Chapter 12-muscle

Temporal summation in neurons means that the __________ of the neuron increases when two depolarizing stimuli occur close together in time.

Strength of the graded potential

What is the function of t-tubules?

T-tubules allow action potentials to travel from the surface of the muscle fiber to its interior.

rigor mortis

Temporary rigidity of muscles occurring after death. when metabolism stops and ATP supplies are exhausted, muscles are unable to bind more ATP, so they remain in the tightly bound rigor state. The muscles "freeze" owing to immovable crossbridges. The tight binding of actin and myosin persists for a day or so after death, until enzymes within the decaying fiber begin to break down the muscle proteins.

calcium-induced calcium release (CICR)

The RyR channel opens in response to Ca2+ entering the cell

Why are skeletal muscles described as striated?

The banding pattern of organized filaments in the sarcomere forms striations in the muscle.

Explain how you vary the strength and effort made by your muscles in picking up a pencil versus picking up a full gallon container of milk.

The body uses different types of motor units and recruits different numbers of motor units. Small movements use motor units with fewer muscle fibers; gross movements use motor units with more fibers.

sarcolemma

The cell membrane of a muscle fiber

What is the chemical signal at a neuromuscular junction?

The chemical signal at a neuromuscular junction is acetylcholine.

Contraction

The creation of tension in a muscle, is an active process that requires energy input from ATP.

In the sliding filament theory of contraction, what prevents the filaments from sliding back to their original position each time a myosin head releases to bind to the next actin binding site?

The crossbridges do not all unlink at one time, so while some myosin heads are free and swiveling, others are still tightly bound.

When tetrodotoxin (TTX), a poison that blocks Na+ channels, is applied to certain types of smooth muscle, it does not alter the spontaneous generation of action potentials. From this observation, what conclusion can you draw about the action potentials of these types of smooth muscle?

The depolarization phase of the action potentials must not be due to Na+ entry. In these muscles, depolarization is due to Ca2+ entry.

Origin

The end of the muscle that is attached closest to the trunk or to the more stationary bone.

creatine kinase

The enzyme that transfers the phosphate group from phosphocreatine to ADP, also known as creatine phosphokinase (CPK). Muscle cells contain large amounts of this enzyme. Consequently, elevated blood levels of creatine kinase usually indicate damage to skeletal or cardiac muscle.

What events are taking place during the latent period before contraction begins?

The events of the latent period include creation of the muscle action potential, release of Ca2+ from the sarcoplasmic reticulum, and diffusion of Ca2+ to the contractile filaments.

Muscle Tension

The force created by contracting muscle.

Recruitment

The force of contraction in a skeletal muscle can be increased by recruiting additional motor units; controlled by the nervous system and proceeds in a standardized sequence.

Tranverse Tubules (T-tubules)

The membranes of t-tubules are a continuation of the muscle fiber membrane, which makes the lumen of t-tubules continuous with the extracellular fluid. The lie transverse to the terminal cisternae. T-tubules allow action potentials to move rapidly from the cell surface into the interior of the fiber so that they reach the terminal cisternae nearly simultaneously. Without t-tubules, the action potential would reach the center of the fiber only by conduction of the action potential through the cytosol, a slower and less direct process that would delay the response time of the muscle fiber.

Insertion

The more distal {distantia, distant} or more mobile attachment.

How does the nervous system increase the force of contraction in a muscle composed of many motor units?

The nervous system increases the force of contraction by recruiting additional motor units.

asynchronous recruitment

The nervous system modulates the firing rates of the motor neurons so that different motor units take turns maintaining muscle tension. The alternation of active motor units allows some of the motor units to rest between contractions, preventing fatigue.

Which part of contraction requires ATP? Does relaxation require ATP?

The release of myosin heads from actin requires ATP binding. Energy from ATP is required for the power stroke. Relaxation does not directly require ATP, but relaxation cannot occur unless Ca2+ is pumped back into the sarcoplasmic reticulum using a Ca2+-ATPase.

Relaxation

The release of tension created by a contraction.

I Band

These are the lightest color bands of the sarcomere and represent a region occupied only by thin filaments. The abbreviation I comes from isotropic, a description from early microscopists meaning that this region reflects light uniformly under a polarizing microscope. A Z disk runs through the middle of every I band, so each half of an I band belongs to a different sarcomere.

M Line

This band represents proteins that form the attachment site for thick filaments, equivalent to the Z disk for the thin filaments. Each M line divides an A band in half. M is the abbreviation for mittel, the German word for "middle."

H Zone

This central region of the A band is lighter than the outer edges of the A band because the H zone is occupied by thick filaments only. The H comes from helles, the German word for "clear."

A Band

This is the darkest of the sarcomere's bands and encompasses the entire length of a thick filament. At the outer edges of the A band, the thick and thin filaments overlap. The center of the A band is occupied by thick filaments only. The abbreviation A comes from anisotropic, meaning that the protein fibers in this region scatter light unevenly.

myosin light chain kinase (MLCK)

This protein is activated by the Ca2+-calmodulin complex to phosphorylate the myosin light chain protein.

