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