Chapter 11: Muscle

load-velocity relationship

(1) when the load is zero, the velocity of shortening is maximal; (2) as the load increases, the velocity of shortening decreases; and (3) when the load is equal to or exceeds the maximum tetanic tension that the muscle can produce, the velocity of shortening is zero and the contraction becomes isometric

F actin

G actin molecules are linked together to form a long polymer called _____________, which is so-named because it has a fibrous structure

smooth muscle contraction

1. Ca2+ binds to calmodulin, a regulatory protein in the sarcoplasm that is similar in structure to troponin. 2. the Ca2+-calmodulin complex activates an enzyme called myosin light chain kinase (MLCK), which is also present in the sarcoplasm. 3. activated MLCK in turn phosphorylates (adds a phosphate group to) light chains in the myosin heads. 4. the phosphorylated myosin heads bind to actin, and muscle contraction begins.

contraction period

Ca2+ binds to troponin, myosin-binding sites on actin are exposed, and myosin crossbridges form; as a result, peak tension develops in the muscle fiber

relaxation period

Ca2+ is actively transported back into the SR, myosin-binding sites are covered by tropomyosin, myosin heads detach from actin, and tension in the muscle fiber decreases

latent period

a brief delay that occurs between application of the stimulus and the beginning of contraction; during this time, the events of excitation-contraction coupling occur: the muscle action potential sweeps along the sarcolemma and into the T tubules, causing the release of Ca2+ from the SR

triad

a complex of three units in a muscle fiber composed of a transverse tubule and the sarcoplasmic reticulum terminal cisternae on both sides of it

actin

a contractile protein that is part of thin filaments in muscle fibers

sarcomere

a contractile unit in a striated muscle fiber extending from one Z disc to the next Z disc; contains an A band, I bands, an H zone, and an M line

dystrophin

a cytoskeletal structural protein that links thin filaments of the sarcomere to integral membrane proteins of the sarcolemma, which are attached in turn to proteins in the connective tissue extracellular matrix that surrounds muscle fibers; with its associated proteins thought to reinforce the sarcolemma and help transmit the tension generated by the sarcomeres to the tendons

nebulin

a long, nonelastic structural protein wrapped around the entire length of each thin filament that holds F-actin strands together; helps anchor the thin filaments to the Z discs and regulates the length of thin filaments during development

motor unit

a motor neuron together with the muscle fibers it stimulates

sarcoplasmic reticulum (SR)

a network of sacs and tubes surrounding myofibrils of a muscle fiber, comparable to endoplasmic reticulum; functions to reabsorb calcium ions during relaxation and to release them to cause contraction

troponin

a regulatory protein that consists of three globular subunits—one that binds to tropomyosin, one that binds to actin, and one that has binding sites for calcium ions (Ca2+); when bound to Ca2+, conformational change moves tropomyosin away from the myosin-binding sites on actin, allowing for muscle contraction

tropomyosin

a rod-shaped regulatory protein that joins with others to form two long strands that wrap around the F actin double helix; covers the myosin-binding sites on actin in a relaxed muscle

aerobic respiration

a series of oxygen-requiring reactions that produce ATP, carbon dioxide, water, and heat; includes the reactions of the Krebs cycle and the electron transport chain

creatine

a small, amino acid-like molecule that is synthesized in the liver, kidneys, and pancreas and then transported to muscle fibers

fused (complete) tetanus

a smooth, sustained contraction in which individual twitches cannot be detected and maximum tension is reached (do not confuse with the disease tetanus!)

pacemaker potential

a spontaneous depolarization that always reaches threshold and therefore triggers the production of an action potential; after repolarization, the ________________ _________________ starts to develop again and the cycle repeats. the ________________ _________________ in an autorhythmic smooth muscle fiber is caused by either an increase in Ca2+ movement into the cell or a decrease in K+ movement out of the cell

smooth muscle tone

a state of continued partial contraction that can sustain long-term tone

flaccid

a state of limpness in which muscle tone is lost when the motor neurons serving a skeletal muscle are damaged or cut