The proper alignment of filaments within a sarcomere is ensured by two proteins:

Titin and Nebulin

How does a calcium signal turn muscle contraction on and off?

Troponin

A single action potential in a muscle fiber evokes a single twitch.

True

Human muscles are a mixture of fiber types, with the ratio of types varying from muscle to muscle and from one individual to another.

True

it is the ratio of MLCK to MLCP activity that determines the contraction state of smooth muscle.

True

Latch State

Under conditions that we do not fully understand, dephosphorylated myosin may remain attached to actin for a period of time. This condition maintains tension in the muscle fiber while consuming little ATP. It is a significant factor in the ability of smooth muscle to sustain contraction without fatiguing.

graded contractions

Variations in the degree of muscle contractions exhibited in smooth muscles. force varies according to the strength of the Ca2+ signal.

Ca2+ entry from the extracellular fluid takes place with the help of membrane channels that are:

Voltage-gated Ca2+ channels: open in response to a depolarizing stimulus. Ligand-gated Ca2+ channels: are also known as receptoroperated calcium channels or ROCC. These channels open in response to ligand binding. Stretch-activated channels: Some smooth muscle cells, such as those in blood vessels, contain stretch-activated channels that open when pressure or other force distorts the cell membrane. (myogenic contraction).

The off-on positioning of tropomyosin is regulated by troponin. Briefly describe what happens when calcium is introduced.

When contraction begins in response to a calcium signal: (1) one protein of the complex—troponin C— binds reversibly to Ca2+ (2) The calcium-troponin C complex pulls tropomyosin completely away from actin's myosin-binding sites. (3) This "on" position enables the myosin heads to form strong, high-force crossbridges and carry out their power strokes (4) , moving the actin filament . (5) Contractile cycles repeat as long as the binding sites are uncovered.

the law of mass action

When cytosolic calcium decreases, Ca2+ unbinds from troponin. (For muscle relaxation to occur, Ca2+ concentrations in the cytosol must decrease).

Briefly describe the contraction cycle molecularly:

(1) ATP binds to myosin. Myosin releases actin. (2) Myosin hydrolyzes ATP. Energy from ATP rotates the myosin head to the cocked position. Myosin binds weakly to actin. (3) Power stroke begins when tropomyosin moves off the binding site (requires presence of Ca2+ in cytosol). Head swivels and P is released. (4) Myosin releases ADP at the end of the power stroke.

Summarize the current understanding of the key points of smooth muscle contraction. In smooth muscle:

(1) An increase in cytosolic Ca2+ initiates contraction. This Ca2+ is released from the sarcoplasmic reticulum but also enters from the extracellular fluid. ( 2) Ca2+ binds to calmodulin, a calcium-binding protein found in the cytosol. (3) Ca2+ binding to calmodulin is the first step in a cascade that ends in phosphorylation of myosin light chains. (4) Phosphorylation of myosin light chains enhances myosin ATPase activity and results in contraction. Thus, smooth muscle contraction is controlled through myosin-linked regulatory processes rather than through tropomyosin. (5) Active myosin crossbridges slide along actin and create muscle tension

Relaxation in a smooth muscle fiber is a multistep process:

(1) Free Ca2+ in cytosol decreases when Ca2+ is pumped out of the cell or back into the sarcoplasmic reticulum by Ca2+-ATPase (2) Ca2+ unbinds from calmodulin (CaM). MLCK activity decreases. (3) Myosin phosphatase removes phosphate from myosin light chains, which decreases myosin ATPase activity. (4) Less myosin ATPase activity results in decreased muscle tension.

Signals that increase cAMP production cause muscle relaxation through the following mechanisms:

(1) Free cytosolic Ca2+ concentrations decrease when IP3 channels are inhibited and the SR Ca2+-ATPase is activated. (2) K+ leaking out of the cell hyperpolarizes it and decreases the likelihood of voltage-activated Ca2+ entry. (3) Myosin phosphatase activity increases, which causes a decrease in muscle tension.

Pathways that increase IP3 cause contraction several ways:

(1) IP3 opens IP3 channels on the SR to release Ca2+. (2) Diacylglycerol (DAG), another product of the phospholipase C signal pathway, indirectly inhibits myosin phosphatase activity. Increasing the MLCK/MLCP ratio promotes crossbridge activity and muscle tension.

Many types of smooth muscle display resting membrane potentials that vary between _____ and _____ mV.

-40 ;-80

Troponin (TN)

A calcium-binding complex of three proteins. Troponin controls the positioning of an elongated protein polymer, tropomyosin

Tropomyosin

A helical protein that winds around actin helices in skeletal and cardiac muscle cells to form the thin filament of the sarcomere. In the absence of Ca2+, tropomyosin covers the myosin-binding sites on actin and prevents muscle contraction. When calcium is present, a conformation change in tropomyosin occurs so that the myosin-binding sites are exposed and muscle contraction can occur.

Which type of runner would you expect to have more slow-twitch fibers, a sprinter or a marathoner?

A marathoner probably has more slow-twitch muscle fibers, and a sprinter probably has more fast-twitch muscle fibers.

