when stimulated by chemical signals, stretch, and electrical changes across the plasma membrane
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What is conductivity?
local electrical excitation sets off a wave of excitation that travels along the muscle fiber
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What is contractility?
shortens when stimulated
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What is extensibility?
capable of being stretched between contractions
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What is elasticity?
returns to its original rest length after being stretched
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What are skeletal muscles?
voluntary, striated muscle usually attached to bones
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what is voluntary?
usually subject to conscious control
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How long are myofibers?
as long as 30 cm
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what are the connective tissue wrappings?
endomysium, perimysium, and epimysium
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What is the endomysium?
connective tissues around muscle cell
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what is the perimysium?
connective tissue around the muscle fascicle
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what is the epimysium?
connective tissue around the entire muscle
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what are tendons?
* attachments between muscle and bone * continuous with collagen fibers of tendons
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what is collagen?
* somewhat extensible and elastic * stretches slightly under tension and recoils when released
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what does collagen do?
* resists excessive stretching and protects muscle from injury * returns muscle to its resting length * contributes to power output and muscle efficiency
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what is the sarcolemma?
plasma membrane of a muscle fiber
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what is the sarcoplasm?
cytoplasm of a muscle fiber
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what are myofibrils?
long protein cords occupying most of sarcoplasm
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what is glycogen?
carbohydrate stored to provide energy for exercise
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what is myoglobin?
red pigment; provides some oxygen needed for muscle activity
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what are multiple nuclei?
flattened muscles pressed against the inside of the sarcolemma
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what are myoblasts?
Stem cells that fused to form each muscle fiber early in development; muscle building cells
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what are satellite cells?
* unspecialized myoblasts remaining between the muscle fiber and endomysium * play a role in regeneration of damaged skeletal muscle tissue
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where are the mitochondria?
packed into spaces between myofibrils
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what is the sarcoplasmic reticulum?
* smooth ER that forms a network around each myofibril * acts as a calcium reservoir; releases calcium through channels to activate contraction
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what is the terminal cisterns?
* dilated end-sacs of SR which cross the muscle fiber form one side to the other
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what are t tubules?
tubular infolding of the sarcolemma which penetrate through the cell and emerge on the other side
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what is a triad?
a t tubule and two terminal cisterns associated with it
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what are thick filaments?
* made up of several hundred myosin molecules * each molecule is shaped like gold club * two chains intertwined to form a shaft-like tail * double globular head
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what are thin filaments?
* Fibrous (F) actin: two intertwined strands * string of globular (G) actin subunits each with an active site that can bind to head of myosin molecule * tropomyosin * when relaxed, each blocking six or seven active sites on G actin subunits * Troponin molecule: small, calcium-binding protein on each troponin molecule
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what are elastic filaments?
* titin: huge, springy protein that makes elastic filament * run through cord of thick filament and anchor it to z disc and M line * help stabilize and position the thick filament * prevent overstretching and provide recoil
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what are contractile proteins?
myosin and actin do the work of contraction
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what are regulatory proteins?
* tropomyosin abd troponin * act like a switch that determines when fiber can (and cannot) contract * contraction activated by release of calcium into scar plasm and its binding to troponin * troponin changes shape and move tropomyosin off the active sites on actin
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what is dystrophin?
* clinically important protein * links actin in outermost myofilamnets to membrane proteins that link to endomysium * transfers forces of muscle contraction to connective tissue ultimately leading to tendon * genetic defects in dystrophin produce disabling disease muscular dystrophy
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what are striations?
* result from the precise organization of myosin and actin in cardiac and skeletal muscle cells * alternate A-bands (dark) and I-bands (light)
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what are A bands?
* darkest parts is where thick filaments overlap a hexagonal array of think filaments
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where is the H band?
* middle of A band * thick filaments only * not as dark
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where is the M line?
middle of H band
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what is the I band?
* the way bands reflect polarized light * Z disc: provides anchorage for thin filaments and elastic filaments * z disc bisects I band and “zig-zags”
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where is the sarcomere?
* segments from z disc to z disc * functional contractile unit of muscle fiber
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why do muscle cells shorten?
their individual sarcomeres shorten
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what changes during muscle shortening?
only the amount of overlap changes
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what pulls on extrcellu.ar proteins during shortening?
* dystrophin and linking proteins * transfers pull to extracellular tissue
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what is the order of skeletal muscles form biggest to smallest?
what happens when nerve connections or severed or poisoned?
muscle is paralyzed
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what is denervation atrophy?
shrinkage of paralyzed muscle when nerve remains disconnected
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what are somatic motor neurons?
nerve cells whose cell bodies are in the brainstem and spinal cord
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what are somatic motor fibers?
* axons lead to the skeletal muscle * each nerve fiber branches out to a number of muscle fibers * each muscle fiber is supplied by only one motor neuron
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what is a motor unit?
one nerve fiber and all the muscle fibers innervated by it
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what does a motor unit do?
* dispersed throughout muscle * contact in unison * produce was contraction over wide area * provide ability to sustain long-term contraction as motor units take turns contracting * effective contraction usually requires contraction of several motor units at once
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what are small motor units?
* control degree of control * three to six muscle fibers per neuron * eye and hand muscles
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what are large motor units?
* control more strength then control * powerful contractions supplied by large motor units with hundreds of fibers
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what is a synapse?
point where a nerve fiber meets its target cell
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what is the neuromuscular junction?
* when target cell is a muscle fiber * each terminal branch of the nerve fiber within the NMJ forms separate synapse with the muscle fiber
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what is a axon terminal?
