Chapter 10: The Muscular System

•Effect of two successive stimuli:

-If a second stimulus occurs during the absolute refractory period, no response would be generated -If a second stimulus occurs after the refractory period of the first stimulus but before the relaxation period, the effect of two stimuli is summed up and a single curve of larger contraction is achieved - wave summation

(blank) binding to troponin moves tropomyosin away from myosin-binding sites & allows muscle contraction to begin as myosin binds to actin

calcium ion

•long, non-elastic, wraps around each actin filament and anchors to Z disc

Nebulin

Contraction Cycle Steps:

•ATP hydrolysis: Hydrolysis of ATP reorients and energizes the myosin head. •Formation of cross-bridges: Myosin head attaches to the myosin-binding site on actin & releases the Pi group •Power stroke: The crossbridge rotates & release the ADP, sliding the thin filaments towards center of sarcomere •Detachment of myosin from actin: As the next ATP binds to the myosin head, myosin head detaches from actin

Aerobic Respiration

•Activity that lasts longer than half a minute depends on aerobic respiration •Pyruvate entering the mitochondria is completely oxidized generating -ATP -Carbon dioxide -Water -Heat •Each molecule of glucose yields about 36 molecules of ATP

Release of ACh:

•Arrival of the action potential at the synaptic bulb triggers exocytosis of the vesicles •The ACh diffuses through the synaptic cleft and goes to the motor end plate

Neuromuscular Junction (NMJ)

•At the NMJ, the motor axon divides into many synaptic end bulbs •Suspended in the cytosol within each bulb are many membrane enclosed sacs - Synaptic vesicles that contain the neurotransmitter(ACh) •The region of the sarcolemma adjacent to the synaptic end bulbs is called as the 'motor end plate' -It has many Acetylcholine (Ach) receptors (around 30-40 millions)

Muscle proteins

•Contractile proteins: Generate force during contraction ex. actin & myosin •Regulatory proteins: Help to regulate 'on and off' of contraction process. ex. troponin & tropomyosin •Structural proteins: Maintain the proper alignment of thick & thin filaments •give the myofibril elasticity & extensibility •link the myofibrils to the sarcolemma & extracellular matrix ex. titin, myomesin, nebulin, dystrophin

Functional Characteristics / Properties of Muscle

•Electrical Excitability: Ability to respond to a stimulus generated by nerves - an electrical signal called action potential •Contractility: Ability of the muscular tissue to contract forcefully when stimulated by an action potential •Extensibility: Ability of the muscular tissue to stretch without being damaged •Elasticity: Ability of muscular tissue to return to its original length & shape after an extension

Creatine Phosphate

•Energy-rich molecule found only in muscle fibers •During rest, muscle fibers utilize excess ATP to synthesize creatine phosphate •During muscle contraction, creatine phosphate transfers its high energy phosphate group to ADP regenerating new ATP •Creatine kinase (CK) catalyze both the reactions

•If sufficient oxygen is present, pyruvate formed by glycolysis enters aerobic respiration pathways producing a large amount of ATP

•If oxygen levels are low, anaerobic reactions convert pyruvate to lactic acid which is carried away by the blood •Another characteristic of the glycogen-lactic acid system is that it can form ATP molecules about 2.5 times as rapidly as can the oxidative mechanism of the mitochondria

Fast Oxidative-Glycolytic (FOG) Fibers

•Intermediate in diameter between the other two types of fibers •Contain large amounts of myoglobin and many blood capillaries, appear dark red •Generate considerable ATP by aerobic cellular respiration •Moderately high resistance to fatigue •Generate some ATP by anaerobic glycolysis, their ATPase activity is 3-5 times faster than SO •Speed of contraction faster •Contribute to activities such as walking and sprinting

Fast Glycolytic (FG) Fibers

•Largest in diameter, generate the most powerful contractions •Have low myoglobin content but highest myofibrils, relatively few blood capillaries •Few mitochondria, appear white in color •Generate ATP mainly by glycolysis, their ATPase activity is fastest •Fibers contract strongly and quickly, fatigue quickly •Adapted for intense anaerobic movements of short duration Weight lifting or throwing a ball