α-actinin

a structural protein in the Z disc; binds actin molecules of the thin filament to titin

myomesin

a structural protein that forms the M line of sarcomere; binds to titin molecules and connects adjacent thick filaments to one another

muscle tone

a sustained, partial contraction of portions of a skeletal or smooth muscle in response to activation of stretch receptors or a baseline level of action potentials in the innervating motor neurons

neuromuscular junction (NMJ)

a synapse between the axon terminals of a motor neuron and the sarcolemma of a muscle fiber

titin

a type of structural protein that connects a Z disc to the M line of the sarcomere, helping stabilize the position of the thick filament; accounts for much of the extensibility and elasticity of myofibrils (probably helps the sarcomere return to its resting length after a muscle has contracted or been stretched, may help prevent overextension of sarcomeres, and maintains the central location of the A bands)

contractile proteins

actin and myosin; main components of thick and thin filaments, generate force during contraction

end plate potential (EPP)

action potential generated at the muscle membrane

oxygen debt

added oxygen, over and above the resting oxygen consumption, that is taken into the body after exercise; important for converting lactic acid to glycogen, tissue repair, resynthesizing creatine phosphate

contraction cycle step 3 (power stroke)

after a crossbridge forms, the myosin head pivots, changing its position from a 90° angle to a 45° angle relative to the thick and thin filaments; it pulls the thin filament past the thick filament toward the center of the sarcomere, generating tension (force) in the process. this event is known as the power stroke, the energy required for the power stroke is derived from the energy stored in the myosin head from the hydrolysis of ATP in step 1; once the power stroke occurs, ADP is released from the myosin head

sarcopenia

age-related progressive muscle wasting

hinge

allows the myosin head to pivot during the contraction process, located where the myosin heads join the myosin tail

terminal cisternae (of SR)

also known as lateral sacs, the dilated ends of the sarcoplasmic reticulum found on either side of a t-tubule; important in calcium release for muscle contraction

striations

alternating dark and light bands found on skeletal and cardiac muscle (caused by alternating dark A bands and light I bands)

IP3 (inositol trisphosphate)-gated channels

although some neurotransmitters and hormones increase the sarcoplasmic Ca2+ concentration in smooth muscle by opening receptor-activated channels, others can increase the sarcoplasmic Ca2+ concentration by activating a second messenger pathway that opens these channels. During this process, the neurotransmitter or hormone binds to a G protein-coupled receptor that activates the enzyme phospholipase C, which in turn causes the production of the second messengers, inositol trisphosphate (IP3) and diacylglycerol (DAG). Once IP3 is generated, it binds to and opens these channels in the SR membrane, causing the release of Ca2+ ions from the SR into the sarcoplasm.

hyperplasia

an increase in the number of normal cells (myofibers) in a tissue or organ, increasing its size; can occur in limited types of smooth muscle

muscle fiber (myocyte)

an individual multinucleated muscle cell surrounded by a sheath of connective tissue

contraction cycle step 4 (detachment of myosin from actin)

at the end of the power stroke, the crossbridge remains firmly attached to actin until it binds another molecule of ATP; as ATP binds to the ATP-binding site on the myosin head, the myosin head detaches from actin

muscle

bodily analog to effort in lever system

joint

bodily analog to fulcrum in lever system

bone

bodily analog to lever in lever system

fascicles

bundles of muscle fibers wrapped in connective tissue

calsequestrin

calcium-binding protein within the sarcoplasmic reticulum which aids in storage of intracellular Ca2+

H zone

in the center of each A band, contains thick filaments only

M line

in the middle of the sarcomere, supporting proteins that hold the thick filaments together in the H zone

muscle fatigue

inability of a muscle to maintain its strength of contraction or tension; may be related to insufficient oxygen, depletion of glycogen, and/or lactic acid buildup

hypertonia

increased muscle tone that is expressed as spasticity or rigidity

stress-relaxation response

increased tension initially when stretched but then after a minute or so will release the tension; allows smooth muscle to undergo great changes in length while retaining the ability to contract effectively

length-tension relationship

indicates how the forcefulness of muscle contraction (tension) depends on the length of the sarcomeres within a muscle fiber before contraction begins. at a sarcomere length of about 2.0-2.4 μm, the zone of overlap in each sarcomere is optimal, and the muscle fiber can develop maximum tension (100%). As the sarcomeres of a muscle fiber are overstretched, the zone of overlap shortens, and fewer myosin heads can make contact with thin filaments; the tension the fiber can produce decreases. When a skeletal muscle fiber is stretched to 170% of its optimal length, there is no overlap between the thick and thin filaments; none of the myosin heads can bind to thin filaments, the muscle fiber cannot contract, and tension is zero. As sarcomere lengths become understretched, the tension that can develop again decreases