Myosin

A motor protein with the ability to create movement. They are the "thick filaments" Each myosin molecule is composed of protein chains that intertwine to form a long tail and a pair of tadpole-like heads. The rodlike tail is stiff, but the protruding myosin heads have an elastic hinge region where the heads join the rods. This hinge region allows the heads to swivel around their point of attachment.

What are the three anatomical elements of a neuromuscular junction?

A neuromuscular junction consists of axon terminals from one somatic motor neuron, the synaptic cleft, and the motor end plate on the muscle fiber.

Myoglobin

A red oxygen-binding pigment with a high affinity for oxygen. This affinity allows myoglobin to act as a transfer molecule, bringing oxygen more rapidly to the interior of the fibers. Lots of myoglobin produces a red color.

Twitch

A single contraction-relaxation cycle in a skeletal muscle.

myosin light chain

A small regulatory protein chain at the base of the myosin head. Phosphorylation and dephosphorylation of the myosin light chain control contraction and relaxation in smooth muscle.

Load

A weight or force that opposes contraction of a muscle

isotonic contraction

Any contraction that creates force and moves a load.

Skeletal Muscles

Are attached to the bones of the skeleton, enabling these muscles to control body movement. Fibers are large, multinucleate cells that appear striped or striated under the microscope.

Botulism

Botulinum toxin acts by decreasing the release of acetylcholine from the somatic motor neuron. Injections of botulinum toxin as a treatment for writer's cramp and used as Botox to reduce wrinkles.

There are many ways to categorize the different types of smooth muscle, but we will consider three:

By location: vascular, gastrointestinal, urinary, respiratory, reproductive, and ocular. By contraction pattern: (1) phasic: undergo periodic contraction and relaxation, some cycle rhythmically through contractions alternating with relaxation (eg: wall of lower esophagus). (2) tonic: continuously contracted (spinchters). The tonic smooth muscle in the walls of some blood vessels maintain an intermediate level of contraction (tonic control). By their communication with neighboring cells: (1) single-unit smooth muscle, or unitary smooth muscle:the cells are electrically connected by gap junctions, and they contract as a coordinated unit. also called visceral smooth muscle. (2) multi-unit smooth muscle, the cells are not linked electrically an each muscle cell functions independently. In the iris and ciliary body of the eye, in part of the male reproductive tract, and in the uterus except just prior to labor and delivery.

ryanodine receptors (RyR)

Ca2+ release channels in the sarcoplasmic reticulum. When the depolarization of an action potential reaches a DHP receptor, the receptor changes conformation. The conformation change opens the RyR Ca2+ release channels in the sarcoplasmic reticulum. Stored Ca2+ then flows down its electrochemical gradient into the cytosol, where it initiates contraction.

Contraction-relaxation cycle

Can be explained by the sliding filament theory of contraction. In intact muscles, one contraction-relaxation cycle is called a muscle twitch.

slow wave potentials

Cells that exhibit cyclic depolarization and repolarization of their membrane potential. Both slow wave and pacemaker potentials are due to ion channels in the cell membrane that spontaneously open and close.

Satellite Cells

Committed stem cells that lie just outside the muscle fiber membrane. They activate and differentiate into muscle when needed for muscle growth and repair.

Arrange the following skeletal muscle components in order, from outermost to innermost: sarcolemma, connective tissue sheath, thick and thin filaments, myofibrils.

Connective tissue, sarcolemma, myofibrils, thick and thin filaments

Sarcomere

Contractile unit of muscle/ Any of the repeating structural units of striated muscle fibrils. One repeat of the pattern of the arrangement of thick and thin filaments in a myofibril creates a repeating pattern of alternating light and dark bands.

electromechanical coupling

Contraction caused by electrical signaling

Myogenic Contraction

Contraction in this instance originates from a property of the muscle fiber itself. Myogenic contractions are common in blood vessels that maintain a certain amount of tone at all times.

When the circular muscle layer of the intestine contracts, what happens to the shape of the tube? When the longitudinal layer contracts, what happens to the shape?

Contraction of the circular layer decreases the diameter of a tube. Contraction of the longitudinal layer shortens the tube.

pharmacomechanical coupling

Contractions initiated by chemical signals without a significant change in membrane potential

isometric (static) contractions

Contractions that create force without moving a load.

What happens to contraction if a smooth muscle is placed in a saline bath from which all calcium has been removed?

Without ECF Ca2+, contraction decreases because smooth muscle depends on ECF Ca2+ for contraction.

According to the convention for naming enzymes, what does the name creatine kinase tell you about this enzyme's function?

Creatine is the substrate, and kinase tells you that this enzyme phosphorylates the substrate.

sarcoplasm

Cytoplasm of a muscle fiber

The dense bodies that anchor smooth muscle actin are analogous to what structure in a sarcomere?