Distribution & Recruitment of Different Types of Fibers

•Most muscles are a mixture of all three types of muscle fibers but in most of the skeletal muscles, half of the fibers are SO fibers •Proportions vary, depending on the action of the muscle, the person's training regiment, and genetic factors •Within a particular motor unit, all of the skeletal muscle fibers are of the same type

Two major criteria for types of skeletal muscle fibers:

•Muscle fibers vary in their composition like content of myoglobin - structural classification •Muscle fibers differ in their functional characteristics like muscle fibers contract at different speeds, and vary in how quickly they fatigue- functional classification

Muscle contraction occurs when

•Myosin heads attach to and "walk" along the thin filaments at both ends of a sarcomere, progressively pulling the thin filaments toward the center of the sarcomere (M line) •Thin filaments slide inward and meet in center of sarcomere, sometimes their ends overlap •Z discs come closer together and the sarcomere shortens but the lengths of the individual thick and thin filaments do not change! Leads to shortening of the entire muscle

Phosphocreatine-Creatine System

•Phosphocreatine can decompose to creatine and phosphate ion and in doing so release large amounts of energy - 10,300 calories per mole •Most muscle cells have two to four times as much phosphocreatine as ATP •All the energy stored in the muscle phosphocreatine is almost instantaneously available for muscle contraction, just as is the energy stored in ATP

Classification based on their myoglobin content (structural)

•Red muscle fibers -Have a high myoglobin content -Appear darker (dark meat in chicken legs and thighs) -Contain more mitochondria -Supplied by more blood capillaries » •White muscle fibers -Have a low content of myoglobin -Appear lighter (white meat in chicken breasts)

The Contraction Cycle

•The onset of contraction begins with the SR releasing calcium ions into the cytosol of the muscle cell •Where the calcium ions bind to troponin & cause the troponin-tropomyosin complexes to move away from the myosin binding sites on actin •Once the binding sites become free, the contraction cycle begins

Complete (fused) tetanus:

•When the frequency of multiple stimuli is such that the next successive stimulus falls from second half of latent period to contraction phase of the previous twitch, then due to complete summation effect, the muscle will produce sustained, smooth and forceful contraction without any period of relaxation in between - complete tetanus

Incomplete tetanus or clonus:

•When the frequency of multiple stimuli is such that the next successive stimulus falls on the relaxation phase of the previous twitch then the succeeding contraction obtained will be superimposed over the previous twitch due to incomplete summation of waves -So the series of wavering contractions with period of incomplete relaxation period in between is referred as the incomplete (unfused) tetanus or clonus

The (blank) points toward the M line in the center of the sarcomere -Tails of two neighboring myosin molecules lie parallel to one another, forming the (blank) of the thick filament

myosin tail, shaft

Aerobic respiration supplies ATP for (blank)

prolonged activity

Functions of muscular tissue

1. Producing body movements 2. Stabilizing body positions 3. Storing and moving substances within the body 4. Generating heat- thermogenesis

Sequence of Muscle stimulation

1. release of ACh 2. activation of ACh receptors on muscle 3. production of muscle action potential 4. termination of ACh activity

Muscle fibers have three ways to produce ATP

1.From creatine phosphate 2.By anaerobic cellular respiration 3.By aerobic cellular respiration

Muscle tissue has two sources of oxygen

1.Oxygen from hemoglobin in the blood 2.Oxygen released by myoglobin in the muscle cell

In skeletal muscle, around (blank) form one thick filament

300 myosin molecules- each myosin molecule is shaped like two golf clubs twisted together

A cross section through a sarcomere shows that: each myosin can interact with ? actin filaments, and each actin can interact with ? myosin filaments

6, 3

Aerobic respiration provides more than (what %) of the needed ATP in activities lasting more than 10 minutes (at the end of an endurance event such as marathon race

90%

extends the entire length of thick filaments

A band

•The contraction cycle repeats as long as (blank) is available & the (blank) level is sufficiently high

ATP, Ca2+

•Main component of thin filaments, anchored on the Z discs •Actin molecules provide a site where a myosin head can attach (cross-bridges)