G actin

individual actin molecules are known as ____________ because they are globular proteins, each contains a myosin-binding site

muscular dystrophy

inherited muscle destroying diseases, characterized by degeneration of muscle fibers (cells), which causes progressive atrophy of the skeletal muscle

promote relaxation

inhibitory regulation of smooth muscle activity

fast oxidative-glycolytic (FOG) fibers

intermediate in diameter between the other two types of fibers, contain large amounts of myoglobin and many blood capillaries and have a dark red appearance. can generate considerable ATP by aerobic respiration, which gives them a moderate resistance to fatigue, their intracellular glycogen level is high so they also generate ATP by anaerobic glycolysis. fast because the ATPase in their myosin heads hydrolyzes ATP three to five times faster than the myosin ATPase in SO fibers, which makes their speed of contraction faster; twitches reach peak tension more quickly than those of SO fibers but are briefer in duration. contribute to activities such as walking and sprinting

intercalated discs

irregular transverse thickenings of the sarcolemma that connect the ends of cardiac muscle fibers to one another; they contain desmosomes, which hold the fibers together, and gap junctions, which allow muscle action potentials to spread from one cardiac muscle fiber to another

fast glycolytic (FG) fibers

largest in diameter and contain the most myofibrils, can generate the most powerful contractions. have low myoglobin content, relatively few blood capillaries, few mitochondria, and appear white in color. contain large amounts of glycogen and generate ATP mainly by anaerobic glycolysis. due to their large size and their ability to hydrolyze ATP rapidly, they contract strongly and quickly. adapted for intense anaerobic movements of short duration, such as weight lifting or throwing a ball, but fatigue quickly

mechanical advantage (lever system)

load is closer to the fulcrum than the effort

mechanical disadvantage (lever system)

load is farther from the fulcrum than the effort

functional syncytium

mass of interconnected muscle fibers that acts as a single, coordinated unit; because cardiac muscle fibers are electrically coupled by gap junctions, when an action potential is generated in a mass of cardiac muscle fibers, it quickly spreads to all of the muscle fibers in that mass and then the muscle fibers contract together

slow oxidative, fast oxidative-glycolytic, fast glycolytic

muscle fiber types

agonist

muscle that makes the movement

antagonist

muscle that produces opposite action at the same joint

antagonistic muscles

muscles that promote opposite actions at the same joint; when one muscle contracts, the other muscle relaxes (if both such muscles contracted at the same time with equal force, there would be no net movement)

smooth muscle

not striated and involuntary, found in hollow organs; fibers are spindle-shaped and have a single, centrally located nucleus

I band

on either side of the A band and contains a Z disc at the center of each, contains thin filaments only

sarcolemma and T tubules

once an action potential is generated in a skeletal muscle fiber, it propagates along the _______________ and ________________ via continuous conduction.

motor end plate

region of the sarcolemma of a muscle fiber that includes acetylcholine (ACh) receptors, which bind ACh released by synaptic end bulbs of somatic motor neurons

calmodulin (CaM)

regulatory protein in the sarcoplasm that is similar in structure to troponin, binds to Ca2+ to activate MLCK

second class lever

resistance is between the fulcrum and the effort (FRE)

-90 mV

resting membrane potential of a skeletal muscle fiber is approximately __________.

sphincter

ringlike bands of smooth muscle which prevent outflow of the contents of a hollow organ

caveolae

small pouch-like invaginations of the plasma membrane that contain extracellular Ca2+ that can be used for muscle contraction

transverse (T) tubules

small, cylindrical invaginations of the sarcolemma of striated muscle fibers that conduct muscle action potentials toward the center of the muscle fiber

slow oxidative (SO) fibers

smallest in diameter and are the least powerful type of muscle fibers, appear dark red because they contain large amounts of myoglobin and many blood capillaries. have many large mitochondria, generate ATP mainly by aerobic respiration, which is why they are called oxidative fibers. ATP hydrolysis by myosin head is slow, contraction relatively slow and takes longer to reach peak tension. resistant to fatigue and are capable of prolonged, sustained contractions for many hours, are adapted for maintaining posture and for aerobic, endurance-type activities such as running a marathon

ANS (autonomic nervous system)

smooth muscle is innervated by motor neurons of the ________________.

aerobically and anaerobically

smooth muscle produces ATP through...

receptor-activated channels

some neurotransmitters and hormones open these channels in the sarcolemma of a smooth muscle fiber; examples include ligand-gated channels and channels associated with G protein-coupled receptors. when these channels open, Ca2+ moves into the sarcoplasm from extracellular fluid.

