chapter 11 skeletal muscle physiology

excitability (responsiveness)

to chemical signals, stretch, and electrical changes across the plasma membrane

conductivity

local electrical excitation sets off a wave of excitation that travels along the muscle fiber

contractility

shortens when stimulated

extensibility

capable of being stretched between contractions

elasticity

returns to its original rest length after being stretched

skeletal muscle

voluntary, striated muscle usually attached to bones

striations

alternating light and dark transverse bands

voluntary

usually subject to conscious control

endomysium

connective tissue around muscle cell

perimysium

connective tissue around muscle fascicle

epimysium

connective tissue surrounding entire muscle

tendons

attachments between muscle and bone matrix

continuous with collagen fibers

collagen

extensible and elastic

stretches slightly under tension and recoils when released, contributes to power output and muscle efficiency

sarcolemma

plasma membrane of a muscle fiber

sarcoplasm

cytoplasm of a muscle fiber

myofibril

long protein cords occupying most of sarcoplasm

glycogen

carbohydrate stored to provide energy for exercise

myoglobin

red pigment, provides some oxygen needed for muscle activity

myoblasts

stem cells that fused to form each muscle fiber early in development

satellite cells

unspecialized myoblasts remaining between the muscle fiber and endomysium

\*plays role in regeneration of damaged skeletal muscle tissue

sarcoplasmic reticulum

smooth ER that forms a network around each myofibril

\*acts as calcium reservoir

terminal cistern

dilated end sacs of sarcoplasmic reticulum which cross the muscle fiber from one side to the other

t tubules

tubular infoldings of the sarcolemma which penetrate through the cell and emerge on the other side

triad

a t tubule and two terminal cisterns associated with it

thick filament

made of myosin molecules

tails linked together by hydrogen bonds

makes dark bands

thin filament

made of actin molecule

has tropomyosin and troponin

tropomyosin

each blocking six or seven active sites on G actin subunit

troponin

small, calcium binding protein on each tropomyosin molecule

\*binds to calcium will change shape that pulls on tropomyosin

elastic filament

run through core of thick filament and anchor it to Z disc and M line

helps stabilize and prevent overstretching

titin

huge, springy protein that makes elastic filament

contractile proteins

myosin and actin

regulatory proteins

tropomyosin and troponin

dystrophin

links actin in outermost myofilament to membrane proteins that link to endomysium

forces muscle contraction to connective tissue

\*doesn’t work causes muscular dystrophy

A band

darkest part is where thick filaments overlap a hexagonal array of thin filament

H band

not as dark, middle of A band, thick filament only

M line

middle of H band

I band

light band, the way the band reflects polarized light

Z disc

provides anchorage for thin filaments and elastic filament

sarcomere

segment from z disc to z disc

\*functional and contractile unit of muscle fiber

denervation atrophy

shrinkage of paralyzed muscle when nerve remains disconnected

somatic motor neurons

nerve cells whose cell bodies are in the brainstem and spinal cord that serve skeletal muscle

somatic motor fibers

axons that lead to the skeletal muscle

each nerve fiber branches out to bunch of muscle fibers

each muscle fiber supplied by one motor neuron

synapse

point where a nerve fiber meets its target cell

neuromuscular junction

when target cell is a muscle fiber

axon terminal

swollen end of nerve fiber

\*acetylcholine neurotransmitter

synaptic cleft

gap between axon terminal and sarcolemma

schwann cell

envelops and isolates neuromuscular junction

voltage (electrical potential)

a difference in electrical change from one point to another

resting membrane potential

about -90mV in skeletal muscle cells

\*maintained by sodium potassium pumps

where is the -90 mV concentration gradient

inside the cell

depolarization

inside of the plasma membrane becomes positive

what ion goes into the cell

3 sodium

what ion goes out of the cell

2 potassium

repolarization

loss of positive potassium ions turns the membrane negative again

impulse

wave of excitation

what does cholinesterase inhibitor do

binds to acetylcholinesterase and prevent it from degrading ACh

spastic paralysis

a state of continual contraction of the muscles, possible suffocation

tetanus

glycine in the spinal cord normally stops motor neurons from producing unwanted muscle contractions but tetanus toxin blocks glycine release and causes overstimulation

