muscles!!

prefixes for muscle

myo, mys, sarco

elongated cells of muscle

muscle fibers

which type of muscle requires nervous system stimulation

skeletal

which types of muscle do not require nervous system stimulation

cardiac and smooth

excitability

ability to receive and respond to stimuli

contractility

ability to shorten forcibly without stimulation

extensibility

ability to be stretched

elasticity

ability to recoil to resting length

4 major functions of muscle

movement of bones or fluids

stabilizing joints

maintaining posture and body position

heat generation (esp skeletal muscle)

what is each skeletal muscle served by

one artery, one nerve, one or more veins

function of nerve ending in skeletal muscle

controls its activity

function of connective tissue sheaths of skeletal muscle

support cells, reinforce muscle

epimysium

dense irregular connective tissue surrounding entire muscle, may blend in with fascia

perimysium

fibrous connective tissue surrounding fasicles

endomysium

fine areolar connective tissue surrounding each muscle fiber

fasicles

groups of muscle fibers

insertion (type of attachment)

attachment to movable bone

origin (type of attachment)

attachment to immovable, or less movable bone

direct attachment

epimysium fused to periosteum of bone or perichondrium of cartilage

indirect attachment

connective tissue wrappings extend beyond muscle as ropelike tendon or sheetlike aponeuroses

sarcolemma

plasma membrane of a muscle fiber

structural characteristics of a muscle fiber

long, cylindrical cell, multiple peripheral nuclei, sarcolemma, sarcoplasm, myofibrils, sarcoplasmic reticulum, T tubules

sarcoplasm

cytoplasm of muscle fiber

glycosomes

cell in the sarcoplasm for glycogen storage

myoglobin

cell in the sarcoplasm for O2 storage

myofibrils

each strand of a muscle fiber

characteristics of myofibrils

densely packed, rod-like elements, about 80% of cell volume, contain sarcomeres, exhibit striations

H zone

lighter region in the midsection of dark A band where filaments do not overlap

M line

line of protein myomesin bisects H zone

Z disc (line)

coin-shaped sheet of proteins on midline of light I band that anchors thin filaments and connects myofibrils to one another

thick filaments

composed of myosin, run entire length of A band

thin filaments

composed of actin, run length of I band and partway into A band

sarcomere

region between 2 successive Z discs (purple lines), smallest contractile unit of muscle fiber!!

characteristics of sarcomeres

smallest contractile unit of muscle fiber, align along myofibril like boxcars of train, contains A band with ½ I band at each end, composed of thick and thin filaments made of contractile proteins

actin myofilaments

thin filaments, extend across I band and partway in A band, anchored to Z discs

myosin myofilaments

thick filaments, extend length of A band, connected at M line

structure of thick filaments

composed of myosin, composed of 2 heavy and 4 light polypeptide chains, myosin head contain 2 light polypeptide chains that act as cross bridges during contraction, binding sites of actin, ATP, ATPase enzymes

structure of thin filaments

composed of double strand of F actin. (globular actin subunits), G actin has active sites for myosin attachment during contraction

regulatory proteins bound to actin

troponin, tropomyosin

sarcoplasmic reticulum

network of smooth endoplasmic reticulum surrounding each myofibril (run longitudinally), pairs of terminal cisterns form perpendicular cross channels, functions in regulation of intracellular Ca levels (stores and releases Ca)

t tubules

continuations of sarcolemma down into the myofibril

function of t tubules

increase surface area of muscle fiber, penetrate cell’s interior at each A & I band junction, form triads with paired terminal cisterns that encircle each sarcomere

cross bridges

when myosin heads bind to actin

initial steps for muscle contraction

activation and excitation-contraction coupling

location of activation

neuromuscular junction

characteristics of activation

needs nervous system stimulation, must generate action potential in the sarcolemma

characteristics of e-c coupling

action potential propagated along sarcolemma, intracellular Ca levels must rise briefly

neuromuscular junction

joining of an axon ending with a single muscle fiber, situated midway long the length of the muscle fiber

synaptic cleft

gel-filled space separating the axon terminal and muscle fiber

acetylcholine (ACh)

neurotransmitter contained in the synaptic vesicles

location of ACh receptors

junctional folds of sarcolemma

components of the neuromuscular junction

axon terminals, synaptic cleft, junctional folds

3 steps of action potential generation

end plate potential (local depolarization), depolarization, repolarization

end plate potential

local depolarization of sarcolemma

ACh binding opens chemically gated ion channels

diffusion of Na in and K out

more Na diffuses in so sarcolemma becomes less negative

depolarization

generation and propagation of an action potential

critical voltage in depolarization

threshold

steps of depolarization

end plate potential spreads to adjacent membrane areas'

