Chapter 9 Section A: Skeletal Muscle

terminal cisternae

(sometimes also referred to as "lateral sacs") At the end of each segment of sarcoplasmic reticulum are two enlarged regions, known as terminal cisternae (sometimes also referred to as "lateral sacs"), that are connected to each other by a series of smaller tubular elements. Ca2+ is stored in the terminal cisternae and is released into the cytosol following membrane excitation. T-tubules and terminal cisternae sur- round the myofibrils at the region of the sarcomeres where the A bands and I bands meet. T-tubule is yellow thing in between two terminal cisternae

Slow-oxidative fibers

(type I) combine low myosin- ATPase activity with high oxidative capacity. very resistant to fatigue

Fast-oxidative-glycolytic fibers

(type IIa) combine high myosin-ATPase activity with high oxidative capacity and intermediate glycolytic capacity.

Fast-glycolytic fibers

(type IIb) combine high myosin- ATPase activity with high glycolytic capacity. fatigue rapidly

Mechanism of Cytosolic Increase in Ca2+ from sarcoplasmic retic

1) The T-tubules are in intimate contact with the terminal cisternae of the sarcoplasmic reticulum, connected by structures known as junctional feet, or foot processes. 2) During a T-tubule action potential, charged amino acid residues within the DHP receptor protein (on the T-tubule) induce a conformational change, which acts via the foot process to open the ryanodine receptor channel. Ca2+ is then released from the terminal cisternae of the sarcoplasmic reticulum into the cytosol, where it can bind to troponin. this is typicall enough to briefly saturate all troponin-binding sites on the thin filaments

isotonic contraction

A contraction in which the muscle changes length while the load on the muscle remains constant (either shortening or lengthening)

disuse atrophy.

A muscle can also atrophy with its nerve supply intact if the muscle is not used for a long period of time, as when a broken arm or leg is immobilized in a cast

motor neurons

AKA somatic efferent neurons The neurons whose axons innervate skeletal muscle fibers cell bodies are located in the brainstem and the spinal cord axons of motor neurons are myelinated and are the largest-diameter axons in the body. They are therefore able to propagate action potentials at high velocities, allowing signals from the central nervous system to travel to skeletal muscle fibers with minimal delay axon of a motor neuron divides into many branches, each branch forming a single synapse with a muscle fiber. myelin sheath surrounding the axon of each motor neu- ron ends near the surface of a muscle fiber, and the axon divides into a number of short processes that lie embedded in grooves on the muscle fiber surface vesicles at axon terminals use ACh (acetylcholine)

What happens to an isotonic twitch with a heavier load?

At heavier loads, (1) the latent period is longer, (2) the velocity of short- ening (distance shortened per unit of time) is slower, (3) the duration of the twitch is shorter, and (4) the distance shortened is less. shortening does not begin until enough cross-bridges have attached and the muscle tension just exceeds the load on the fiber. Thus, before shortening, there is a period of isometric contraction during which the tension increases. The heavier the load, the longer it takes for the tension to increase to the value of the load, when shortening will begin. If the load on a fiber is increased, eventually a load is reached that the fiber is unable to lift, the velocity and distance of shortening decrease to zero, and the contraction will become completely isometric.

unfused tetanus

At low stimulation frequencies, the tension may oscillate as the muscle fiber partially relaxes between stimuli

latent period

Following the action potential, there is an interval of a few milliseconds known as the latent period before the tension in the muscle fiber begins to increase. During this latent period, the processes associated with excitation-contraction coupling are occurring.

size differences among different fibers and metabolism

Glycolytic fibers generally have larger diameters than oxidative fibers. This fact has significance for tension development. The number of thick and thin filaments per unit of cross-sectional area is about the same in all types of skeletal muscle fibers. Therefore, the larger the diameter of a muscle fiber, the greater the total number of thick and thin filaments acting in parallel to produce force, and the greater the maximum tension it can develop

antagonists

Groups of muscles that produce oppositely directed movements at a joint Sets of antagonistic muscles are required not only for flexion-extension but also for side-to-side movements or rotation of a limb. The contraction of some muscles leads to two types of limb movement, depending on the contractile state of other muscles acting on the same limb. when the biceps shortens 1 cm, the hand moves through a distance of 7 cm. Because the muscle shortens 1 cm in the same amount of time that the hand moves 7 cm, the velocity at which the hand moves is seven times greater than the rate of muscle shortening.

