load), there is no movement of the muscle. This type of muscle contraction is called an isometric (ī-sō-met′rik; iso = same, metron = measure) contraction. Thus, the skeletal muscle contracts and muscle tension increases, but muscle length stays the same. Some examples of isometric contractions include pushing on a wall (a posture for stretching one's leg muscles), holding a very heavy weight in the gym while your arm does not move, attempting to move a shovel load of snow that is too heavy, and holding a baby in one position

Isotonic contraction

When skeletal muscle tension results in movement of the muscle The tone in the skeletal muscle remains the same as the length of muscle changes. Examples of an isotonic contraction include walking, lifting a baby, and swinging a tennis racket. Isotonic contractions are differentiated into two subclasses based on whether the muscle is shortening or lengthening as it contracts

Release of Calcium from the Sarcoplasmic Reticulum

When the action potential reaches the sarcoplasmic reticulum, it (1) stimulates a conformational change to voltage-sensitive Ca2+ channels (dihydropyridine receptors) within the T-tubule membrane, which (2) causes a conformational change in Ca2+ release channels (ryanodine receptors) located in the terminal cisternae of the sarcoplasmic reticulum, causing them to open. This allows Ca2+ to diffuse out of the cisternae of the sarcoplasmic reticulum into the cytosol. Calcium now "mingles" with the thick filaments and thin filaments within myofibrils.

Rigor mortis

Within a few hours after the heart stops beating, ATP levels in skeletal muscle fibers have been completely exhausted. The sarcoplasmic reticulum loses its ability to return Ca2+ from the sarcoplasm and move it back into the sarcoplasmic reticulum by the Ca2+ pumps, which require ATP to function. Remember that ATP is also needed to detach the myosin head of the thick filament from the myosin binding site of actin on the thin filaments. Because ATP is no longer available, the crossbridges between thick and thin filaments cannot detach. As a result, the Ca2+ already present in the sarcoplasm, as well as the Ca2+ that continues to leak out of the sarcoplasmic reticulum, triggers a sustained contraction in the skeletal muscle fibers. All skeletal muscles lock into a contracted position and the body of the deceased individual becomes rigid. This physiologic state, termed rigor mortis (rig′er mōr′tis), continues for about 15 to 24 hours. Rigor mortis gradually disappears because lysosomal enzymes are released within the muscle fibers, causing autolysis (self-destruction and breakdown) of the myofibrils. Forensic pathologists often use the development and resolution of rigor mortis to establish an approximate time of death. Because a number of factors affect the rate of development and resolution of rigor mortis, environmental conditions need to be taken into consideration. For example, a warmer body will develop and resolve rigor mortis much more quickly than a body of normal temperature. The following chart provides rough guidelines for estimating the death interval, assuming that body temperature and ambient (surrounding environment) temperature are within normal range. Death Interval Body Temperature Stiffness Dead less than 3 hours. Warm. No stiffness Dead 3-8 hour Warm, but cooling. Developing stiffness Dead 8-24 hours. Ambient temperature. Stiff, but resolving Dead 24-36 hours. Ambient temperature. No stiffness

Connectin, also called titin

a "cablelike" protein that extends from the Z discs to the M line through the core of each thick filament. It stabilizes the position of the thick filament and maintains thick filament alignment within a sarcomere. Additionally, portions of the connectin molecules are coiled and "springlike" so that during sarcomere shortening they are compressed to produce passive tension. This passive tension is then released to return the sarcomere to its normal resting length. Thus, connectin contributes to skeletal muscle fiber elasticity

tetanus

a form of spastic paralysis caused by a toxin produced by the bacterium Clostridium tetani. The toxin blocks the release of glycine (an inhibitory neurotransmitter in the spinal cord), resulting in overstimulation by motor neurons of the muscles and excessive muscle contractions. Penetrating wounds contaminated with soil and vegetable matter are especially prone to developing C. tetani infection. This condition is potentially life-threatening, and so we routinely are vaccinated against it.

Glycolysis

a metabolic pathway, which involves the breakdown of glucose into two pyruvate molecules, producing a net of 2 ATP molecules. It occurs within the cytosol, and although it can function in the presence of oxygen, oxygen is not required. Glucose is made available either directly from glycogen stores within the muscle fiber (through glycogenolysis) or delivered by the blood. One of the main advantages of producing ATP through glycolysis is that it does not require oxygen (i.e., it is nonoxidative). The other is its rapid rate of ATP production (i.e., the amount of ATP produced per time—at almost twice the rate of aerobic cellular respiration). Although lower total amounts of ATP are produced (compared to aerobic cellular respiration), ATP is produced more quickly, a necessary requirement for short bursts of maximum exercise (e.g., running a 100-meter dash). What happens to the pyruvate molecules that are produced during glycolysis? The fate of pyruvate molecules is dependent upon oxygen availability. Pyruvate molecules either (1) enter a mitochondrion to be broken down through aerobic cellular respiration (if sufficient oxygen is available) or (2) are converted to lactate molecules (under conditions of low oxygen availability).

Creatinine phosphate

a molecule with a high-energy chemical bond between creatine and Pi and is present in tissues with both large and fluctuating energy needs (e.g., muscle, brain). When skeletal muscle is actively contracting, the Pi in creatine phosphate is readily transferred to ADP to form additional ATP (and creatine), an enzymatic reaction catalyzed by creatine kinase. This provides an additional 10 to 15 seconds of energy during maximum exertion. also called creatine phosphokinase, is the enzyme that helps transfer a phosphate between creatine and ATP. Different forms of creatine kinase are present within cardiac muscle and skeletal muscle. The heart muscle form of creatine kinase is found in elevated blood levels in patients suffering from a myocardial infarction (heart attack: "Coronary Heart Disease, Angina Pectoris, and Myocardial Infarction"). This provides a specific diagnostic tool for identifying damage to the heart. In contrast, elevated levels of the skeletal muscle form of creatine kinase are used to diagnose degenerative skeletal muscle disease, such as muscular dystrophy. However, note that elevated levels of the skeletal muscle form of creatine kinase may also occur after intense exercise and therefore is not always a sign of disease. Later during times of rest, the limited stores of ATP and CP within skeletal muscle are replenished. ATP is formed through cellular respiration, and some of those ATP molecules are used to regenerate creatine phosphate. The process that happens at rest is the reverse of the process that happens during exercise. The Pi in ATP is transferred to creatine to form additional creatine phosphate and ADP, an enzymatic reaction also catalyzed by creatine kinase. These enzymatic reactions involving Pi transfer to ADP to form ATP are not dependent upon the presence of oxygen.