* the swollen end of nerve fiber * contains synaptic vesicles with acetylcholine (ACh)
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what is a synaptic cleft?
gap between axon terminal and sarcolemma
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what does a Schwann cell do?
envelop and isolated NMJ
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what do nerve impulse cause?
synaptic vesicles to undergo exocytosis releasing ACh into synaptic cleft
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what happens during an unstimulated (resting) cell?
* the more anions on the inside of the membrane than on the outside * anions make the inside of the plasma membrane negatively charged then to its outer surface * the plasma membrane is electrically polarized with a negative resting membrane potential * there are excess sodium ions in the extracellular fluid * there are excess potassium ions in the intracellular fluid
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what happens during stimulated (active) muscle?
* sodium gates open in the plasma membrane * sodium flows into the cell downs its electrochemical gradient * these cations override the negative charges in the ICF * depolarization: inside of the plasma membrane becomes positive * immediately potassium gates open and potassium rushes out of the cell partly repelled by positive sodium charge and partly because of its concentration * loss of positive potassium ions turns the membrane negative again (repolarization)
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what does an action potential do?
* one point causes another one to happen immediately in front of it, which triggers another one a little further along and so forth * this wave of excitation is called an impulse
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what causes muscles to become paralyzed?
* toxins interfering with synaptic function * cholinesterase inhibitors bind to acetylcholinesterase and prevent it form degrading ACh
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what is spastic paralysis?
state of continual contraction of the muscles; possible suffocation
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what is tetanus?
* form of spastic paralysis caused by toxin clostridium tetani * glycine in the spinal cord normally stops motor neurons form producing unwanted muscle contractions * tents toxin blocks glycine release in the spinal cord and causes overstimulation and spastic paralysis of the muscles
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what is flaccid paralysis?
state in which the muscles are limp and cannot contract
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what is curare?
competes with ACh for receptor sites, but does not stimulate the muscles
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what is botulism?
* type of food poisoning caused by a neuromuscular toxin * blocks release of ACh causing flaccid paralysis * botox cosmetic injections used for wrinkle removal
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what are the four major phases of contraction and relaxation?
process un which nerve action potentials lead to muscle action potentials
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what is excitation-contraction coupling?
events that link the action potentials on the sarcolemma to activation of the myofilaments, thereby preparing them to contract
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what is contraction?
step in which muscle fiber develops tension and may shorten
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what is relaxation?
when stimulation ends, a muscle fiber relaxes and returns to its resting length
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what is length-tension relationship?
the amount of tension generated by a muscle depends on how stretched or shorten it was before it was stimulated
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what happens if a muscle is overly shortened before stimulated?
a weak contraction results, as thick filaments just butt against z discs
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what happens when a muscle is too stretched before stimulated?
* a weak contractions results, as the minimal overlap between thick and thin filaments, results in minimal cross-bridge formation * optimum resting length produces greatest force when muscle contracts
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what is rigor mortis?
* hardening of muscles and stiffening of the body beginning 3 to 4 hours after death * muscle relaxation requires ATP, and ATP production is no longer produced after death * rigor mortis peaks about 12 hours after death, then diminishes over the next 48 to 60 hours
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how does rigor mortis occur?
* deteriorating sarcoplasmic reticulum releases calcium * deteriorating activates sarcolemma allows calcium to enter cytosol * calcium activates myosin-actin cross-bridging * muscle contracts, but cannot relax
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what is a myogram?
a chart of the timing and strength of a muscle’s contraction
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what is the muscle threshold?
minimum voltage necessary to generate an action potential in the muscle fiber and produce a contraction
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what is a twitch?
a quick cycle of contraction and relaxation when stimulus is at threshold or higher
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what is the latent period?
* very brief delay between stimulus and contraction * time required for excitation, excitation-contraction coupling, and tensing of elastic components of muscle
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what is the contraction phase?
* time when muscle generate external tension * force generated can overcome the load and cause movement
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what is the relation phase?
* time when tension declines to baseline * SR reabsorbs calcium, myosin releases actin and tension decreases * takes longer than contraction
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why do different stimuli cause twitches varying in strength?
* the muscles starting length influences tension generation * muscles fatigue after continual use * warmer muscles’ enzymes work more quickly * muscle cell’s hydration level influences cross-bridge formation
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what are the different contraction strength of twitches?
* with subthreshold stimuli- no contraction at all * at threshold intensity and above- twitch produced
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what do higher voltages create?
* cites more nerve fibers which stimulate more motor units to contract * more motor units come play with stronger stimuli
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what is the muscle size principle?
weak stimuli recruit small units, while strong stimuli recruit small and large units for powerful movements
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what does a low frequency create?
produce identical twitches
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what does a higher frequency create?
produce temporal (wave) summation
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what does a unnaturally high stimulus frequencies create?
cause a steady, contraction called complete (fused) tetanus (only created in lab experiments)
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what is isometric muscle contraction (“same length”)?
* muscle produces internal tension but external resistance causes it to stay the same length * important in postural muscle function and antagonistic muscle joint stabilization
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what is a isotonic muscle contraction (“same tension”)?
* muscle changes in length with not change in tension * creates concentric and eccentric contraction
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what is concentric contraction?
muscle shortens as it maintains tension
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what is eccentric contraction?
muscle lengthens as it maintains tension
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what happens at the beginning of a contraction?
* isometric phase * muscle tension rises but muscle does not shorten
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what happens when tension overcomes resistance of the load?
* tension stopes increasing * muscle begins to shorten and move the load (isotonic phase)
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what does ATP supply depend on?
oxygen and organic energy sources (i.e. glucose and fatty acids)
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what are the two main pathways of ATP synthesis?
* anaerobic fermentation * aerobic respiration
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what is anaerobic fermentation?
* enables cells to produce ATP in the absence of oxygen * yields little ATP and lactate, which needs to be disposed of by the liver