AP in a contractile smooth muscle fiber

spike potential or an action potential with a plateau

autorhythmic fibers (pacemaker cells)

spontaneously generate action potentials, but are unable to contract because they contain essentially no myofibrils

rigor mortis

state of partial contraction of muscles after death due to lack of ATP; myosin heads (crossbridges) remain attached to actin, thus preventing relaxation

cardiac muscle

striated and involuntary, fibers are branched and usually have only one centrally located nucleus, have intercalated discs; muscle tissue of the heart

skeletal muscle

striated and voluntary; muscle that is attached to the bones of the skeleton, provides force that moves the bones

unfused (incomplete) tetanus

sustained but wavering contraction due to only partial relaxation between stimuli (do not confuse with the disease tetanus!)

varicosities

swollen regions of autonomic motor neuron axons found in smooth muscle, points of release of neurotransmitters; receptors are not confined to motor end plate but on the entire surface of the cell

muscular system

system that consists of all of the skeletal muscles of the body

series static elements

tendons are called the __________ ____________ ___________ of a muscle, they are important because they transfer the tension generated by the sliding filaments to bone

isometric contraction

tension increases greatly without a change in muscle length

Ca2+ release channels

the Ca2+ that enters a smooth muscle fiber through L-type voltage-gated Ca2+ channels also serves as trigger Ca2+ that binds to and opens these channels in the membrane of the SR; as a result, more Ca2+ is released into the sarcoplasm from the SR. the process by which extracellular Ca2+ triggers the release of additional Ca2+ from the SR through these channels is called Ca2+ -induced Ca2+ release (CICR). in cardiac muscle, it provides the majority of the Ca2+ needed for contraction since cardiac muscle fibers have a moderately extensive SR with a large intracellular reserve of Ca2+. in smooth muscle, CICR provides only a small amount of the Ca2+ required for contraction because the SR is present in small amounts and therefore has only a small intracellular Ca2+ reserve; most of the Ca2+ needed for contraction in smooth muscle comes from extracellular fluid.

contractility

the ability of cells or parts of cells to actively generate force to undergo shortening for movements

autorhythmicity

the ability to repeatedly generate spontaneous action potentials

origin

the attachment of a muscle's tendon to the more stationary bone (remains relatively fixed during muscle contraction)

insertion

the attachment of the muscle's other tendon to the more movable bone

twitch

the brief contraction of a group of muscle fibers within a muscle in response to a single action potential

sarcolemma

the cell membrane of a muscle fiber, especially of a skeletal muscle fiber

myosin

the contractile protein that makes up the thick filaments of muscle fibers, functions as a motor protein in all three types of muscle tissue; consists of six polypeptide chains: two large heavy chains and four small light chains

contraction cycle step 2 (attachment of myosin to actin)

the energized myosin head attaches to the myosin-binding site on actin and releases the previously hydrolyzed phosphate group; when a myosin head attaches to actin during the contraction cycle, the myosin head is referred to as a crossbridge (although a single myosin molecule has a double head, only one head binds to actin at a time)

contraction cycle step 1 (ATP hydrolysis)

the energy generated from hydrolysis of ATP to ADP is stored in the myosin head for later use during the contraction cycle (myosin head is said to be energized when it contains stored energy); the energized myosin head assumes a "cocked" position, like a stretched spring, it is perpendicular (90°) relative to the thick and thin filaments and has the proper orientation to bind to an actin molecule (products of ATP hydrolysis—ADP and a phosphate group—are still attached to the myosin head)

Ca2+-ATPase pump

the membrane of the sarcoplasmic reticulum (SR) contains active transport proteins called ___________________ that constantly transport Ca2+ from the sarcoplasm into the SR

myoglobin

the oxygen-binding, iron-containing protein present in the sarcoplasm of muscle fibers; contributes the red color to muscle