glycolysis

If the intensity of exercise exceeds about 70% of the maximal rate of ATP breakdown, glycolysis contributes an increasingly significant fraction of the total ATP generated by the muscle. produces less ATP per glucose, but is faster than oxidative phosphorylation glucose from blood or broken down from glycogen in muscle

denervation atrophy

If the neurons to a skeletal muscle are destroyed or the neuromuscular junctions become nonfunctional, the denervated muscle fibers will become progressively smaller in diameter and the amount of contractile proteins they contain will decrease

muscle cramps

Involuntary tetanic contraction of skeletal muscles action potentials fire at abnormally high rates, a much greater rate than occurs during maximal voluntary contraction may be partly related to electrolyte imbalances in the extracellular fluid surrounding both the muscle and nerve fibers. These imbalances may arise from overexercise or persistent dehydration

hypocalcemia

Low extracellular Ca2+

summation

NOT LIKE NEURONS Here we are observing the effect of additional attached cross-bridges from multiple actions potentials reaching the fiber

dihydropyridine (DHP) receptor

T-tubule protein modified voltage-sensitive Ca2+ channel in the junctional foot between the t-tubule and the terminal cisternae The main function of the DHP receptor, however, is not to conduct Ca2+ but rather to act as a voltage sensor.

muscle tension

The force exerted on an object by a contracting muscle load opposes it For muscle fibers to shorten and thereby move a load, muscle tension must be greater than the opposing load.

neuromuscular junction

The junction of an axon terminal with the motor end plate ACh diffuses from the axon terminal to the motor end plate where it binds to ionotropic receptors of the nicotinic type normal ACh follows, see chpt 6 Most neuromuscular junctions are located near the middle of a muscle fiber, and newly generated muscle action potentials propagate from this region in both directions toward the ends of the fiber. inhibitory potentials do not occur in human skeletal muscle; all neuromuscular junctions are excitatory.

optimal length (L0)

The length at which the fiber develops the greatest isometric active tension the amount of active tension a muscle fiber develops during contraction can also be altered by changing the length of the fiber When a muscle fiber length is 60% of L0 or shorter, the fiber develops no tension when stimulated. Further lengthening leads to a decrease in tension. At lengths of 175% of L0 or greater, the fiber develops no active tension when stimulated. Stretching a fiber to 175% of L0 pulls the filaments apart to the point where there is no overlap. At this point, there can be no cross- bridge binding to actin and no development of tension. When most skeletal muscle fibers are relaxed, passive elastic properties keep their length near L0 and thus near the optimal length for force generation.

twitch

The mechanical response of a muscle fiber to a single action potential Following the action potential, there is an interval of a few milliseconds known as the latent period before the tension in the muscle fiber begins to increase.

recruitment

The process of increasing the number of motor units that are active in a muscle at any given time achieved by activating excitatory synaptic inputs to more motor neurons. the more motor units recruited and the greater the muscle tension The size of a motor neuron refers to the diameter of the neuronal cell body, which usually correlates with the diameter of its axon. smaller neurons will get action potentials first due to surfacearea/volume ratio Because the smallest motor neurons innervate the slow-oxidative motor units, these motor units are recruited first, followed by fast-oxidative-glycolytic motor units, and finally during very strong contractions, by fast-glycolytic motor units

motor end plate

The region of the muscle fiber plasma membrane that lies directly under the terminal portion of the axon

cross-bridge cycle

The sequence of events that occurs between the time a cross- bridge binds to a thin filament, moves, and then is set to repeat the process Each cycle consists of four steps: (1) attachment of the cross-bridge to a thin filament; binding the ADP, Pi, mysoin, and actin (2) movement of the cross-bridge, producing tension in the thin filament, strained conformation of the cross bridge ; causes release of the ADP +Pi (3) detachment of the cross-bridge from the thin filament; when a new ATP binds (4) energizing the cross-bridge so it can again attach to a thin filament and repeat the cycle. by hydrolysis of ATP by ATPase Each cross-bridge undergoes its own cycle of movement independently of other cross-bridges. In resting state, cross brdiges are energized with ATP and are bound with ADP + Pi. ATP binds to cause to detach, then hydrolyzes to energize the cross bridge to allow it to move forward/bind to new actin, then old ADP + Pi releases due to tension from moving forward

contraction time

The time interval from the beginning of tension development at the end of the latent period to the peak tension The total duration of a contraction depends in part on the time that cytosolic Ca21 remains elevated so that cross-bridges can continue to cycle. This is closely related to the Ca2+-ATPase activity in the sarcoplasmic reticulum; activity is greater in fast-twitch fibers and less in slow-twitch fibers.