Botulism

a potentially fatal muscular paralysis, is caused by a toxin produced by the bacterium, Clostridium botulinum. The toxin prevents the release of acetylcholine (ACh) at synaptic knobs and leads to muscular paralysis. Like C. tetani, C. botulinum is common in the environment and produces its toxin under anaerobic conditions. Most cases of botulism poisoning result from ingesting the toxin in canned foods that were not processed at temperatures high enough to kill the botulism spores. Similarly, ingestion of unpasteurized honey by infants in the first year of life can introduce C. botulinum spores into their immature gastrointestinal tracts. The Food and Drug Administration (FDA) approved the use of botulinum toxin type A (Botox) for temporary diminishing of wrinkles (see Clinical View 6.7: "Botox and Wrinkles"). Botox is also used clinically to help reduce overcontraction of muscle (or spasticity) associated with certain disorders or conditions (e.g., cerebral palsy, multiple sclerosis, torticollis, changes following a stroke or spinal cord injury). Botox injections have become one of the most important treatments for spasticity and are most effective 1 to 2 weeks after the injections, with spasticity reduced for up to 3 to 6 months. Treatments may be repeated as often as every 3 months.

Tropomyosin

a short, thin, twisted filament that is a "stringlike" protein. Consecutive tropomyosin molecules cover small regions of the actin strands, including the myosin binding sites in a noncontracting muscle

twitch

a single, brief contraction period and then relaxation period of a skeletal muscle in response to a single stimulation. The minimum voltage needed to stimulate the skeletal muscle to generate a twitch is the threshold. The voltage below the threshold is called a subthreshold stimulus. There is a delay called a latent period (lag period) that occurs after the stimulus is applied and before the contraction of the skeletal muscle fiber begins. There is no change in fiber length during the latent period. This delay can be accounted for by the time necessary for all of the events in excitation-contraction coupling, Ca2+ release from the sarcoplasmic reticulum into the cytosol, and the beginning of tension generation within the skeletal muscle fiber. The contraction period begins as repetitive power strokes pull the thin filaments past the thick filaments, shortening the sarcomeres; muscle tension increases during muscle contraction. The relaxation period begins with release of crossbridges as Ca2+ is returned to its storage within the sarcoplasmic reticulum; muscle tension decreases during muscle relaxation. Relaxation depends upon the elasticity of connectin within muscle tissue to return to its original length following shortening of the muscle. The time required for a twitch varies based on the predominant type of skeletal muscle fibers composing the muscle. The extrinsic eye muscles are predominantly fast-twitch fibers producing a twitch that is as rapid as 7.5 milliseconds, whereas the soleus (deep calf muscle) is predominantly slow-twitch fibers producing a twitch that lasts about 100 milliseconds

tendon

a thick, cordlike structure composed of dense regular connective tissue rope-like extension of three connective tissue layers attaches muscle directly to periosteum of a bone attach a muscle either to a skeletal component (bone or ligament) or to fascia example: achilles tendon attaches gastrocnemius muscle of calf to calcaneus of tarsus

contraction regulation by......?- skeletal muscle

acetylcholine released by somatic motor neurons

what provides most energy for sustained muscle contraction?

aerobic cellular respiration

deep fascia

also called visceral or muscular fascia, is an additional, expansive sheet of dense irregular connective tissue that is external to the epimysium. It separates individual muscles; binds together muscles with similar functions; contains nerves, blood vessels, and lymph vessels; and fills spaces between muscles. The deep fascia is internal or deep to a layer called the superficial fascia (or subcutaneous layer)

myasthenia gravis (MG)

an autoimmune disease that occurs in about 1 in 10,000 people, primarily women between 20 and 40 years of age. A person's own antibodies attack the neuromuscular junctions, binding ACh receptors into clusters. The abnormally clustered ACh receptors are removed from the muscle fiber sarcolemma by endocytosis, thus significantly diminishing the number of receptors within the sarcolemma. The resulting decreased muscle stimulation causes rapid fatigue and muscle weakness. Eye and facial muscles are often attacked first, producing double vision and drooping eyelids. These symptoms are usually followed by swallowing problems, limb weakness, and overall low physical stamina. Some patients with MG have a normal life span, whereas others die within a short time from paralysis of the respiratory muscles.

creatine phosphate

an energy-rich molecule found only in muscle fibers -synthesized in liver, kidneys, and pancreas, then transported to muscle fibers -creatine kinase (enzyme) transfers high-energy phosphate group from excess ATP to creatine when muscle is relaxed

Hypertrophy

an increase in muscle size; results from an increase in the number of mitochondria, greater amounts of myoglobin, and larger glycogen reserves. A slight increase in both ATP and creatine phosphate stores also occurs.

titin

anchors Z disc to M line

Thin filaments (or thin myofilaments)

are approximately half of the diameter of thick filaments (about 5 to 6 nanometers). Thin filaments are primarily composed of two strands of actin protein twisted around each other to form a helical shape

Thick filaments (or thick myofilaments)

are assembled from bundles of 200 to 500 myosin protein molecules

Tropomyosin and troponin

are regulatory proteins associated with thin filaments. Together they form the troponin-tropomyosin complex.