Ca2+ induced Ca2+ release (CICR)

the process by which extracellular Ca2+ triggers the release of additional Ca2+ from the SR

glycolysis

the process of breaking down a glucose molecule into two molecules of pyruvic acid (net gain of 2 ATP)

motor unit recruitment

the process of increasing the number of active motor units when a muscle needs to generate more force during a contraction, smallest motor units are recruited first, with progressively larger motor units added if the task requires more force; one factor responsible for producing smooth movements rather than a series of jerks

myogram

the record or tracing produced by a myograph, an apparatus that measures and records the force of muscular contractions

contraction cycle

the repeating sequence of events that causes the filaments to slide, consisting of 4 steps: 1. ATP hydrolysis 2. attachment of myosin to actin 3. power stroke 4. detachment of myosin from actin

mechanically-gated channels

the sarcolemma of a smooth muscle fiber contains these channels that are sensitive to stretch; therefore, when a smooth muscle fiber is stretched, these channels open, allowing extracellular Ca2+ to move into the sarcoplasm.

L-type voltage-gated Ca2+ channels

the sarcolemma of a smooth muscle fiber contains these channels that open in response to membrane depolarization, allowing Ca2+ to move from extracellular fluid into the sarcoplasm. these channels in smooth muscle open in a graded fashion: as the strength of the depolarization increases, more channels open. in smooth muscle fibers that produce action potentials, the strong depolarization associated with the initial phase of the action potential opens a large number of these channels. this allows a large amount of Ca2+ to enter the sarcoplasm, which in turn causes a strong contraction. in smooth muscle fibers that produce only subthreshold depolarizations, only a few of these channels open. this allows just a small amount of Ca2+ to enter the sarcoplasm, resulting in a weak contraction.

single-unit smooth muscle

consists of a small number of autorhythmic fibers (pacemaker cells), which are usually grouped together, and a large number of contractile fibers that contract together as a single unit. also referred to as visceral smooth muscle (because it is found in the walls of viscera); it is autorhythmic and behaves as a functional syncytium; fibers connect to one another by gap junctions, forming a network through which action potentials can spread rapidly to all of the fibers in the syncytium and the fibers contract in unison. graded contractions occur by increasing the amount of Ca2+ that enters the sarcoplasm of the muscle fibers; recruitment of a larger number of muscle fibers to produce graded contractions is not possible because all of the existing fibers in the syncytium contract at the same time; therefore, no more fibers can be added to increase the amount of tension generated. ex. visceral muscle, intestine, uterus, urinary bladder and blood vessels

multi-unit smooth muscle

consists only of contractile fibers that act independently of each other as multiple units, gap junctions are rare; fibers must be stimulated individually by nerves to contract. graded contractions occur by recruitment of additional muscle fibers; has a richer supply of ANS nerve endings ex. pupillary muscles, airways, iris, ciliary body, hair arrector pili

myosin heads

contain the light chains and the remaining portions of the heavy chains; each has two binding sites: an actin-binding site and an ATP-binding site (ATP-binding site also functions as an ATPase)

myofibrils

contractile elements within muscle fibers, containing overlapping thick and thin filaments

graded contractions

contractions that vary in strength depending on how much force is needed by the muscle to support a particular object

slow-wave potential

cycles of alternating depolarization and repolarization that do not necessarily reach threshold; sometimes, threshold is reached and an action potential is generated; on other occasions, threshold is not reached and an action potential does not occur. the underlying mechanism is thought to involve fluctuations in Na+ movement out of the cell caused by periodic changes in the activity of Na+/ K+ pumps

sarcoplasm

cytoplasm of a muscle fiber

dense bodies

cytoplasmic structure to which thin filaments of a smooth muscle fiber are anchored, functionally similar to Z discs in striated muscle fibers

zone of overlap

dark region where thick and thin filaments overlap, toward ends of A band

A band

dark, middle part of sarcomere that extends entire length of thick filaments and includes those parts of thin filaments that overlap thick filaments