isometric contraction

When a muscle develops tension but does not shorten or lengthen occurs when the muscle supports a load in a constant position or attempts to move an otherwise supported load that is greater than the tension developed by the muscle the bound cross-bridges exert a force on the thin filaments but they are unable to move it. Rather than the filaments sliding, the rotation during the power stroke is absorbed within the structure of the cross-bridge in this circumstance. If isometric contraction is prolonged, cycling cross-bridges repeatedly rebind to the same actin molecule.

muscle fatigue

When a skeletal muscle fiber is repeatedly stimulated, the tension the fiber develops eventually decreases even though the stimulation continues . This decline in muscle tension as a result of previous contractile activity is known as muscle fatigue. decreased shortening velocity and a slower rate of relaxation rest can allow to restore, but not enough rest might casue onset of fatigue sooner upon subsequent activations metabolic changes that occur in active muscle cells include a decrease in ATP concentration and increases in the concentrations of ADP, Pi, Mg2+, H+ (from lactic acid), and oxygen free radicals those metabolic changes have been shown to 1. decrease the rate of Ca2+ release, reuptake, and storage by the sarcoplasmic reticulum; 2. decrease the sensitivity of the thin filament proteins to activation by Ca2+; and 3. directly inhibit the binding and power-stroke motion of the myosin cross-bridges.

eccentric contraction

When an unsupported load is greater than the tension generated by cross-bridges, the result is a lengthening contraction In this situation, the load pulls the muscle to a longer length in spite of the opposing force produced by the cross-bridges. the lengthening of muscle fibers is not an active process produced by the contractile proteins but a consequence of the external forces being applied to the muscle.

concentric contraction

When tension exceeds the load, shortening occurs

Load-Velocity Relation for isotonic stretches

When the initial shorten- ing velocity (slope) of a series of isotonic twitches is plotted as a function of the load on a single fiber, the result is a hyperbolic curve the rate of ATP hydrolysis determines the shortening velocity for the unloaded fiber

contraction

activation of the force-generating sites within muscle fibers—the cross-bridges Following contraction, the mechanisms that generate force are turned off and tension declines, allowing relaxation of muscle fibers

succinylcholine

acts as an agonist to the ACh receptors and produces a depolarizing/desensitizing block similar to acetylcholinesterase inhibitors. used during surgery to prevent muscle mov't

treatment of Myasthenia gravis

administer acetylcholinesterase inhibitors (e.g., pyridostigmine), prolonging the time that acetylcholine is available at the synapse. Other therapies aim at blunting the immune response. Treatment with glucocorticoids is one way that immune function is suppressed-- Removal of the thymus (thymectomy) reduces the production of antibodies and reverses symptoms in about 50% of patients Plasmapheresis treatment that involves replacing the liquid fraction of blood (plasma) that contains the offending antibodies. A combination of these treatments has greatly reduced the mortality rate for myasthenia gravis.

end-plate potential (EPP)

after ACh has been released and binds to it's receptors, allowing channels to open and causing local depolarization of the motor end plate by Na+ mov't magnitude of EPP is larger than a normal EPSP bc neurotransmitter is released over a larger surface area, binding to many more receptors and opening many more ion channels. one EPP is normally more than sufficient to depolarize the muscle plasma membrane adjacent to the end-plate membrane to its threshold potential, initiating an action potential. action potential is then propagated over the surface of the muscle fiber and into the T-tubules by same mech as along axon

striated muscle

both cardiac and skeletal muscle distinct series of alternating light and dark bands perpendicular to the long axis

tendons

bundles of connective tissue consisting of collagen fibers that attach bones to muscle

Costameres

clusters of structural and regulatory proteins that link the Z disks of the outermost myofibrils to the sarcolemma and extracellular matrix Proteins of the costameres serve multiple functions, including lateral transmission of force from the sarcomeres to the extracellular matrix and neighboring muscle fibers, stabilization of the sarcolemma against physical forces during muscle fiber contraction or stretch, and initiation of intracellular signals that link contractile activity with regulation of muscle cell remodeling.