skeletal muscle fibers

are the primary cells forming a skeletal muscle. Like other cells, they contain cytoplasm with the typical cellular structures, such as the Golgi apparatus, ribosomes, and vesicles. Note that the cytoplasm in skeletal muscle fibers is more specifically called sarcoplasm. In addition, skeletal muscle fibers have several specialized features that we describe here, including the details of its contractile proteins.

intercalated discs

are unique to cardiac muscle; they are composed of desmosomes and gap junctions

contraction cycle continues:

as ATP and Ca2+ ions are available, repeating steps 2, 3 and 4.

cardiac muscle appearance

branched cylindrical fiber with one centrally located nuclei intercalated discs join neighboring fibers striated

glycolysis

breakdown of glucose from blood stream or from breakdown of glycogen stores in sarcolemma yields two ATP molecules and two pyruvic acid molecules

recovery oxygen uptake (oxygen debt)

breathing rate and oxygen consumption remains above resting level during recovery period helps restore metabolic conditions ongoing changes after exercise also boost oxygen use

what happens when ACh activity is terminated?

broken down by acetylcholinesterase enzyme (AChE)

Myosin protein

consists of two strands; each strand has a globular head and an elongated tail. The myosin head contains a binding site for actin of the thin filaments. The head also has a catalytic ATPase site where adenosine triphosphate (ATP) attaches and is split into Page 339adenosine diphosphate (ADP) and phosphate (Pi). (It is because the head of myosin functions as an ATPase enzyme that myosin is often referred to more specifically as myosin ATPase.) The tails of two strands of a myosin molecule are intertwined. Each myosin molecule composing a thick filament is oriented so that its tails point toward the center of the thick filaments and its heads point toward the ends of the thick filaments. You may find it helpful to think of the myosin protein molecules as two intertwined golf clubs, where many are grouped together with the golf club shafts in the center and the club heads on each end.

what happens in restoring metabolic conditions?

convert lactic acid back into glycogen stores in liver resynthesizes creatine phosphate and ATP in muscle fibers replace oxygen removed from myoglobin

pyruvic acid

converted to lactic acide in absence of oxygen lactic acid moved into blood and carried to lover for reconversion to glucose (if no O2 is available)

what is only sufficient for short bursts of activity?

creatine phosphate about 15 seconds

Z discs (also called Z lines)

dense protein material that separates one sarcomere from next in myofibril are composed of specialized proteins that are positioned perpendicular to the myofilaments and serve as anchors for the thin filaments. Although the Z disc appears as a flat disc when the myofibril is viewed from its end, only the edge of the disc is visible in a side view, and it sometimes looks like a zigzagged line.

perimysium

dense, irregular connective tissue surronds bundles of muscle fibers called fascicles each fascicle contains ten to one hundred muscle fibers (cells)

skeletal muscle fiber contraction generated

differ in the power, speed, and duration of the muscle contraction generated. Power is related to the diameter of a muscle fiber; large muscle fibers have a larger number of myofibrils in parallel, allowing them to produce a more powerful contraction. Speed has traditionally been described based on whether the skeletal muscle fiber expresses the relatively slow or fast genetic variant of myosin ATPase, the enzyme that splits ATP (see section 10.3c). Those with a fast variant are called fast-twitch fibers, and those with the slow variant are called slow-twitch fibers. However, recent Page 356evidence shows that fast-twitch fibers also have both a fast rate of action potential propagation along the sarcolemma and are quick in their Ca2+ release and reuptake by the sarcoplasmic reticulum in comparison to slow-twitch fibers. Thus, fast-twitch fibers initiate a contraction more quickly following stimulation than a slow-twitch fiber (0.01 milliseconds [msec] versus at least 0.02 msec) and produce a contraction of shorter duration (7.5 msec versus 100 msec). Fast-twitch fibers typically have all three characteristics: They produce a strong contraction, initiate a contraction more quickly following stimulation, and produce a contraction of shorter duration. These characteristics account for why fast-twitch fibers exhibit both power and speed in comparison to slow-twitch fibers.

calmodulin instead of troponin:

easy to contract not a lot of Ca2+

a single nerve impulse:

elicits a single muscle twitch contraction in all the muscle fibers it innervates

Muscle fibers

embryonic development by fusion of many myoblasts one elongated, multinucleated amitotic (doesnt divide) muscle fiber

synaptic end bulb

end of motor neuron axon at NMJ **synaptic vesicles contain acetylcholine (Ach)**

connective tissue components of skeletal muscle

endomysium, perimysium, epimysium

Skeletal muscle is composed primarily of muscle cells that exhibit these characteristics

excitability, conductivity, contractility, extensibility, and elasticity -Excitability is the ability of a cell to respond to a stimulus (e.g., chemical, stretch). The stimulus causes a local change in the resting membrane potential by triggering the movement of ions across the plasma membrane of the excitable cell. -A skeletal muscle cell responds when its receptors bind neurotransmitter (acetylcholine), which is released from a motor neuron. -Conductivity involves an electrical signal that is propagated along the plasma membrane as voltage-gated channels open sequentially during an action potential. These electrical signals functionally connect the plasma membrane of the muscle cell (where stimulation occurs) to the interior of the muscle cell. -Contractility is exhibited when contractile proteins within skeletal muscle cells slide past one another. Contractility is what enables muscle cells to cause body movement and to perform the other functions of muscles. -Extensibility is the lengthening of a muscle cell. This lengthening is possible because the contractile proteins slide past one another to decrease their degree of overlap. Muscle's extensibility is exhibited when we stretch our muscles, such as before exercising. -Elasticity is the ability of a muscle cell to return to its original length following either shortening or lengthening of the muscle. Elasticity of muscle cells is dependent upon the release of tension in the springlike connectin protein associated with contractile proteins

smooth muscle appearance

fiber is thickest in the middle tapered at the end has one centrally positioned nucleus not striated

sarcoplasmic reticulum

fluid-filled, membranous organelle similar to smooth endoplasmic reticulum of non-muscle cells wraps around each myofibril dilated end sacs are terminal cisterns, associated with either side of a T-tubule to form a triad stores and releases calcium ions (CA2+) that trigger muscle contraction an internal membrane complex that is similar to the smooth endoplasmic reticulum of other cells (see section 4.6a). Segments of the sarcoplasmic reticulum (SR) fit around the myofibril like a sleeve of membrane netting. At either end of individual sections of the sarcoplasmic reticulum are blind sacs called terminal cisternae (sis-ter′nē; sing., sis-ter′nă; cista = a box), which are much like the hem of a sleeve.