Actin

narrow zone in the middle of each A band which contains only thick filament

H zone

•large muscles of limbs e.g. biceps - 2000-3000 fibers/motor unit •All muscle fibers in a motor unit contract & relax at the same time

Large Motor units

middle of the sarcomere, it is made by supporting proteins which hold the thick filaments together at the center of the H zone

M line

•muscles that require precise control. Have few muscle fibers/motor unit -e.g. muscles of larynx - 2-4 fibers/motor unit muscles of eye - 10-20 fibers/motor unit

Small motor units

•Two regulatory proteins - (blank and blank) are also part of the thin filament -Strands of (blank), held in place by troponin, cover the myosin-binding sites -In relaxed muscle, myosin is blocked from binding to actin

tropomyosin and troponin, tropomyosin

Muscle Tone

•A small amount of tautness or tension in the muscle due to weak involuntary contractions of motor units •Small groups of motor units are alternatively active and inactive in a constantly shifting pattern to sustain muscle tone •Muscle tone keeps skeletal muscles firm •Keep the head from slumping forward on the chest, smooth muscle tone helps maintain blood pressure

Activation of ACh receptors:

•ACh attaches to the receptor on the motor end plate and opens ion channels that causes an action potential to occur on the sarcolemma

•The ATP inside muscle fibers powers contraction for only a few seconds

•ATP must be produced by the muscle fiber after reserves are used up

Termination of ACh:

•Acetylcholinesterase enzyme degrades the ACh into acetyl and choline

Anaerobic Respiration

•Series of ATP producing reactions that do not require oxygen •Glucose is used to generate ATP when the supply of creatine phosphate is depleted •Glycolysis breaks down glucose into pyruvate and produces two molecules of ATP •The stored glycogen in muscle can be split into glucose that can be used for energy •The initial stage - glycolysis - occurs without use of oxygen and is therefore also known as anaerobic metabolism

Functionally muscle fibers are classified into three main types:

•Slow oxidative fibers •Fast oxidative-glycolytic fibers •Fast glycolytic fibers

Slow Oxidative Fibers (SO fibers)

•Smallest in diameter & least powerful type of muscle fibers, appear dark red (more myoglobin) •Generate ATP mainly by aerobic cellular respiration Slow ATPase activity compare to fast fibers, have a slow speed of contraction •Very resistant to fatigue, capable of prolonged, sustained contractions for many hours •Adapted for maintaining posture and for aerobic, endurance-type activities such as running a marathon

Production of muscle action potential:

•The action potential then travels along the sarcolemma and into the T tubules (going deep into the cells)

Twitch Contraction

•The brief contraction of all the muscle fibers in a motor unit in response to a single action potential is known as twitch contraction •Twitches of skeletal muscle fibers last anywhere from 20 to 200 msec •A record of muscle contraction is called a myogram

Dilated end sacs of the sarcoplasmic reticulum butt against the T tubule from both the sides

Terminal Cisterns

•anchors myosin to M line and Z disc -Helps the muscle to return to resting length after contraction

Titin

Several key structural proteins are

Titin, α-actinin, myomesin, nebulin and dystrophin

Once fusion occurs, the muscle fiber loses its ability to undergo (blank)

cell division

Muscle fibers are contained in (blank) and form bundles of fibers (fascicles) that are all bound in connective tissue to form muscle

connective tissue sheet

•Muscle fibers (blank) and (blank) at different speeds, and vary in metabolic reactions which they use to generate ATP and in how quickly they fatigue

contract and relax

•As the contraction cycle continues, movement of (blank) applies the force that draws the Z discs toward each other, and the sarcomere shortens

crossbridges

The effect of repeated stimuli on the contractile response of a skeletal muscle depends upon the (blank)

frequency of stimuli

The total strength of a contraction depends on (blank) & (blank) are contracting at the same time

how large the motor units and how many motor units

During a (blank), the distance between two Z discs can decrease to half the resting length

maximal muscle contraction

Skeletal muscles are made up of hundreds to thousands of long, thin cells called

muscle cells or muscle fibers

Each muscle fiber arises from the fusion of a hundred or more small mesodermal cells known as (blank), each mature skeletal muscle fiber has a hundred or more nuclei

