Skeletal Muscle Contraction

skeletal muscle fiber structure

multinucleated cell containing myofibrils surrounded by sarcoplasmic reticulum and sarcolemma with T tubules

myofibril definition

long parallel bundles within muscle fiber composed of repeating sarcomeres

sarcomere definition

functional contractile unit of muscle between two Z discs

Z disc function

anchors thin filaments and defines boundary of sarcomere

A band definition

region containing thick filaments with overlap of thin filaments

I band definition

region containing only thin filaments

H zone definition

central region with only thick filaments

M line function

anchors thick filaments at center of sarcomere

thin filament composition

actin tropomyosin and troponin

thick filament composition

myosin

role of tropomyosin

covers myosin binding sites on actin at rest

role of troponin

binds Ca2+ causing conformational change to expose binding sites

role of Ca2+ in contraction

binds troponin allowing cross bridge formation

source of Ca2+ in muscle

sarcoplasmic reticulum releases Ca2+ into sarcoplasm

trigger for Ca2+ release

action potential in T tubules activates DHP receptor which opens ryanodine receptor

SERCA function

actively pumps Ca2+ back into sarcoplasmic reticulum for relaxation

cross bridge formation

myosin heads bind to exposed actin sites after Ca2+ binding

power stroke

myosin head pivots pulling thin filament toward center of sarcomere

role of ATP in cross bridge cycle

ATP binding causes detachment of myosin from actin

effect of absence of ATP

myosin remains tightly bound to actin

sliding filament theory

thin filaments slide over thick filaments shortening sarcomere without changing filament length

sarcomere changes during contraction

Z discs move closer I band and H zone shorten A band remains constant

muscle relaxation mechanism

Ca2+ pumped back into SR tropomyosin covers binding sites and cross bridges stop

fast twitch fibers characteristics

contract rapidly generate high force fatigue quickly rely on glycolysis

slow twitch fibers characteristics

contract slowly resistant to fatigue rely on aerobic metabolism

fiber type for endurance activities

slow twitch fibers

phosphagen system

uses creatine phosphate to rapidly regenerate ATP for short duration

anaerobic glycolysis

fast ATP production from glucose producing lactate but limited duration

aerobic metabolism

slow but sustained ATP production using glucose and fats in mitochondria

primary energy source at rest

aerobic oxidative system

role of creatine phosphate

provides immediate high energy phosphate to regenerate ATP

voluntary muscle control

controlled by somatic nervous system

involuntary muscle control

controlled by autonomic nervous system

isometric contraction

muscle generates force without change in length

isotonic contraction

muscle changes length while generating force

concentric contraction

muscle shortens during contraction

eccentric contraction

muscle lengthens while contracting

triad structure

one T tubule with two terminal cisternae of sarcoplasmic reticulum

role of T tubules

transmit action potential deep into muscle fiber

role of ryanodine receptor

Ca2+ release channel in sarcoplasmic reticulum

excitation contraction coupling

process linking action potential to Ca2+ release and muscle contraction

calcium role in postsynaptic membrane

does not directly stimulate contraction but triggers neurotransmitter release at presynaptic terminal via vesicle fusion

isotonic contraction definition

muscle changes length while tension remains constant either shortening or lengthening

motor end plate definition

specialized region of muscle membrane with high density of acetylcholine receptors where end plate potential is generated

fast twitch muscle fibers

rapid contraction high force production fatigue quickly rely mainly on anaerobic glycolysis

slow twitch muscle fibers

slow contraction low force production high fatigue resistance rely mainly on aerobic metabolism

fiber type for endurance exercise

slow twitch oxidative fibers predominantly used during prolonged activity like marathon running

excitation contraction coupling

process linking muscle membrane depolarization to calcium release from sarcoplasmic reticulum leading to contraction