motor end plate

folded region of sarcolemma opposite synaptic end bulb increases surface area abundant acetylcholine receptors (receive transmitter) is a specialized region of the sarcolemma of a skeletal muscle fiber. (It is so named because "motor end" reflects that it is located at the end of a motor neuron and "plate" describes its large, saucerlike appearance.) It has numerous folds and indentations (junction folds) to increase the membrane surface area covered by the synaptic knob. The motor end plate has vast numbers of ACh receptors. These plasma membrane protein channels are chemically gated ion channels. Binding of ACh opens these channels, allowing Na+ entry into the muscle fiber and K+ to exit. ACh receptors are like doors; ACh is the only "key" to open these receptor doors.

fibrosis

formation of excess fibrous connective tissue in an organ or tissue in a reparative or reactive process Muscle mass is often replaced by adipose connective tissue and dense regular (fibrous) connective tissue. The increasing amounts of this connective tissue decrease the flexibility of muscle; an increase in collagen fibers can restrict movement and circulation.

unfused tetanus

frequency of multiple stimuli allows only partial muscle relaxation between each stimulus *complete contraction of muscle and does not relax

junctions between fibers- smooth muscle

gap junctions in visceral smooth muscle; none in multiunit smooth muscle

Blood vessels

generally, each muscle has one artery and two veins plentiful microscopic blood capillaries bring oxygen and nutrients and remove metabolic wastes each muscle fiber is in close contact with **one or more microscopic blood capillaries**

lactic acids turns into _____ by _______.

glucose glyconeogenosis

location of cardiac muscle

heart

muscle fatigue

inability of muscle to contract forcefully after prolonged activity the reduced ability or the inability of the skeletal muscle to produce muscle tension. The primary cause of muscle fatigue during excessive or sustained exercise (e.g., running a marathon) is a decrease in glycogen stores. However, there are many other causes of muscle fatigue, which are still being debated. Here they are organized by the specific physiologic event of muscle contraction that is affected. Excitation at the neuromuscular junction. Muscle fatigue may be caused either by insufficient free Ca2+ at the neuromuscular junction to enter the synaptic knob or by a decreased number of synaptic vesicles to release neurotransmitter (see section 10.3a). Both limit the ability of somatic motor neurons to stimulate a skeletal muscle. Excitation-contraction coupling. Muscle fatigue may be due to a change in ion concentration (e.g., Na+, K+) that interferes with the ability of the muscle fiber to conduct an action potential along the sarcolemma (see section 10.3b). This interferes with stimulating release of Ca2+ from the sarcoplasmic reticulum. Crossbridge cycling. Muscle fatigue may result from increased phosphate ion (Pi) concentration. Elevated Pi concentration in the muscle sarcoplasm interferes with Pi release from the myosin head during crossbridge cycling, and this slows the rate of cycling. Muscle fatigue also may occur when lower amounts of Ca2+ are available for release from the sarcoplasmic reticulum (part of which is due to its binding with the excess Pi). Lower Ca2+ levels result in less Ca2+ binding to troponin, reducing crossbridge formation, which results in a weaker muscle contraction (see section 10.3c). Thus, both an increase in Pi concentration and lower Ca2+ levels result in a weaker force generated during muscle contraction. Lack of ATP is not currently thought to be a primary cause of muscle fatigue. This is because ATP levels are generally maintained through aerobic cellular respiration in mitochondria during sustained exercise. It remains to be determined if ATP may still be a factor because of its location in the cell—that is, within the mitochondria and not in proximity to myofilaments.

Muscular paralysis

inability of skeletal muscles to contract) may occur if either nervous system function at the neuromuscular junction or excitation-contraction coupling is impaired. This damage may be the result of neurotoxins, which are toxins that damage nervous system components. Two paralysis conditions caused by toxins are tetanus and botulism.

hyperplasia

increase in number of muscle fibers from satellite cells Recent evidence suggests that some (limited) increase in the number of muscle fibers also may occur, a process called *demand/need to create more body does'nt do this much because it does'nt like it and takes up a lot of energy

what happens during exercise that boosts oxygen?

increased body temperature speeds up all metabolism heart and respiratory muscles still working harder than at rest tissue repair processes are increased

cardiac muscle cells

individual muscle cells arranged in thick bundles within the heart wall. Cardiac muscle cells branch and are both shorter and thicker than skeletal muscle fibers. (They typically have a diameter of about 15 µm and range in length from 50 to 100 µm.) Individual cells are joined to adjacent muscle cells at junctions termed intercalated discs. These cells have only one or two nuclei. Cardiac muscle cells are striated because, like skeletal muscle fibers, cardiac muscle cells also contain sarcomeres. Cardiac muscle cells contain a large number of mitochondria, and they use aerobic cellular respiration almost exclusively to generate the ATP required for their unceasing work. In addition, the sarcoplasmic reticulum is not as well developed as in skeletal muscle, so most Ca2+ enters the cardiac muscle fibers from interstitial fluid instead. Cardiac muscle cells are stimulated by a specialized autorhythmic pacemaker. This feature is responsible for the repetitious, rhythmic heartbeat. The autonomic nervous system (a division of the nervous system that controls cardiac muscle contraction, smooth muscle contraction, and gland secretion, which is discussed in chapter 15) controls both the rate and the force of contraction of cardiac muscle