myofibrils

composed of thick and thin filaments, which are cytosolic proteins Most of the cytoplasm of a muscle fiber is filled with myofibrils, each extending from one end of the fiber to the other and linked to the tendons at the ends of the fiber.

myosin

composes the majority of thick filaments composed of two large polypeptide heavy chains and four smaller light chains form a molecule that consists of two globular heads (containing heavy and light chains and looks like testicles...) and a long tail formed by the two intertwined heavy chains. The tail of each myosin molecule lies along the axis of the thick filament, and the two globular heads extend out to the sides, forming cross-bridges. Each head contains two binding sites, one for attaching to the thin filament and one for ATP (4 binding sites total, 2 for each) ATP bind- ing site also functions as an enzyme (called myosin-ATPase) that hydrolyzes the bound ATP, harnessing its energy for contraction the bottom thing in the diagram

junctional feet/foot processes

connect the T-tubules to the terminal cisternae of the sarcoplasmic reticulum This junction involves two integral membrane proteins, one in the T-tubule membrane and the other in the membrane of the sarcoplasmic reticulum. The T-tubule protein is a modified voltage-sensitive Ca2+ channel known as the dihydropyridine (DHP) receptor the protein embedded in the sarcoplasmic reticulum membrane is known as the ryanodine receptor

Fast-twitch fibers

contraction times as short as 10 msec greater Ca2+-ATPase activity which allows for Ca2+ to be sucked back into the sarc retic quickly

thick filament

cytosolic proteins one of two types of filament. The larger are thick filaments and the smaller are thin filaments. Both are part of cylindrical bundles called myofibrils composed almost entirely of the protein myosin

thin filament

cytosolic proteins one of two types of filament. The larger are thick filaments and the smaller are thin filaments. Both are part of cylindrical bundles called myofibrils half diameter of thick composed of actin with cords of tropomyosin wrapping around and troponin molecules

curare

deadly arrowhead poison still used by some indigenous peoples of South America, binds strongly to nicotinic ACh receptors. It does not open their ion channels, however, and is resistant to destruction by acetylcholinesterase. When a receptor is occupied by curare, ACh cannot bind to the receptor. Therefore, although the motor neurons still conduct normal action potentials and release ACh, there is no resulting EPP in the motor end plate and no contraction. Because the skeletal muscles responsible for breathing, like all skeletal muscles, depend upon neuromuscular transmission to initiate their contraction, curare poisoning can cause death by asphyxiation.

endurance training

decrease in the number of fast-glycolytic fibers and an increase in the number of fast- oxidative-glycolytic fibers as the oxidative capacity of the fibers increases.

Cross section of myofibril

each thin filament is surrounded by a triangle of thick filaments each thick filament is surrounded by a hexagon of 6 thin filaments

Flexion

ending of a limb at a joint

oxidative fibers

fibers that contain numerous mitochondria and thus have a high capacity for oxidative phosphorylation. Most of the ATP such fibers produce is dependent upon blood flow to deliver oxygen and fuel molecules to the muscle. Therefore, these fibers are surrounded by many small blood vessels. They also contain large amounts of an oxygen-binding protein known as myoglobin which increases the rate of oxygen diffusion into the fiber and pro- vides a small store of oxygen. --> give fibers red color

formation/development of the muscle fiber

formed during development by the fusion of a number of undifferentiated, mononucleated cells known as myoblasts into a single, cylindrical, multinucleated cell. skeletal muscle differentiation is completed around the time of birth, and these differentiated fibers continue to increase in size from infancy to adulthood. large compared to other cell types

Oxidative Phosphorylation

forms most of ATP used for muscle contraciton in mitochondria from breakdown of muscle glycogen to glucose, then fatty acids more important

rigor mortis

gradual stiffening of skeletal muscles that begins several hours after death and reaches a maximum after about 12 hours The ATP concentration in cells, including muscle cells, declines after death thus, the breakage of the link between actin and myosin does not occur

glycolytic fibers

have few mitochondria but possess a high concentration of glycolytic enzymes and a large store of glycogen less myoglobin== less red== called WHITE MUSCLE FIBERS

insulin-like growth factor-1 and exercise

helps mediate exercise induced changes in muscle

sarcoplasmic reticulum

homologous to the endoplasmic reticulum found in most cells forms a series of sleevelike segments around each myofibril At the end of each segment are two enlarged regions, known as terminal cisternae (sometimes also referred to as "lateral sacs"), that are connected to each other by a series of smaller tubular elements. Ca2+ is stored in the terminal cisternae and is released into the cytosol following membrane excitation.