Dense bodies are analogous to Z disks.

myosin light chain phosphatase (MLCP)

Dephosphorylation of the myosin light chain by the enzyme decreases myosin ATPase activity.

slow-twitch fibers (also called ST or type I)

Develop tension slower and have slower twitches than fast twitch. Rely primarily on oxidative phosphorylation for production of ATP. Because oxidative fibers have more myoglobin and more capillaries to bring blood to the cells and are smaller in diameter, they maintain a better supply of oxygen . Use: posture Dark Red

In 1954, however, scientists Andrew Huxley and Rolf Niedergerke

Discovered that the length of the A band of a myofibril remains constant during contraction. Because the A band represents the myosin filament, Huxley and Niedergerke realized that shortening of the myosin molecule could not be responsible for contraction. Subsequently, they proposed an alternative model, the sliding filament theory of contraction. In this model, overlapping actin and myosin filaments of fixed length slide past one another in an energy-requiring process, resulting in muscle contraction.

Each myosin molecule has binding sites for what molecules?

Each myosin molecule has binding sites for ATP and actin.

Why are the ends of the A band the darkest region of the sarcomere when viewed under the light microscope?

Ends of the A bands are darkest because they are where the thick and thin filaments overlap.

What is the difference between F-actin and G-actin?

F-actin is a polymer filament of actin made from globular G-actin molecules.

Distal

Farther from the trunk of the body.

Cardiac Muscle

Found only in the heart and moves blood through the circulatory system. Fibers are also striated but they are smaller, branched, and uninucleate. Cells are joined in series by junctions called intercalated disks.

Most smooth muscle neurotransmitters and hormones bind to ______________ receptors

G protein-linked

Each myosin head has two protein chains:

Heavy chain: the motor domain that binds ATP and uses the energy from ATP's high-energy phosphate bond to create movement. Because the motor domain acts as an enzyme, it is considered a myosin ATPase. The heavy chain also contains a binding site for actin. small chain:

Nebulin

Helps Titin. An inelastic giant protein that lies alongside thin filaments and attaches to the Z disk, but does not extend to the M line. Nebulin helps align the actin filaments of the sarcomere

myofibrils

Highly organized bundles of contractile and elastic proteins that carry out the work of contraction.

List the letters used to label the elements of a sarcomere. Which band has a Z disk in the middle? Which is the darkest band? Why? Which element forms the boundries of a sarcomere? Name the line that dives the A band in half. What is the function of this line?

Z disk—ends of a sarcomere. I band—Z disk in the middle. A band (thick filaments)—darkest; H zone—lighter region of A band. M line divides A band in half; thick filaments link to each other.

In three-dimensional array, the actin and myosin molecules form a lattice of parallel, overlapping thin and thick filaments, held in place by their attachments to the _________ and __________ proteins, respectively

Z-disk (actin); M-line (myosin)

Muscle Cramp

a "charley horse," a sustained painful contraction of skeletal muscles. Many muscle cramps are caused by hyperexcitability of the somatic motor neurons controlling the muscle. As the neuron fires repeatedly, the muscle fibers of its motor unit go into a state of painful sustained contraction.

Most joints in the body have both flexor and extensor muscles, because

a contracting muscle can pull a bone in one direction but cannot push it back.

sarcoplasmic reticulum (SR)

a form of modified endoplasmic reticulum that wraps around each myofibril like a piece of lace. Consists of terminal cisternae. It concentrates and sequesters Ca2+ with the help of a Ca2+-ATPase in the SR membrane. Calcium release from the SR creates calcium signals that play a key role in contraction in all types of muscle.

Titin

a huge elastic molecule and the largest known protein, composed of more than 25,000 amino acids. A single titin molecule stretches from one Z disk to the neighboring M line. Two functions: (1) it stabilizes the position of the contractile filaments and (2) its elasticity returns stretched muscles to their resting length

phosphocreatine

a molecule whose high energy phosphate bonds are created from creatine and ATP when muscles are at rest. Used as a back-up energy source. When muscles become active, such as during exercise, the high-energy phosphate group of phosphocreatine is transferred to ADP, creating more ATP to power the muscles.

Fatigue

a reversible condition in which a muscle is no longer able to generate or sustain the expected power output. It is influenced by the intensity and duration of the contractile activity, by whether the muscle fiber is using aerobic or anaerobic metabolism, by the composition of the muscle, and by the fitness level of the individual

In smooth muscle, SR Ca2+ release is mediated both by

a ryanodine receptor (RyR) calcium release channel and by an IP3-receptor channel.

latent period

a short delay between the muscle action potential and the beginning of muscle tension development. This delay represents the time required for calcium release and binding to troponin .

dihydropyridine (DHP) receptor

a voltage-sensing L-type calcium channel contained in the t-tubule membrane. They open ryanodine receptors, or RyR.

store-operated Ca2+ channels

allow more Ca2+ into the cell when the cell detects it SR Ca2+ levels are low.

McArdle's Disease

also known as myophosphorylase deficiency, is a condition in which the enzyme that converts glycogen to glucose 6-phosphate is absent in muscles. As a result, muscles lack a usable glycogen energy supply, and exercise tolerance is limited.

Striations

alternate dark and light bands found on skeletal and cardiac muscle

peripheral fatigue

arise anywhere between the neuromuscular junction and the contractile elements of the muscle . For example, if ACh is not synthesized in the axon terminal fast enough to keep up with neuron firing rate, neurotransmitter release at the synapse decreases. In extended submaximal exertion, fatigue is associated with the depletion of muscle glycogen stores. Elevated phosphate levels. Ion imbalances.

central fatigue

arise from the central nervous system. Includes subjective feelings of tiredness and a desire to cease activity. Low pH from acid production during ATP hydrolysis is often mentioned as a possible cause of fatigue, but only in cases of maximum exertion. failure of the CNS command neurons.