junctions between fibers- cardiac muscle

intercalated discs contain gap junctions and desmosomes

fast oxidative-glycolytic (FOG)- fiber diameter

intermediate

fast oxidative-glycolytic (FOG)

intermediate contraction moderate fatigue resistance aerobic respiration and glycolysis *walking and sprinting **muscles in legs

nervous control- cardiac muscle

involuntary

nervous control- smooth muscle

involuntary

isotonic contraction

involves a change in muscle length without a change in its tension concentric or eccentric

resistance exercise

involves producing forceful muscle contractions (e.g., weight lifting or power lifting), primarily results in stronger skeletal muscles. Resistance exercise stimulates skeletal muscle fibers to increase contractile proteins (myosin, actin), especially in fast glycolytic muscle fibers.

Endurance (or aerobic) exercise

involves sustained, moderate activity that increases heart rate (e.g., running several miles). This type of exercise causes changes that primarily alter how skeletal muscle fibers are supplied with energy. The changes that specifically occur to skeletal muscle fibers include (1) an increase in the number of mitochondria and the enzymes within mitochondria, which enhances ATP production through aerobic cellular respiration; (2) an increase in enzymes for using fatty acids in aerobic cellular respiration; and (3) an increase in the amounts of lactate dehydrogenase enzyme (for converting lactate back to pyruvate). The greater availability of fatty acids and pyruvate delays glycogen depletion within skeletal muscle fiber and, thus, fatigue. Endurance exercise also induces changes to the cardiovascular system. The heart wall thickens, which increases the amount of blood that can be pumped by the heart, and additional blood vessels form within skeletal muscle through angiogenesis. Both of these changes provide more efficient delivery of blood, and additional oxygen is supplied to skeletal muscle. These changes also enhance ATP production through aerobic cellular respiration.

Action potential

involves two events: depolarization, which causes the inside of the sarcolemma of the skeletal muscle fiber to become positive due to the influx of Na+, and repolarization, which is the returning of the inside of the sarcolemma to its relatively negative resting membrane potential due to the outward flow of K+.

Troponin

is a globular, or "ball-like," protein attached to tropomyosin. Troponin contains the binding site for Ca2+.

Synaptic knob of a motor neuron

is an expanded tip of an axon. Where the axon nears the sarcolemma of a muscle fiber, the synaptic knob enlarges and flattens to cover a relatively large surface area of the sarcolemma. The synaptic knob cytosol houses numerous synaptic vesicles (small membrane sacs) filled with molecules of the neurotransmitter acetylcholine First, Ca2+ pumps are embedded within the plasma membrane of the synaptic knob. Prior to the arrival of the electrical signal (nerve signal) at the synaptic knob, Ca2+ pumps within its plasma membrane have established a Ca2+ concentration gradient, with more Ca2+ outside the synaptic knob than inside it. Second, voltage-gated Ca2+ channels are also embedded in the membrane of the synaptic knob. Opening of these channels allows Ca2+ to flow down its concentration gradient from the interstitial fluid into the synaptic knob, which will trigger exocytosis of acetylcholine from the vesicles. Third, vesicles are normally repelled from the synaptic knob plasma membrane.

Synaptic cleft

is an extremely narrow (30 nanometers), fluid-filled space separating the synaptic knob and the motor end plate. The enzyme acetylcholinesterase (AChE) resides within the synaptic cleft and quickly breaks down ACh molecules following their release into the synaptic cleft.

Skeletal muscle function

is an organ -Body movement. Contraction of your skeletal muscles generates large body movements, such as those of walking, and the smaller, more precise body movements such as picking up an object. It is also responsible for the highly developed movements involved in communicating that occur when speaking, writing, and changing facial expressions; the movements associated with breathing; and those involved in the voluntary phase of swallowing -Maintenance of posture. Contraction of specific skeletal muscles stabilizes your trunk, pelvis, legs, neck, and head to keep you erect. These postural muscles contract continuously when you are awake to keep you from collapsing. -Protection and support. Skeletal muscle is arranged in layers within the walls of the abdominal cavity and the floor of the pelvic cavity. These layers of muscle protect the internal organs and support their normal position within the abdominopelvic cavity. -Regulating elimination of materials. Circular muscle bands, called sphincters (sfingk′ter; sphincter = a band) contract and relax to regulate passage of material. These skeletal muscle sphincters at the orifices of the gastrointestinal and urinary tracts allow you to voluntarily control the expulsion of feces and urine, respectively. -Heat production. Energy is required for muscle tissue contraction, and heat is always produced by this energy use (the second law of thermodynamics). Thus, muscles are like small furnaces that continuously generate heat and function to help maintain your normal body temperature. You shiver when you are cold because involuntary skeletal muscle contraction gives off heat. Likewise, you sweat during exercise to release the additional heat produced by your working muscles

Superficial fascia

is composed of areolar connective tissue and adipose connective tissue that separates muscle from skin.

multiunit smooth muscle

is found within the eye in both the iris and ciliary muscles, composing the arrector pili muscles in the skin, the wall of larger air passageways within the respiratory system, and the walls of larger arteries in these body structures are arranged into motor units, and they have a neuromuscular junction. These two features are similar to skeletal muscle, except that the motor neuron here is a component of the autonomic nervous system. The degree of contraction of this smooth muscle is dependent upon the number of motor units activated, thus facilitating increasing degrees of tension as more motor units are stimulated.