myoblasts

Two projections of each myosin molecule are called

myosin heads

plasma membrane of a muscle cell

sarcolemma

The filaments are arranged in compartments known as (blank) - the basic structural and functional units of the myofibrils/muscle fibers

sarcomeres

The Z discs in turn pull on neighboring sarcomeres, and the whole muscle fiber (blank)

shortens

Besides regulatory and contractile proteins, muscle contains about a dozen (blank)

structural proteins

Creatine is synthesized in

the body (liver, kidneys & pancreas) and also derived from foods like milk, red meat & some fish

Latent period (2 msec)

-A brief delay between the application of stimulus and beginning of muscular contraction -The action potential sweeps over the sarcolemma and Ca++ is released from the SR

A huge amount of ATP is needed to

-Power the contraction cycle -Pump Ca++ into the SR

Cardiac muscles

-Striated -Single nucleus -Involuntary

Isotonic contraction

-The tension developed remains constant while the muscle changes its length -Used for body movements and for moving objects -Picking a book up off a table

Three layers of connective tissue extend from the deep fascia to protect & strengthen the skeletal muscle:

-epimysium -perimysium -endomysium

Types of Muscles

-skeletal -cardiac -smooth

Skeletal muscles

-striated -multinucleated -voluntary

contractile organelles of skeletal muscle which extend throughout the length of the skeletal muscle fiber

Myofibrils

(blank) and (blank) are oxygen-binding proteins

Myoglobin and hemoglobin

form the M line

Myomesin

•Thick filaments that functions as a motor protein in all three types of muscle tissue -Push or pull various cellular structures to achieve movement by converting the chemical energy in ATP to the mechanical energy of motion or the production of force

Myosin

narrow plate-shaped region which separated one sarcomere from the next

Z discs

•cytoskeletal protein, links actin filaments to sarcolemma -Re-enforces sarcolemma and assists in transmission of tension to the tendons Muscular dystropy

Dystrophin

•A single muscle fiber has only 1 NMJ •But an axon of a motor neuron branches out & forms many NMJ's with different muscle fibers •The motor neuron & all the skeletal muscles supplied by that 1 axon is called as a motor unit

Motor Unit

cytoplasm of a muscle fiber, includes a large amount of glycogen that can be used for synthesis of ATP

Sarcoplasm

Fluid filled system of membranous sacs surround each myofibril

Sarcoplasmic reticulum

tiny invaginations of the sarcolemma, filled with interstitial fluid

Transverse (T) Tubules

myofibrils consist of myofilaments:

-Actin (Thin filaments): 8nm diameter, 1-2 µm long -Myosin (Thick filaments): 16nm diameter, 1-2 µm long -Both thick and thin filaments are directly involved in the contractile process

These structural proteins contribute to the

-Alignment -Stability -Elasticity, and -Extensibility of myofibrils

Contraction period (10-100 msec)

-Ca++ binds to troponin -Myosin-binding sites on actin are exposed -Cross-bridges form -Peak tension develops in the muscle fiber

•Relaxation period (10-100 msec)

-Ca++ is actively transported back into the SR -Myosin-binding sites are covered by tropomyosin -Myosin heads detach from actin

Smooth Muscles

-Non-striated -Single nucleus -Involuntary

•The recruitment of different motor units in a muscle depends on the need of the muscle

-Postural muscles of the neck, back, and legs have a high proportion of SO fibers -Muscles of the shoulders and arms have a high proportion of FG fibers -Leg muscles have large numbers of both SO and FOG fibers

Isometric contraction

-The tension generated is not enough for the object to be moved and the muscle does not change its length -Holding a book steady using an outstretched arm

Refractory period

-When a muscle fiber contracts, it temporarily cannot respond to another action potential

skeletal muscle is composed of

-connective tissue -blood vessels and nerves

Types of muscle contraction

-muscle contraction: isotonic (movement) and isometric (no movement) -isotonic: concentric (muscle shortens) and eccentric (muscle lengthens)

lighter, less dense area, contains rest of the thin filaments, but no thick filaments

I band