Muscle Contraction Physiology

Neuromuscular junction Motor end plate Acetylcholine Depolarization Sarcoplasmic reticulum T‑tubules & triad Calcium release Actin, myosin, troponin, tropomyosin Sarcomere structure Cross‑bridge cycle Power stroke & recovery stroke ATP role

motor neuron function

carries command from nervous system to skeletal muscle

motor end plate

folded region of muscle fiber membrane where neuron communicates with muscle

synaptic cleft

space between axon terminal and motor end plate

neurotransmitter at NMJ

acetylcholine (ACh)

effect of ACh on muscle cell

opens sodium channels → sodium rushes in → depolarization

depolarization definition

loss of resting membrane polarity due to sodium influx

T-tubules

invaginatinos of sarcolemma that carry action potential into muscle fiber

sarcoplasmic reticulum (SR)

specialized ER storing calcium; releases Ca2+ during contraction

terminal cisternae

enlarged SR chambers adjacent to T-tubules

triad

one T-tubule + two terminal cisternae

calcium gradient in SR

SR stores Ca2+ at ~2000x higher concentration than sarcoplasm

exitation-contraction coupling

depolarization triggers SR to release Ca2+ into muscle cell

Sacromere

contractile unit of muscle between two Z lines

Z lines

boundaries of sarcomere; anchor actin

M line

center of sarcomere; contains only myosin

actin

thin filament; double helix protein

myosin

thick filament with globular heads

troponin-tropomyosin complex

regulatory proteins blocking myosin binding sites on actin

what calcium binds to

troponin

effect of calcium binding

moves tropomyosin to expose myosin binding sites on actin

cross-bridge

myosin head binding to actin

power stroke

myosin pulls actin toward M line

recovery stroke

myosin returns to original position after releasing actin

ATP role in contraction

ATP binds myosin → allows release from actin → powers next cycle

ATPase activity

myosin head breaks down ATP into ADP + phosphate for energy

steps of cross-bridge cycle

bind → power stroke → release → recovery

energy storage in resting muscle

myosin heads store energy from ATP while waiting for contraction

sliding filament theory

acting & myosin slide past each other to shorten sarcomere

uncontracted vs contracted sarcomere

uncontracted: actin & myosin barely overlap; contracted: filaments slide, sarcomere shortens