Dystrophin

is part of a protein complex that anchors myofibrils that are adjacent to the sarcolemma to proteins within the sarcolemma. These proteins of the sarcolemma also extend to the connective tissue of the endomysium that encloses the muscle fiber. Thus, dystrophin links internal myofilament proteins of a muscle fiber to external proteins. The genetic disorder of muscular dystrophy is caused by abnormal structure, or amounts, of dystrophin protein

Duchenne's muscular dystrophy (DMD)

is the most common form of the illness. It is almost exclusively a disease of males and occurs in about 1 in 3500 live births. DMD results from the expression of a sex-linked recessive allele, which is the gene that directs the synthesis of dystrophin. In DMD, dystrophin either has an abnormal structure or is produced in insufficient amounts. The defective, reduced, or absent dystrophin results in an unstable sarcolemma, which is susceptible to damage by forces generated during muscle contraction. Excess calcium ions then enter the muscle fibers, damaging the contractile proteins with an accompanying loss of muscle fibers. For these individuals, muscular difficulties become apparent in early childhood. Walking is a problem; the child falls frequently and has difficulty standing up again. The hips are affected first, followed by the lower limbs, and eventually the abdominal and vertebral muscles. Muscular atrophy causes shortening of the muscles, which results in postural abnormalities such as scoliosis (lateral curvature of the spine) DMD is an incurable disease, with patients confined to a wheelchair by adolescence. An individual with DMD rarely lives beyond the age of 30, and death typically results from respiratory or heart complications.

Skeletal muscle

is vascularized by an extensive network of blood vessels. The blood vessels extend through both the epimysium and the perimysium to reach the endomysium, which ensheathes each muscle fiber. The smallest blood vessels called, capillaries, are associated with the endomysium and function as the site of exchange of substances (e.g., oxygen, glucose, waste products) between the blood and the skeletal muscle fibers.. Skeletal muscle is innervated by motor neurons, which control skeletal muscle contraction. Somatic motor neurons extend from the brain and spinal cord to skeletal muscle fibers. Each motor neuron has a long extension called an axon (nerve fiber) that branches extensively at its terminal end. The axon extends through all three connective tissue layers to almost make contact with an individual muscle fiber (i.e., there is a very small gap of about 30 nanometers between the motor neuron and muscle fiber). The junction between the axon and the muscle fiber itself is called a neuromuscular junction. Skeletal muscle is classified as voluntary muscle because the skeletal muscle fibers can be consciously controlled by the nervous system.

types of contractions (2)

isotonic isometric

fiber diameter of cardiac muscle

large

fast oxidative-glycolytic (FOG)- myoglobin content

large amount

slow oxidative (SO)- myoglobin content

large amount

fast glycolytic (FG)- fiber diameter

largest

eccentric contraction

lengthening of muscle; During an eccentric contraction, the muscle exerts less force than that needed to move the load, and the muscle lengthens. If you are holding a 10-pound weight in your hand, and the muscles of your anterior upper arm (e.g., biceps brachii) exert 5 pounds of force, then the biceps muscle lengthens in an eccentric contraction (as your arm extends). This also occurs, for example, in the biceps brachii when placing a baby into a crib.

capacity for regeneration?- cardiac muscle

limited, under certain conditions

capacity for regeneration?- skeletal muscle

limited, via satellite cells

dystrophin

links sarcomere to sarcolemma

skeletal muscle appearance

long cylindrical fiber with many peripherally located nuclei; striated

flaccid

lose muscle tone when motor neurons are damaged

fast oxidative-glycolytic (FOG)- capillaries

many

fast oxidative-glycolytic (FOG)- mitochondria

many

slow oxidative (SO)- capillaries

many

slow oxidative (SO)- mitochondria

many

contraction control

many small motor units control small, precise movements (10 to 20 muscle fibers per motor unit) -concise movement= neat handwriting few, larger motor units control large, powerful movements (up to 3,000 muscle fibers per motor unit) -moving entire arm

speed of contraction- cardiac muscle

moderate

visceral tissue type (single unit)

most common, found in skin, walls of blood vessels, and hollow organs autorhythmic (no control/involuntary) fibers connected by gap junctions action potential spreads so all cells contract as single unit **group**

nervous control- skeletal muscle

mostly voluntary

The first physiologic event of skeletal muscle contraction

muscle fiber excitation by a somatic motor neuron—an event that occurs at the neuromuscular junction and results in release of ACh and its subsequent binding to ACh receptors.

smooth muscle tissue

non-striated, under involuntary control, and moves substances in hollow internal organs is located throughout the body, typically composing approximately 2% of the weight of an adult. is found in the walls of organs of many different body systems. Smooth muscle function is determined by its location. The following are examples (figure 10.28): Cardiovascular system: Blood vessels alter blood pressure and the distribution of blood flow. Respiratory system: Bronchioles (air passageways) control the resistance to airflow that enters and exits the air sacs (alveoli) of the lungs. Digestive system: The stomach, small intestine, and large intestine mix and propel ingested material as it is moved along the gastrointestinal tract. Urinary system: Ureters propel urine from the kidney to the urinary bladder, which eliminates urine from the body. Female reproductive system: The uterus helps expel the baby during delivery.

total tension in whole skeletal muscle organ depends on:

number of individual muscle fibers contracting at the same time

Lactate formation from pyruvate

occurs under conditions of low oxygen availability. This occurs, for example, during intense exercise when skeletal muscle's oxygen demands for aerobic cellular respiration cannot be met. The pyruvate molecules are instead converted to lactate molecules, an enzymatic reaction catalyzed by lactate dehydrogenase. What happens to lactate following its formation? Lactate can either enter a mitochondrion within a skeletal muscle fiber, where it is converted back to pyruvate and oxidized to carbon dioxide through aerobic cellular respiration, or leave the skeletal muscle fiber and enter the blood. Lactate that enters the blood can then either be (1) taken up by cardiac muscle of the heart, where (as in skeletal muscle) it is converted back to pyruvate and oxidized through aerobic cellular respiration or (2) taken up by the liver to be converted to glucose through gluconeogenesis. Glucose molecules are then released by the liver back into the blood, where they may be taken up into a skeletal muscle fiber. This cycling of lactate to the liver, where it is converted to glucose, and the subsequent transport of glucose from the liver to the muscle is called the lactic acid cycle (or Cori cycle). Observe that lactate does not accumulate in skeletal muscles and thus is not the cause of muscle soreness