flaccid paralysis

a state in which the muscles are limp and cannot contract

curare

competes with ACh for receptor sites, but does not stimulate the muscles

botulism

type of food poisoning

blocks release of ACh causing flaccid paralysis

botox

cosmetic injections used for wrinkle removal

excitation

process in which nerve action potentials lead to muscle action potentials

excitation contraction coupling

events that link the action potentials on the sarcolemma activation of the myofilaments, thereby preparing them to contract

contraction

step in which the muscle fiber develops tension and may shorten

relaxation

when stimulation ends, a muscle fiber relaxes and returns to its resting length

length tension relationship

the amount of tension generated by a muscle depends on how stretched or shortened it was before it was stimulated

rigor mortis

hardening of muscles and stiffening of body beginning 3 to 4 hours after death

\*muscle relaxation requires ATP and ATP production is no longer produced after death

myogram

a chart of the timing and strength of a muscles contraction

threshold

minimum voltage necessary to generate an action potential in the muscle fiber and produce a contraction

twitch

a quick cycle of contraction and relaxation when stimulus is at threshold or higher

latent period

very brief delay between stimulus and contraction

contraction phase

time when muscle generates external tension

force can overcome the load and cause movement

relaxation phase

time when tension declines to baseline

SR reabsorbs Ca myosin releases actin and tension decreases, takes longer than contraction

example of slow twitch activity

marathon

example of fast twitch activity

sprint

what influences twitch strength

muscles starting length

muscles fatigue

warm muscles enzymes work quicker

hydration level influences cross bridges

size principle

weak stimuli recruit small units, while strong stimuli recruit small and large units for powerful movements

low frequency

stimuli produce identical twitches

high frequency

stimuli produce temporal summation

each new wave rides on the previous one

unnaturally high stimulus frequency

\*in lab experiments

cause a steady, contraction called complete (fused) tetanus

isometric muscle contractions

produces internal tension but external resistance causes it to stay the same length

\*does not change length

isotonic muscle contraction

muscle changes in length with no change in tension

concentric contraction

muscle shortens as it maintains tension

eccentric contraction

muscle lengthens as it maintains tension

anaerobic fermentation

enables cells to produce ATP in the absence of oxygen

yields little ATP and lactate, needs to be disposed of by the liver

aerobic respiration

produces more ATP, does not generate lactate, requires supply of oxygen

myokinase

transfers inorganic phosphate from one ADP to another, making ATP

creatine kinase

obtains inorganic phosphate from a phosphate storage molecule creatine phosphate and gives it to ADP to make ATP

phosphagen system

the combination of ATP and CP which provides nearly all energy for short bursts of activity

anaerobic threshold

point at which lactate becomes detectable in the blood

glycogen lactate system

the pathway from glycogen to lactate

muscle fatigue

progressive weakness from prolonged use of muscles

potassium accumulation

in the t tubules reduces excitability

excess ADP and inorganic phosphate

slow cross bridge movements, inhibits calcium release and decreases force production in myofibrils

fuel depletion

glycogen and glucose levels decline, electrolyte loss through sweat can decrease muscle excitability

central fatigue

when less motor signals are issued from brain

VO2 max

the point at which the rate of oxygen consumption plateaus and does not increase further with added workload

excess post exercise oxygen consumption

meets a metabolic demand also known as oxygen debt

difference between the elevated rate of oxygen consumption following exercise and the usual resting rate

slow twitch, slow oxidative, type I fibers

endurance, stores lots of O2 and carbs

abundant mitochondria, capillaries, myoglobin \*deep red color

thin fibers, slow ATPase, releases calcium slowly, small motor units