hypertrophy

increase in size of skeletal muscle cell satellite cell-mediated compensation for a loss of muscle tissue also occurs in response to heavy exercise

oxygen debt

increased oxygen consumption following exercise repays increased production of ATP by oxidative phosphorylation following exercise is used to restore the energy reserves in the form of creatine phosphate and glycogen

Hypocalcemic tetany

involuntary tetanic contraction of skeletal muscles that occurs when the extracellular Ca2+ concentration decreases to about 40% of its normal value. The effect of changes in extracellular Ca2+ is exerted not on the sarcoplasmic reticulum Ca2+ but directly on the plasma membrane. Low extra- cellular Ca21 (hypocalcemia) increases the opening of Na1 chan- nels in excitable membranes, leading to membrane depolarization and the spontaneous firing of action potentials.

skeletal muscle

is attached to bone, and its contraction is responsible for supporting and moving the skeleton contraction of skeletal muscle is initiated by action potentials in neurons of the somatic motor division of the peripheral nervous system and is usually under voluntary control distinct series of alternating light and dark bands perpendicular to the long axis

actin

is the main component of thin filaments globular protein composed of a single polypeptide (a monomer) that polymerizes with other actin monomers to form a polymer made up of two intertwined, helical chains that are the core of the thin filament Each actin molecule contains a binding site for myosin.

transverse tubule (T-tubule)

lies directly between—and is intimately associated with—the terminal cisternae of adjacent segments of the sarcoplasmic reticulum perpendicular with the direction of the myofibril T-tubules and terminal cisternae surround the myofibrils at the region of the sarcomeres where the A bands and I bands meet. are continuous with the plasma membrane (which in muscle cells is sometimes referred to as the sarcolemma), and action potentials propagating along the surface membrane also travel throughout the interior of the muscle fiber by way of the T-tubules. The lumen of the T-tubule is continuous with the extracellular fluid surrounding the muscle fiber.

tetanus

maintained contrac- tion in response to repetitive stimulation (tetanic contraction) As the frequency of action potentials increases, the level of tension increases by summation until a maximal fused tetanic ten- sion is reached, beyond which tension no longer increases even with further increases in stimulation frequency this maximal tetanic tension is about three to five times greater than the isometric twitch tension-- persistent elevation of Ca concentration causes all actin binding sites to remain open which allows, over time for all cross bridges to have time to attach tetanic contractions are beneficial when maximal, sustained work is required such as holding a heavy object in place; they are also responsible for much of our ability to maintain our posture.

Effect of Aging on muscle

maximum force a muscle generates decreases by 30% to 40% between the ages of 30 and 80. due primarily to a decrease in average fiber diameter

central command fatigue

may cause a person to stop exercising even though the muscles are not fatigued occurs when the appropriate regions of the cerebral cortex fail to send excitatory signals to the motor neurons ability to initiate central commands to muscles during a period of increasingly distressful sensations

slow-twitch fibers

may take 100 msec or longer. less Ca2+-ATPase activity which allows for Ca2+ to be sucked back into the sarc retic over a longer period of time and causes the twitch to take longer

motor unit

motor neuron and the muscle fibers it innervates The muscle fibers in a single motor unit are located in one muscle, but they are distributed throughout the muscle and are not necessarily adjacent to each other When an action potential occurs in a motor neuron, all the muscle fibers in its motor unit are stimulated to contract.

Myasthenia gravis

neuromuscular disorder characterized by muscle fatigue and weakness that progressively worsen as the muscle is used. Myasthenia gravis affects about one out of every 7500 Americans, occurring more often in women than men. The most common cause is the destruction of nicotinic ACh-receptor proteins of the motor end plate, mediated by antibodies of a per- son's own immune system magnitude of the end-plate potential is markedly reduced because of the decreased availability of receptors

isometric twitch

no shorten/lengthening latent period in an isotonic twitch contraction is longer than that in an isometric twitch contraction, however the event of shortening/lengthening is shorter than the duration of force generation in this isometric twitch In the isometric twitch experiment, tension begins to increase as soon as the first cross-bridge attaches, so the latent period is due only to the excitation-contraction coupling delay.

muscle

number of skeletal muscle fibers bound together by connective tissue usually attached to bones by bundles of connective tissue consisting of collagen fibers known as tendons In some muscles, the individual fibers extend the entire length of the muscle, but in most, the fibers are shorter, often oriented at an angle to the longitudinal axis of the muscle.