Aerobic Cellular Respiration

occurs within mitochondria and requires oxygen, which is made available from the blood or released from myoglobin (figure 10.17c). It involves three stages, including the intermediate step, the citric acid cycle, and the electron transport system. One of the primary advantages of producing ATP through aerobic cellular respiration is the variety of nutrients that can be oxidized, which include pyruvate (made available through glycolysis), fatty acids, and amino acids (which are deaminated (NH2 is removed. The other advantage is that, although the rate of ATP formation is slower (than in glycolysis), greater amounts of ATP are produced. The specific amounts are dependent upon the nutrient that is oxidized (e.g., pyruvate generates 17 ATP molecules; the fatty acid palmitate generates 129 ATP molecules). These higher amounts are a necessary requirement for longer, more moderate levels of activity (e.g., jogging several miles).

when does contraction begin?

once an action potential is propagated along the sarcolemma and into the T-tubules

motor units

one motor neuron and all the skeletal muscle fibers the motor neuron stimulates

Each nerve impulse elicits?

one muscle action potential 1 nerve impulse can make one twitch.

structure of cardiac muscle fibers

one nucleus, branched, striated and *involuntary* interconnected by intercalated discs (desmosomes and gap junctions), so action potential spreads cell-to-cell *without interruption

white muscle fibers

pale with low myoglobin

fascicle

perimysium and muscle fibers within

contraction period

power strokes generate tension

motor unit recruitment

process of increasing the number of contracting motor units -force of muscle contraction becomes greater as more motor units are activated -smooth, involuntary movement from asynchronously stimulated motor units of a skeletal muscle

electrical excitability

produce electrical action potentials (impulses)in response to certain stimuli shared property with neurons *stim machine

3 layers of connective tissue in organ

protect and strengthen skeletal muscle fibers epimysium perimysium endomysium

series of oxidation reactions of nutrients

pyruvic acid from gycolysis, or breakdown of fatty acids or amino acids yields 50 ATP molecules, carbon dioxide, water, and heat

myogram

record of a twitch contraction -duration varies with type of muscle fiber (10 to 100 msec)

fast oxidative-glycolytic (FOG)- color

red-pink

Crossbridge Cycling

refers to a four step process that is repeated (step 3b-e): (1) crossbridge formation (attaching of myosin head to actin), (2) power stroke (pulling thin filament by movement of myosin head), (3) release of myosin head from actin, and (4) resetting of myosin head: Crossbridge formation Myosin heads, which are in the "cocked," or ready, position attach to exposed myosin binding sites of actin. Binding of each myosin head results in formation of a crossbridge between the thick and thin filament (step 3b). Page 349Power stroke After forming a crossbridge, the myosin head swivels (or ratchets) in what is called a power stroke. The swiveling of a myosin head pulls the thin filament a small distance past the thick filament toward the center of the sarcomere. ADP and Pi are released during this process, and the ATP binding site becomes available again (step 3c). Release of myosin head ATP then binds to the ATP binding site of a myosin head, which causes the release of the myosin head from the binding site on actin (step 3d). Resetting of myosin head Myosin ATPase splits ATP into ADP and Pi, providing the energy to reset the myosin head in the cocked position (step 3e). If Ca2+ is still present, and the myosin binding sites are still exposed, then these four steps involving the myosin heads continue: attach, pull, release, and reset. It is the repetitive action of these steps that results in sarcomere shortening, and a sarcomere moves from its relaxed state into a contracted state. Note that calcium levels remain elevated because the skeletal muscle fiber is repeatedly stimulated by the motor neuron at a very rapid rate.

Source of Ca2+ for contraction- cardiac muscle

sarcoplasmic reticulum and interstitial fluid

** Some myoblasts persist as:

satellite cells that can fuse and regenerate muscle fibers in damaged mature skeletal muscle organs

wave summation

second stimulus arrives before muscle fiber has fully relaxed, causing second, stronger contraction The rapidly restimulated muscle displays a summation of contractile forces as the effect of each new wave is added to the previous wave. This effect is often called either wave summation because contraction waves are added together, or "summed," or temporal summation because it depends upon increasing frequency (tempo, or timing) of stimulation.

aerobic cellular respiration

series of oxygen-requiring reactions in mitochondria **O2 is required

shortening of individual sarcomeres causes:

shortening of whole muscle fiber, which in turn can lead to contraction of the whole skeletal muscle

contraction strength

size of a skeletal muscle's motor unit number of motor units activated

3 type of muscular tissue:

skeletal cardiac smooth

A motor neuron stimulates

skeletal muscle fibers. This stimulation ultimately results in the interaction between myofilaments within the skeletal muscle fibers to produce tension. The resulting tension is exerted on the portions of the skeleton (or other body structures) where the muscle is attached to cause movement in the body. The anatomic structures and associated physiologic processes of skeletal muscle contraction include the events that occur at the (1) neuromuscular junction; (2) sarcolemma, T-tubules, and sarcoplasmic reticulum; and (3) sarcomeres.

speed of contraction- smooth muscle

slow

types of muscle fibers (3)

slow oxidative (SO) fast oxidative-glycolytic (FOG) fast glycolytic (FG)

slow oxidative (SO)

slowest contraction fatigue resistant aerobic respiration (source of ATP) *posture and endurance

fiber diameter of smooth muscle

small

fast glycolytic (FG)- myoglobin content

small amount

fast glycolytic- mitochondria

small amount

slow oxidative (SO)- fiber diameter

smallest

all-or-none law

states that if a skeletal muscle fiber contracts in response to stimulation, it will contract completely (all)—and if the stimulus is not sufficient, it will not contract (none). This means that the skeletal muscle fiber contracts maximally or not at all.