Contents of cytosol between myofibrils

contains many glycogen granules and mitochondria. Glycogen, the storage form of glucose found in animals, is a reserve source of energy. Mitochondria provide much of the ATP for muscle contraction through oxidative phosphorylation of glucose and other biomolecules.

IP3 triggers ___________ and cAMP promotes _________.

contraction; relaxation

fast-twitch oxidative-glycolytic fibers (FOG or type IIA)

develop tension two to three times faster than slow-twitch fibers (type I). The speed with which a muscle fiber contracts is determined by the isoform of myosin ATPase present in the fiber's thick filaments. Fast-twitch fibers pump Ca2+ into their sarcoplasmic reticulum more rapidly than slow-twitch fibers do, so fast-twitch fibers have quicker twitches. Fast-twitch oxidative-glycolytic fibers exhibit properties of both oxidative and glycolytic fibers. They are smaller than fasttwitch glycolytic fibers and use a combination of oxidative and glycolytic metabolism to produce ATP. Because of their intermediate size and the use of oxidative phosphorylation for ATP synthesis, fast-twitch oxidative-glycolytic fibers are more fatigue resistant than their fast-twitch glycolytic cousins. Fast-twitch oxidative-glycolytic fibers, like slow-twitch fibers, are classified as red muscle because of their myoglobin content. . Use: Standing, walking Red

fast-twitch glycolytic fibers (FG or type IIB)

develop tension two to three times faster than slow-twitch fibers (type I). The speed with which a muscle fiber contracts is determined by the isoform of myosin ATPase present in the fiber's thick filaments. Fast-twitch fibers pump Ca2+ into their sarcoplasmic reticulum more rapidly than slow-twitch fibers do, so fast-twitch fibers have quicker twitches. Glycolytic fibers (fast-twitch type IIB) rely primarily on anaerobic glycolysis to produce ATP. Glycolytic fibers fatigue more easily than do oxidative fibers, which do not depend on anaerobic metabolism. are described as white muscle because of their lower myoglobin content. The combination of larger size, less myoglobin, and fewer blood vessels means that glycolytic fibers are more likely to run out of oxygen after repeated contractions. Use: jumping; quick, fine movements

Most experimental evidence suggests that muscle fatigue arises from

excitation-contraction failure in the muscle fiber rather than from failure of control neurons or neuromuscular transmission.

The fibers in a given muscle are arranged with their long axes in parallel. Each skeletal muscle fiber is sheathed in connective tissue, with groups of adjacent muscle fibers bundled together into units called ____________.

fascicles

Antagonistic Muscle Groups

flexor/extensor pair (when some muscles pull one way and others pull other way). a pair of antagonistic muscles in the arm: the biceps brachii {brachion, arm}, which acts as the flexor, and the triceps brachii, which acts as the extensor. When you do a "dumbbell curl" with a weight in your hand, the biceps muscle contracts and the hand and forearm move toward the shoulder. When you lower the weight, the triceps contracts, and the flexed forearm moves away from the shoulder. In each case, when one muscle contracts and shortens, the antagonistic muscle must relax and lengthen.

According to the sliding filament theory, tension generated in a muscle fiber is directly proportional to the number of

high-force crossbridges between the thick and thin filaments.

Name some examples of paracrines found in smooth muscle.

histamine: asthma is a condition in which smooth muscle of the airways constricts in response to histamine release (can be reversed with epinephrine). nitric oxide or endothelium-derived relaxing factor: This gas is synthesized by the endothelial lining of blood vessels and relaxes adjacent smooth muscle that regulates the diameter of the blood vessels.

gastrocnemius muscle

in the calf of the leg; has about 2000 muscle fibers in each motor unit (gross motor actions)

Describe the compositions of the I band, H zone, M Line and A band. What is the purpose of the Z disc?

I band: Actin only. H zone: Myosin only. M line: Myosin linked with accessory proteins. A band: (outer edge) Actin and myosin overlap. Z disc: has accessory proteins that link the thin filaments together, similar to the accessory proteins shown for the M line.

One study found that many world-class athletes have muscle insertions that are farther from the joint than in the average person. Why would this trait translate into an advantage for a weight lifter?

If the muscle insertion point is farther from the joint, the leverage is better and a contraction creates more rotational force.

excitation-coupled Ca2+ entry

In recent years it has become apparent that there is some limited Ca2+ entry through the DHP receptor. Skeletal muscle contraction can still take place if there is no ECF Ca2+, however, so the physiological role of excitation-coupled Ca2+ entry is still being investigated.

Thin Filament

In skeletal and cardiac muscle tissue, a filament composed of actin, tropomyosin, and troponin. Thin filaments are attached to the Z lines of the sarcomeres and slide over thick filaments during muscle contraction.

Thick Filament

In skeletal and cardiac muscle tissue, a filament composed of bundles of myosin molecules. The myosin head groups attach to the thick filaments and pull them toward the center of the sarcomere during muscle contraction.