voltage gated Na channels open

Na influx decreases membrane voltage toward threshold

if threshold is reached, action potential initiated - muscle contraction

repolarization

restoring electrical conditions of RMP

steps of repolarization

Na channels close and voltage gated K channels open

K efflux rapidly restores resting polarity

fiber cannot be stimulated - refractory period until repolarization complete

conditions restored by Na - K pump

excitation-contraction (E-C) coupling

events that transmit AP along sarcolemma and lead to sliding of myofilaments, causes rise in intracellular Ca which causes contraction, AP ends before contraction

latent period

time when E-C coupling events occur, time between AP initiation and beginning of contraction

steps of E-C coupling

AP propagates along the sarcolemma and down the T tubules

calcium ions are released

calcium binds to troponin and removes blocking action of tropomyosin

contraction begins

steps of cross bridge cycle

cross bridge formation

the power (working) stroke

cross bridge detachment - ATP pulls actin and myosin apart

cocking of myosin head - returns to its original position

working (power) stroke of cross bridge cycle

myosin head pivots and pulls thin filament toward M line

3 phases of muscle twitch

latent period, period of contraction, period of relaxation

latent period (muscle twitch)

shortest period, events of E-C coupling occur, no muscle tension

period of contraction (muscle twitch)

cross bridge formation, tension increases

period of relaxation (muscle twitch)

longest period, Ca reentry into sarcoplasmic reticulum, tension declines to zero

graded muscle responses

varying strength of contraction for different demands

required for proper control of skeletal movement

what are muscle responses graded by?

changing frequency of stimulation

changing strength of stimulation

wave (temporal) summation

increased stimulus frequency (muscle does not completely relax between stimuli) - 2nd contraction of greater force

produces smooth, continuous contractions

unfused (incomplete) tetanus

further increase in stimulus frequency

fused (complete) tetany

if stimuli are given quickly enough, muscle reaches maximum tension (smooth, sustained contraction)

muscle fatigue

no muscle relaxation, muscle cannot contract, zero tension

recruitment

multiple motor unit summation, controls force of contraction

subthreshold stimuli

results in no observable contractions

threshold stimulus

stimulus strength causing first observable contraction

maximal stimulus

strongest stimulus that increases contractile force, no increase in force of contraction beyond this

size principle

recruitment works on this

motor units with the smallest muscle fibers recruited first, motor units with larger fibers recruited as stimulus intensity increases, largest motor units activated only for most powerful contractions

isometric contraction

no shortening, muscle tension increases but does not exceed load

isotonic contraction

muscle shortens because muscle tension exceeds load

types of isotonic contractions

concentric and eccentric

concentric contractions

muscle shortens and does work

eccentric contractions

muscle generates force as it lengthens (about 50% more forceful)

function of isometric contractions

maintain posture, stabilize joints

characteristics of isometric contractions

occur when load is greater than the tension muscle can develop

tension increases to muscle’s capacity, but muscle neither shortens nor lengthens

cross bridges generate force but do not move actin filaments

muscle tone

constant, slightly contracted state of all muscles - due to spinal reflexes (groups of motor units alternately activated in response to input from stretch)

function of muscle tone

keeps muscles firm, healthy, and ready to respond

function of ATP in contractile activities

move and detach cross bridges, calcium pumps in sarcoplasmic reticulum, return of Na and K after EC coupling

ways ATP is regenerated

direct phosphorylation of ADP by creatine phosphate

anaerobic pathway (glycolysis to lactic acid)

aerobic respiration

only energy source for muscle contraction

ATP

how quickly are ATP stores depleted?

4-6 seconds

anaerobic pathway (ATP regeneration)

glycolysis (doesn’t require oxygen)

when does the anaerobic pathway occur?

at 70% of maximum contractile activity (bulging muscles compress blood vessels, so oxygen delivery is impaired, pyruvic acid is converted to lactic acid)

function of lactic acid

used as fuel by the liver, kidneys, and heart

converted back to pyruvic acid or glucose by the liver

advantages of anaerobic respiration

only yield 5% as much ATP as aerobic respiration, but produces ATP 2 ½ times faster

where does the aerobic pathway occur?

in the mitochondria

characteristics of the aerobic pathway

produces 95% of ATP during rest and light to moderate exercise, slow, requires oxygen, breaks glucose into CO2, H2O, and a large amount of ATP

fuel options for the aerobic pathway

stored glycogen, bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids

what determines the force of contraction?

the number of cross bridges attached, which is affected by the number of muscle fibers stimulated (recruitment), relative size of the fibers, frequency of stimulation, and degree of muscle stretch