M line

of the sarcomere A narrow, dark band in the center of the H zone corresponds to proteins that link together the central region of adjacent thick filaments

H zone

of the sarcomere H zone is a narrow, light band in the center of the A band where the thin filaments don't overlap the thick filaments

A band

of the sarcomere The thick filaments are located in the middle of each sarcomere, where they create a wide, dark band known as the A band.

I band

of the sarcomere light band known as the I band lies between the ends of the A bands of two adjacent sarcomeres and contains the portions of the thin filaments that do not overlap the thick filaments. The I band is bisected by the Z line.

titin

of the sarcomere the protein that makes up filaments that extend from the Z line to the M line are linked to both the M-line proteins and the thick filaments. ATTACHED AT Z-LINE maintain the alignment of thick filaments in the middle of each sarcomere responsible for most of the passive elastic properties of relaxed muscle fibers

Z line

of the sarcomere two sets of thin filaments coming from each direction and intermingling with thick filaments the outside end that's anchored and not interacting with thick filament is the Z-line, it's connected to proteins outer edge of the sarcomere --> thin filaments from two adjacent sarcomeres are anchored to the two sides of each Z line. (actually more like z-disks bc myofibrils are cylindrical)

myosin-ATPase

on the globular myosin head which hangs off the thick filament. ATP binding site that ALSO hydrolyzes the bound ATP, harnessing its energy for contraction

poliomyelitis

once-common viral disease that destroys motor neurons, leading to the paralysis of skeletal muscle, and may result in death due to respiratory failure.

sarcomere

one unit of repeating pattern of thick and thin filaments contains two sets of thin filaments, one at each end

sliding-filament mechanism

overlapping thick and thin filaments in each sarcomere move past each other, propelled by movements of the cross-bridges. swiveling motion of many cross-bridges forces the thin filaments attached to successive Z lines to move toward the center of the sarcomere, thereby shortening the sarcomere During this shortening of the sarcomeres, there is no change in the lengths of either the thick or thin filaments. a common pattern of muscle shortening involves one end of the muscle remaining at a fixed position while the other end shortens toward it. as the sarcomere shortens, the center of each sarcomere also slides toward the fixed end of the muscle

red muscle fibers

oxidative fiber with lots of myoglobin that appear red

myoglobin

oxygen-binding protein known as myoglobin, which increases the rate of oxygen diffusion into the fiber and pro- vides a small store of oxygen.

3 ways muscles make ATP

phosphorylation of ADP by creatine phosphate (a small molecule produced from three amino acids and capable of functioning as a phosphate donor) oxidative phosphorylation of ADP in the mitochondria phosphorylation of ADP by the glycolytic pathway in the cytosol

creatine phosphate

phosphorylation of creatine phosphate is a rapid means of forming ATP at the onset of contractile activity When the chemical bond between creatine (C) and phosphate is broken, the amount of energy released is about the same as that released when the terminal phosphate bond in ATP is broken--> transferred to ADP + Pi to form ATP, catalyzed by creatine phosphate replenishes ATP quickly at beginning of contraction limited by the initial concentration of creatine phosphate in the cell.

myostatin

produced by skeletal muscle cells and binds to receptors on those same cells exert a negative feedback effect to prevent excessive muscle hypertrophy and thus prevents muscle growth

ryanodine receptor

protein embedded in the sarcoplasmic reticulum membrane This large molecule not only includes the foot process that connects to the DHP receptor but also forms a Ca2+ channel. in muscle fatigue, channels become leaky to Ca2+, and persistent elevation of cytosolic Ca2+ activates proteases that degrade contractile proteins. muscle soreness and weakness that lasts until the synthesis of new proteins can replace those that are damaged

Muscular dystrophy

relatively common genetic disease, affecting an estimated one in every 3500 males (but many fewer females) caused by the absence or defect of one or more proteins that make up the costameres in striated muscle.

muscle soreness

result of structural damage to muscle cells and their membranes, which activates the inflammation response Soreness most often results from lengthening contractions, indicating that the lengthening of a muscle fiber by an external force produces greater muscle damage than does either shortening or isometric contraction

tropomyosin

rod-shaped molecule composed of two intertwined polypeptides with a length approximately equal to that of seven actin monomers. Chains of tropomyosin molecules are arranged end to end along the actin thin filament. partially cover the myosin-binding site on each actin monomer, thereby preventing the cross-bridges from making contact with actin