Motor neurons in nerve

stimulate skeletal muscles to contract *movement* thread-like axon process of neuron extends from brain or spinal cord to a group of muscle fibers axon end typically branches to extend to different skeletal muscle fibers **stimulates at the same time ex: neat handwriting*

skeletal muscle tissue

striated, under voluntary control, and moves bones in skeleton

M line

supporting protein in middle of sarcomere

connective tissue attaches muscle

tendon aponeurosis

oxygen debt

the amount of additional oxygen that is consumed following exercise to restore pre-exercise conditions. This additional oxygen is required primarily by Skeletal muscle fibers to replace oxygen on myoglobin molecules, replenish ATP and creatine phosphate, and replace glycogen stores Liver cells to convert lactate back to glucose through gluconeogenesis Additional oxygen is also required by the respiratory muscles that are engaging in forced breathing, the heart as it pumps more blood through the body, and the overall higher metabolic rate. The next time you see individuals breathing hard following exercise, you will realize that they are "paying off" their oxygen debt, helping to return the body to its state prior to exercise.

One essential feature of skeletal muscle fibers is the electrical charge difference across the sarcolemma;

the cytosol right inside the plasma membrane is relatively negative in comparison to the interstitial fluid outside of the cell. This electrical charge difference when the cell is at rest is called the resting membrane potential (RMP). Skeletal muscle fibers have an RMP of about −90 millivolts (mV). An RMP is established and maintained by both leak channels and Na+/K+ pump. The primary function of the Na+/K+ pumps is to maintain the concentration gradients for Na+ (with more Na+ outside the cell) and K+ (with more K+ inside the cell). The acetylcholine receptors (chemically gated ion channels) within the motor end plate and the voltage-gated Na+ channels and voltage-gated K+ channels in the sarcolemma and T-tubules are closed, Ca2+ ions are stored within the terminal cisternae of the sarcoplasmic reticulum, and the contractile proteins (myofilaments) within the sarcomeres are in their relaxed position.

Calcium Entry at Synaptic Knob: Nerve signal (or nerve impulse)

the electrical signal that is propagated down an axon, is sent along a motor neuron of the somatic nervous system. The nerve signal triggers the opening of voltage-gated Ca2+ channels within the synaptic knob plasma membrane, and calcium moves down its concentration gradient from the interstitial fluid through the open channels into the synaptic knob. Calcium binds with membrane proteins (synaptotagmin) exposed on the external surface of synaptic vesicles

Three layers of connective tissue are within muscles:

the epimysium, the perimysium, and the endomysium. These layers provide protection and support, a means of attachment of the muscle to the skeleton or other structures within the body, and sites for distribution of blood vessels and nerves: -The epimysium is a layer of dense irregular connective tissue that surrounds the whole skeletal muscle. This fibrous tissue ensheathes the entire skeletal muscle to protect and support it like a tough leather sleeve. -The perimysium is a layer of dense irregular connective around each fascicle. These tough, fibrous connective tissue sleeves also provide protection and support, but to each bundle of muscle fibers. -The endomysium is composed of areolar connective tissue that surrounds each muscle fiber. These more delicate coverings function to electrically insulate the muscle fibers. The epimysium, perimysium, and endomysium collectively extend past the muscle fibers to form either a tendon or an aponeurosis.

Muscle tension

the force generated when a skeletal muscle is stimulated to contract. The term tension is used to describe the force that a muscle exerts because a muscle can only pull on a structure. The tension generated by the contractile proteins within a muscle is transferred to its connective tissue coverings, which move a body part Muscle tension produced in a contracting muscle is measured in several classic laboratory experiments. One variation of these experiments uses the specimen of a gastrocnemius (calf) muscle with an attached sciatic nerve that is excised from a frog. The gastrocnemius muscle is then anchored to an apparatus that produces a myogram, a graphic recording of changes in muscle tension when it is stimulated. Here we describe the generation and graphic recording of (1) a muscle twitch; (2) motor unit recruitment; and (3) wave summation, incomplete tetany, and tetany.

muscle tone

the resting tension in a skeletal muscle generated by involuntary somatic nervous stimulation of the muscle. Limited numbers of motor units within a muscle are usually stimulated randomly at any given time to maintain a constant tension; the specific motor units being stimulated during rest change continuously so motor units do not become fatigued.

Terminal cistern

which are much like the hem of a sleeve. Terminal cisternae serve as the reservoirs for calcium ions (Ca2+) and are immediately adjacent to each T-tubule. Together, two terminal cisternae and a centrally located T-tubule form a structure called a triad. Within the triad, the T-tubule membrane contains voltage-sensitive Ca2+ channels (dihydropyridine receptors), which are responsive to electrical signals (i.e., action potentials). The terminal cisternae membrane of the sarcoplasmic reticulum contain Ca2+ release channels (ryanodine receptors). It is here in the triad that the connection occurs between the electrical signals (action potentials) propagated along the sarcolemma and T-tubule and the release of calcium from the sarcoplasmic reticulum

contractile proteins organized into sarcomeres in cardiac muscle.... (yes or no)

yes

contractile proteins organized into sarcomeres in skeletal muscle.... (yes or no)

yes

contractile proteins organized into sarcomeres in smooth muscle.... (yes or no)

yes

transverse tubules present?- skeletal muscle

yes, aligned with each A-I band junction

transverse tubules present?- cardiac muscle

yes, aligned with each Z disc

Autorythmicity?- smooth muscle

yes, in visceral smooth muscle