hypotonia

decreased or lost muscle tone in which muscles appear flaccid

myosin phosphatase

dephosphorylates myosin heads

action potential with a plateau

depolarizing phase: opening of L-type voltage-gated Ca2+ channels plateau phase: prolonged opening of L-type voltage-gated Ca2+ channels, along with the partial opening of voltage-gated K+ channels repolarizing phase: closure of L-type voltage-gated Ca2+ channels and the opening of voltage-gated K+ channels

spike potential

depolarizing phase: opening of L-type voltage-gated Ca2+ channels repolarizing phase: closure of L-type voltage-gated Ca2+ channels and the opening of voltage-gated K+ channels

cachexia

disease-associated muscle wasting

third class lever

effort is between the fulcrum and the resistance (FER)

myoblast

embryonic cell that develops into a cell of muscle fiber

hypertrophy

enlargement of existing cells (increase in size of cells and increase in number of myofibrils); occurs in skeletal, cardiac, and smooth muscle, can help repair damaged tissue

promote contraction

excitatory regulation of smooth muscle activity

creatine phosphate (CP)

first source of ATP during muscle contraction, an energy-rich molecule found in myofibers made by creatine kinase

excitability, contractility, extensibility, elasticity

four major properties of muscle

first class lever

fulcrum is between the effort and resistance (EFR)

myostatin

gene controlling musculature, blocks muscle development

contractile fibers

have the necessary myofibrils to contract, but do not have the ability to initiate action potentials; they become excited and then contract together in response to action potentials conducted to them from autorhythmic fibers via gap junctions

rigidity

hypertonia characterized by increased muscle tone, but reflexes are not affected

spasticity

hypertonia characterized by increased muscle tone, increased tendon reflexes, and pathological reflexes (Babinski sign)

excitation-contraction (EC) coupling

the sequence of events that links the muscle action potential to muscle contraction and occurs at the triads of the skeletal muscle fiber. during relaxation, the level of Ca2+ in the sarcoplasm is low because DHP receptors in the T tubule membrane block Ca2+ release channels in the terminal cisternal membrane of the SR. when the Ca2+ concentration in the sarcoplasm is low, tropomyosin covers the myosin-binding sites on actin, preventing myosin and actin from interacting. a muscle potential propagating along a T tubule causes the DHP receptors to undergo a conformational change that leads to the opening of the Ca2+ release channels; Ca2+ is released from the SR into the sarcoplasm. the released calcium ions bind to troponin, which in turn undergoes a conformational change that causes tropomyosin to move away from the myosin-binding sites on actin; myosin molecules subsequently bind to actin and the contraction cycle begins

isotonic contraction

the tension developed by the muscle remains almost constant while the muscle changes its length; can be concentric or eccentric

concentric (isotonic contraction)

the tension generated is great enough to exceed the load and the muscle shortens, pulling on another structure (such as a tendon) to produce movement; the load and the velocity of muscle shortening are inversely related.

refractory period

the time following an action potential during which a new action potential cannot be initiated

action

the type of movement that occurs when the muscle contracts (e.g. flexion, extension)

troponin

thin filaments in smooth muscle lack _______________.

latent period, contraction period, relaxation period

three phases of a twitch

skeletal, cardiac, smooth

three types of muscle tissue

structural proteins

titin, α-actinin, myomesin, nebulin and dystrophin; keep the thick and thin filaments in the proper alignment, give the myofibril extensibility and elasticity, and link the myofibrils to the sarcolemma and extracellular matrix

whole muscle tension

total force a whole muscle can produce; depends on the number of muscle fibers that are contracting in unison, which is determined by the size and number of motor units that are activated

muscle fiber tension

total force that a single muscle fiber can produce; depends on the frequency of stimulation, the length of the muscle fiber before contraction begins, and the diameter of the muscle fiber

regulatory proteins

troponin and tropomyosin; help switch the contraction process on and off

smooth, cardiac, skeletal

types of muscle fibers in order of amount of sarcoplasmic reticulum (smallest to largest)

smooth, cardiac, skeletal

types of muscle fibers in order of fiber diameter (smallest to largest)

smooth, cardiac, skeletal

types of muscle fibers in order of speed of contraction (slowest to fastest)

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

types of skeletal muscle fibers in order of amount of creatine kinase (lowest to highest)

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

types of skeletal muscle fibers in order of capacity for generating ATP (lowest to highest)

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

types of skeletal muscle fibers in order of fatigue resistance (lowest to highest)