How can a neuron alter the amount of neurotransmitter it releases?

Increased frequency of action potentials in the neuron increases neurotransmitter release.

What is the response of a muscle fiber to an increase in the firing rate of the somatic motor neuron?

Increased motor neuron firing rate causes summation in a muscle fiber, which increases the force of contraction.

power stroke

Initiated by a calcium signal, when myosin crossbridges swivel and push the actin filaments toward the center of the sarcomere. At the end of a power stroke, each myosin head releases actin, then swivels back and binds to a new actin molecule, ready to start another contractile cycle. The power stroke repeats many times as a muscle fiber contracts. The myosin heads bind, push, and release actin molecules over and over as the thin filaments move toward the center of the sarcomere.

Excitation-Contraction Coupling and Relaxation:

Initiation: (1) Somatic motor neuron releases ACh at neuromuscular junction. (2) Net entry of Na+ through ACh receptor-channel initiates a muscle action potential. E-C Coupling: (3) Action potential in t-tubule alters conformation of DHP receptor. (4) DHP receptor opens RyR Ca2+ release channels in sarcoplasmic reticulum, and Ca2+ enters cytoplasm. (5) Ca2+ binds to troponin, allowing actin-myosin binding. (6) Myosin heads execute power stroke. (7) Actin filament slides toward center of sarcomere. Relaxation: (8) Sarcoplasmic Ca2+-ATPase pumps Ca2+ back into SR. (9) Decrease in free cytosolic [Ca2+] causes Ca2+ to unbind from troponin. (10) Tropomyosin re-covers binding site. When myosin heads release, elastic elements pull filaments back to their relaxed position.

Myosin is an ATPase:

It Hydrolyzes ATP to ADP and inorganic phosphate (Pi). The energy released by ATP hydrolysis is trapped by myosin and stored as potential energy in the angle between the myosin head and the long axis of the myosin filament. Myosin heads in this position are said to be "cocked," or ready to rotate. The potential energy of the cocked heads becomes kinetic energy in the power stroke that moves actin.

lengthening (eccentric) contraction.

Lengthening contractions are thought to contribute most to cellular damage after exercise and to lead to delayed muscle soreness.

Terminal Cisternae

Longitudinal tubules with enlarged end regions that make up the SR. These are storage "tanks" for calcium

Tendons

Made out of collagen, they attach muscles to bones.

Actin

Makes up the thin filaments of the muscle fiber. One actin molecule is a globular protein (G-actin). Usually, multiple G-actin molecules polymerize to form long chains or filaments, called F-actin. In skeletal muscle, two Factin polymers twist together like a double strand of beads, creating the thin filaments of the myofibril.

Explain how hyperpolarization decreases the likelihood of contraction in smooth muscle.

Many Ca2+ channels open with depolarization; therefore, hyperpolarization decreases the likelihood that these channels open. The presence of Ca2+ is necessary for contraction.

Flexion

Moves bones closer together. A bending movement around a joint in a limb (as the knee or elbow) that decreases the angle between the bones of the limb at the joint.

_________ converts the chemical bond energy of ATP into the mechanical energy of crossbridge motion.

Myosin

Smooth muscle exhibits a variety of electrical behaviors:

it can hyperpolarize as well as depolarize. Smooth muscle can also depolarize without firing action potentials. Contraction may take place after an action potential, after a subthreshold graded potential, or without any change in membrane potential.

extraocular muscles

move the eyes; one motor unit contains as few as three to five muscle fibers (fine motor actions)

The sliding filament theory explains how a muscle can contract and create force without creating _____________.

movement

Extension

moves bones away from each other. An unbending movement around a joint in a limb (as the knee or elbow) that increases the angle between the bones of the limb at the join.

Muscle fibers

muscle cells; a collection of these form skeletal muscles. Each skeletal muscle fiber is a long, cylindrical cell with up to several hundred nuclei near the surface of the fiber. The largest cells in the body, created by the fusion of many individual embryonic muscle cells

IP3 Channels

open when G protein-coupled receptors activate phospholipase C signal transduction pathways. Inositol trisphosphate (IP3) is a second messenger created in that pathway. When IP3 binds to the SR IP3-receptor channel, the channel opens and Ca2+ flows out of the SR into the cytosol.

pacemaker potentials

oscillating membrane potentials have regular depolarizations that always reach threshold and fire an action potential. they create regular rhythms of contraction. Pacemaker potentials are found in some cardiac muscles as well as in smooth muscle. Both slow wave and pacemaker potentials are due to ion channels in the cell membrane that spontaneously open and close.

mechanics

refers to how muscles move loads and how the anatomical relationship between muscles and bones maximizes the work the muscles can do.

Z disc

One sarcomere is composed of two Z disks and the filaments found between them. Z disks are zigzag protein structures that serve as the attachment site for thin filaments. The abbreviation Z comes from zwischen, the German word for "between."

Compare Ca2+ release channels in skeletal and smooth muscle sarcoplasmic reticulum.

Skeletal muscle Ca2+-release (RyR) channels are mechanically linked to DHP receptors. Smooth muscle also has Ca2+-release channels that are activated by IP3.