Duchenne muscular dystrophy

sex-linked recessive disorder caused by a mutation in a gene on the X chromosome that codes for the protein dystrophin The defective gene can result in either a nonfunctional or missing protein.

isotonic twitch

shortening/lengthening latent period in an isotonic twitch (this) contraction is longer than that in an isometric twitch contraction, however the event of shortening/lengthening in this isotonic twitch is shorter than the duration of force generation in a isometric twitch in the isotonic twitch experiment, the latent period includes both the time for excitation-contraction coupling and the extra time it takes to accumulate enough attached cross-bridges to lift the load off of the platform. (bottom graph) at the end of the twitch, the isotonic load comes back to rest on the platform well before all of the cross-bridges have detached in the isometric experiment.

muscle fiber

skeletal muscle cell name due to its elongated shape and the presence of multiple nuclei

troponin

small globular molecule holds tropomyosin in place over the myosin binding sites on the actin molecules composed of three subunits designated by the letters I (inhibitory), T (tropomyosin-binding) and C (Ca2+-binding) One troponin binds to a molecule of tropomyosin and regulates the access to myosin-binding sites on the seven actin monomers in contact with that tropomyosin. This is a resting muscle fiber; troponin and tropomyosin cooperatively block the interaction of cross-bridges with the thin filament. when Ca2+ binds to specific binding sites on the Ca2+ binding subunit of troponin this produces a change in the shape of troponin (i.e., its tertiary structure), which relaxes its inhibitory grip and allows tropomyosin to move away from the myosin-binding site on each actin molecule. Conversely, the removal of Ca2+ from troponin reverses the process, turning off contractile activity.

extension

straightening of a limb

cardiac muscle

striated contains intercalated disks

how large are muscle fibers? how does this affect them?

super large!! up to 20 cm length. retention of the nuclei from the original myoblasts-- Spread throughout the length of the muscle fiber, each nucelus participates in regulation of gene expression and protein synthesis within its local domain.

smooth muscle

surround various hollow organs and tubes derives its name from the fact that it lacks this striated appearancec contraction is not normally under voluntary control. It occurs autonomously in some cases, but frequently it occurs in response to signals from the autonomic nervous system, hormones, and autocrine or paracrine signals

Dystrophin

the first costamere protein discovered to be related to a muscular dystrophy extremely large protein that normally forms a link between the contractile filament actin and proteins embedded in the overlying sarcolemma. In its absence, fibers subjected to repeated structural deformation during contraction are susceptible to membrane rup- ture and cell death.

load

the force exerted on the muscle by an object (usually its weight) muscle tension opposes it

cross-bridges

the heads of the myosin make contact with the thin filament and exert force during muscle contraction

ending high concentrations of Ca2+ in the cytosol

the membranes of the sarcoplasmic reticulum contain primary active-transport proteins—Ca2+-ATPases—that pump calcium ions from the cytosol back into the lumen of the reticulum pumping of the released Ca21 back into the reticulum requires a much longer time than the action potential. Therefore, the cytosolic Ca2+ concentration remains elevated, and the contraction continues for some time after a single action potential

sarcolemma

the plasma membrane of the muscle cell, action potentials propagate down it

Excitation-contraction coupling

the sequence of events by which an action potential in the plasma membrane activates the force-generating mechanisms. The electrical activity in the plasma membrane does not directly act upon the contractile proteins but instead pro- duces a state of increased cytosolic Ca21 concentration, which continues to activate the contractile apparatus long after the electrical activity in the membrane has ceased.

troponin and tropomyosin

tropomyosin wraps around the actin of thin filament important function regulating cont

satellite cells

undifferentiated stem cells help repair muscle fibers if they're damaged/destroyed normally quiescent, located between the plasma membrane and surrounding basement membrane along the length of muscle fibers. In response to strain or injury, they become active and undergo mitotic proliferation. Daughter cells then differentiate into myoblasts that can either fuse together to form new fibers or fuse with stressed or damaged muscle fibers to reinforce and repair them. may not restore a severely damaged muscle to the original number of muscle fibers mediates hypertrophy

fused tetanus

with no oscillations in the tension in muscle, is produced at higher stimulation frequencies