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

types of skeletal muscle fibers in order of fiber diameter (smallest to largest)

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

types of skeletal muscle fibers in order of glycogen stores (lowest to highest)

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

types of skeletal muscle fibers in order of recruitment (first to last)

crossbridge

when a myosin head attaches to actin during the contraction cycle, the myosin head is referred to as a _________________.

store-operated channels

when the intracellular reserves of Ca2+ in the SR are depleted, a signal is relayed from the SR to the sarcolemma, where it causes these channels to open. opening these channels allows Ca2+ to enter the sarcoplasm from extracellular fluid. The entering Ca2+ can be used for contraction or to replenish the depleted Ca2+ stores in the SR.

eccentric (isotonic contraction)

when the length of a muscle increases during a contraction; the tension exerted by the myosin crossbridges resists movement of a load and slows the lengthening process

asynchronous recruitment

while some motor units are contracting, others are relaxed; this pattern of motor unit activity delays muscle fatigue and allows contraction of a whole muscle to be sustained for long periods

tendon

white fibrous cord of connective tissue that attaches muscle to bone

long refractory period

prevents wave summation and tetanic contractions in cardiac muscle

anaerobic glycolysis

process by which the breakdown of glucose gives rise to lactic acid when oxygen is absent or at a low concentration; each molecule of glucose catabolized via this method yields two molecules of lactic acid and two molecules of ATP

lactic acid

product of anaerobic glycolysis, accumulates in active skeletal muscle fibers and in the bloodstream

thermogenesis

production of heat through muscle contraction

troponin I and troponin T

proteins found exclusively in cardiac muscle and released during myocardial damage (e.g. heart attack)

motor proteins

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

electromyography (EMG)

recording the strength of muscle contraction as a result of electrical stimulation

creatine phosphate, anaerobic glycolysis, aerobic respiration

ATP production methods of skeletal muscle fibers

aerobically

cardiac muscle produces ATP almost exclusively through...

creatine kinase (CK)

catalyzes the transfer of one of the high-energy phosphate groups from ATP to creatine, forming creatine phosphate and ADP

tetanus

caused by a toxin (tetanospasmin) produced by endospores of the bacterium Clostridium tetani; tetanospasmin can act at motor end plates, in the spinal cord, or in the brain to affect the signals controlling muscle contractions, producing the commonly seen body contorting spasms

satellite cells

cells that retain the capacity to fuse with one another or with damaged muscle fibers to regenerate functional muscle fibers (forming myoblasts, then a myotube)

voltage-gated, Ca2+ release, receptor-activated, IP3-gated, store-operated, and mechanically-gated

channels that allow for an increase in sarcoplasmic Ca2+ concentration

repolarizing phase (of skeletal muscle AP)

closure of the voltage-gated Na+ channels and the opening of voltage-gated K+ channels

myosin tail

composed of portions of the heavy chains that wrap around each other to form a double-stranded helix

sliding filament mechanism

myosin heads attach to and "walk" along the thin filaments at both ends of a sarcomere, progressively pulling the thin filaments toward the M line, the thin filaments slide inward and meet at the center of a sarcomere; as the thin filaments slide inward, the I band and H zone narrow and eventually disappear altogether when the muscle is maximally contracted. however, the width of the A band and the individual lengths of the thick and thin filaments remain unchanged. since the thin filaments on each side of the sarcomere are attached to Z discs, when the thin filaments slide inward, the Z discs come closer together and the sarcomere shortens; shortening of the sarcomeres causes shortening of the whole muscle fiber, which in turn leads to shortening of the entire muscle

Z discs

narrow, plate-shaped regions of dense material that separate one sarcomere from the next

depolarizing phase (of skeletal muscle AP)

opening of voltage-gated Na+ channels

AP in an autorhythmic smooth muscle fiber

pacemaker potential or slow-wave potential depolarizing phase: opening of L-type voltage-gated Ca2+ channels repolarizing phase: closure of the L-type voltage-gated Ca2+ channels and opening of voltage-gated K+ channels

wave summation

phenomenon in which stimuli arriving at different times cause contractions with greater tension; i.e. if a second stimulus occurs in a skeletal muscle fiber after the refractory period of the first stimulus is over but before the muscle fiber has relaxed, the second contraction will actually be stronger than the first