During contraction, H zone and I band both __________, while A band _____________ __________. The Z disks of the sarcomere move _______ _________ as the sarcomere shortens. These changes are consistent with the sliding of thin actin filaments along the thick myosin filaments as the actin filaments move toward the __ ______ in the center of the sarcomere.

shorten; remains constant; closer together; M line

The sarcomere ________ during contraction. As contraction takes place, actin and myosin do not change length but:

shortens; instead slide past one another

The human body has three types of muscle tissue:

skeletal muscle, cardiac muscle, and smooth muscle.

action potentials in skeletal muscle are conducted more ________ than action potentials in myelinated axons

slowly

Myosin Crossbridges

Small bridges on thick filaments that extend to thin filaments. Each G-actin molecule has a single myosin-binding site, and each myosin head has one actinbinding site and one binding site for ATP. Crossbridges form when the myosin heads of thick filaments bind to actin in the thin filaments. Crossbridges have two states: lowforce (relaxed muscles) and high-force (contracting muscles).

Name one way actin and its associated proteins differ in skeletal and smooth muscle.

Smooth muscle actin lacks troponin.

Name two ways smooth muscle myosin differs from skeletal muscle myosin.

Smooth muscle myosin is longer and has heads the entire length of the filament.

Identify as many pairs of antagonistic muscle groups in the body as you can. If you cannot name them, point out the probable location of the flexor and extensor of each group.

Some examples are biceps/triceps in the upper arm; hamstring (flexor)/quadriceps (extensor) in the upper leg; tibialis anterior (flexor)/gastrocnemius (extensor) for foot movement at the ankle.

Skeletal muscles are unique in that they contract only in response to a signal from a _________ __________ ________. They cannot initiate their own contraction, and their contraction is not influenced directly by hormones.

somatic motor neuron

Summation in muscle fibers means that the ________ of the fiber increases with repeated action potentials.

tension

Extensor

the bones move away from each other when the muscle contracts, and the movement is called extension.

Each myofibril is composed of several types of proteins:

the contractile proteins myosin and actin, the regulatory proteins tropomyosin and troponin, and the giant accessory proteins titin and nebulin.

What effects the strength of tension in a muscle fiber?

the length of individual sarcomeres before contraction begins (longer > shorter), frequency at which muscle action potentials stimulate the muscle fiber (summation) (more > less).

tetanus

the muscle fiber achieves a state of maximal contraction

Flexor

the muscle that closes/bends a joint such as on the arm or leg. the centers of the connected bones are brought closer together when the muscle contracts, and the movement is called flexion.

Smooth muscle

the primary muscle of internal organs and tubes, such as the stomach, urinary bladder, and blood vessels. Its primary function is to influence the movement of material into, out of, and within the body. An example is the passage of food through the gastrointestinal tract. Viewed under the microscope, smooth muscle lacks the obvious cross-bands of striated muscles.Its lack of banding results from the less organized arrangement of contractile fibers within the muscle cells.

Excitation-contraction (E-C) coupling

the process in which muscle action potentials initiate calcium signals that in turn activate a contraction-relaxation cycle.

complete, or fused, tetanus

the stimulation rate is fast enough that the muscle fiber does not have time to relax. Instead, it reaches maximum tension and remains there

incomplete, or unfused, tetanus

the stimulation rate of the muscle fiber is not at a maximum value, and consequently the fiber relaxes slightly between stimuli.

Duchenne muscular dystrophy

the structural protein dystrophin, which links actin to proteins in the cell membrane, is absent. In muscle fibers that lack dystrophin, extracellular Ca2+ enters the fiber through small tears in the membrane or possibly through stretch-activated channels. Calcium entry activates intracellular enzymes, resulting in breakdown of the fiber components. The major symptom of Duchenne dystrophy is progressive muscle weakness, and patients usually die before age 30 from failure of the respiratory muscles.

The downward rotational force on the forearm is proportional to

the weight of the forearm (F2) times the distance from the fulcrum to the forearm's center of gravity (the point along the lever at which the forearm load exerts its force).

series elastic elements

this component of muscle (i.e. tendons) does not actively generate force, it rather passively transmits the force generated by the contractile elements to the skeletal components to which it is attached.

Name an elastic fiber in the sarcomere that aids relaxation.

titin

Our muscles have two common functions: What is a special function of skeletal muscles?

to generate motion and to generate force. Our skeletal muscles also generate heat and contribute significantly to the homeostasis of body temperature. When cold conditions threaten homeostasis, the brain may direct our muscles to shiver, creating additional heat.

. One t-tubule and its two flanking terminal cisternae are called a __________.

triad

Skeletal muscles are often described as __________ muscles, and smooth and cardiac muscle as ___________. However, this is not a precise classification.

voluntary; involuntary

Rigor State

where the myosin heads are tightly bound to G-actin molecules. No nucleotide (ATP or ADP) is bound to myosin. In living muscle, the rigor state occurs for only a very brief period.

List the ways smooth muscle is more variable than skeletal muscle:

(1) Smooth muscles must operate over a range of lengths. (2) Within an organ, the layers of smooth muscle may run in several directions. (3) smooth muscles contract and relax much more slowly than skeletal or cardiac muscle. (4) Smooth muscle uses less energy to generate and maintain a given amount of force (rely more on glycolysis). (5) Smooth muscle can sustain contractions for extended periods without fatiguing. (6) Smooth muscles have small, spindle-shaped cells with a single nucleus (in contrast to the large multinucleated fibers of skeletal muscles). (7) the contractile fibers are not arranged in sarcomeres. (8) Contraction in smooth muscle may be initiated by electrical or chemical signals or both (Sk is always AP). (9) Smooth muscle is controlled by the autonomic nervous system. (10) Smooth muscle lacks specialized receptor regionssuch as the motor end plates found in skeletal muscle synapses. Instead, receptors are found all over the cell surface. (11) Ca2+ for contraction comes from the extracellular fluid as well as from the sarcoplasmic reticulum. (12) Ca2+ signal initiates a cascade that ends with phosphorylation of myosin light chains and activation of myosin ATPase (Smooth muscle has no troponin).

The total rotational force* created by the biceps depends on two things:

(1) the force of muscle contraction and (2) the distance between the fulcrum and the point at which the muscle inserts onto the radius.

E-C coupling has four major events:

(1_Acetylcholine (ACh) is released from the somatic motor neuron. (2) ACh initiates an action potential in the muscle fiber. (3) The muscle action potential triggers calcium release from the sarcoplasmic reticulum. (4) Calcium combines with troponin and initiates contraction

Compare the following aspects of skeletal and smooth muscle contraction: (a) signal for crossbridge activation (b) source(s) of calcium for the Ca2+ signal (c) signal that releases Ca2+ from the sarcoplasmic reticulum

(a) Skeletal muscle: Ca2+ binds to troponin. Smooth muscle: myosin phosphorylated. (b) Skeletal muscles: all Ca2+ comes from the sarcoplasmic reticulum. Smooth muscle: Ca2+ from both SR and ECF. (c) Skeletal muscle: depolarization signal. Smooth muscle: IP3 signal.

Which of the following statement(s) is (are) true about skeletal muscles? (a) They constitute about 60% of a person's total body weight. (b) They position and move the skeleton. (c) The insertion of the muscle is more distal or mobile than the origin. (d) They are often paired into antagonistic muscle groups called flexors and extensors.

(a) false (b) true (c) true (d) true

Where does energy for the power stroke come from?

ATP

Smooth muscle has the same contractile elements as skeletal muscle—actin and myosin that interact through crossbridges— as well sarcoplasmic reticulum that stores and releases Ca2+. However, details of the structural elements differ in the two muscle types. Explain

Actin is more plentiful in smooth muscle than in striated muscle. Myosin is less and longer. The less numerous myosin filaments are surrounded by actin filaments and are arranged so that each myosin molecule is in the center of a bundle of 12-15 actin molecules. These contractile units are arranged so that they run parallel to the long axis of the cell. Smooth muscle actin is associated with tropomyosin, as in skeletal muscle. However, unlike skeletal muscle, smooth muscle lacks troponin. Smooth muscle cells have an extensive cytoskeleton consisting of intermediate filaments and protein dense bodies in the cytoplasm and along the cell membrane. Protein fibers in the extracellular matrix tie the smooth muscle cells of a tissue together and transfer force from a contracting cell to its neighbors. The amount of SR in smooth muscle varies from one type of smooth muscle to another. The arrangement of smooth muscle SR is less organized than in skeletal muscle. There are no t-tubules, but caveolae, which apparently participate in cell signaling.

List six proteins that make up the myofibrils. Which protein creates the power stroke for contraction?

Actin, myosin, troponin, tropomyosin, titin, and nebulin. Myosin produces the power stroke.

Why is the muscle usually not in rigor state?

Although the contractile cycle began with the rigor state in which no ATP or ADP was bound to myosin, relaxed muscle fibers remain mostly in step 2. The rigor state in living muscle is normally brief because the muscle fiber has a sufficient supply of ATP that quickly binds to myosin once ADP is released in step 4.

Briefly describe events at the neuromuscular junction leading up to a muscle contraction.

An acetylcholine signal is converted from a somatic motor neuron into an electrical signal in the muscle fiber.

Motor unit

Basic unit of contraction in a contact skeletal muscle; composed of a group of muscle fibers that function together and the somatic motor neuron that controls them

The reaction shows that creatine kinase catalyzes the creatine-phosphocreatine reaction in both directions. What then determines the direction that the reaction goes at any given moment?

Because creatine kinase catalyzes the reaction in both directions, the relative concentrations of the reactants and products determine the direction of the reaction. The reaction obeys the law of mass action and goes to equilibrium.

How do pacemaker potentials differ from slow wave potentials?

Pacemaker potentials always reach threshold and create regular rhythms of contraction. Slow wave potentials are variable in magnitude and may not reach threshold each time.

Ca2+-ATPase

Pumps that are always on and are responsible for pumping calcium back into the SR. Necessary to end contraction.

. What causes relaxation in skeletal muscle?

Relaxation in skeletal muscle occurs when troponin releases Ca2+ and tropomyosin moves back to block actin's binding site for myosin.