💪 Lecture 7 Flashcards – Skeletal Muscle: Contraction

sacromere

the repeating structural unit of a myofibril; extends from one Z line to the next

thin filaments

twisted strands of actin plus proteins tropomyosin, troponin, nebulin, and actinin (at Z-line)

A band

region containing the length of thick filaments; remains constant during contraction

I band

region containing only thin filaments; shortens during contraction

Z line

boundary between sacromeres; anchors thin filaments

M line

center of the sarcomere; stabilizes thick filaments

titin

elastic protein tethering thick filaments to the Z-line, maintaining alignment during contraction

calcium (Ca2+)

released from SR via DHPRR and RYR; binds to troponin causing tropomysion to move and expose actin’s active stress

crossbridge formation

myosin head binds to the exposed active site on actin, forming a crossbridge

power stroke

myosin neck pivots, pulling the thin filament toward the M line as ADP and phosphate (Pi) are released

crossbridge detachment

a new ATP molecule binds to the myosin head, causing it to release from actin

myosin reactivation

ATP is hydrolyzed to ADP and Pi, re-cocking the myosin head to its starting position

cycle continuation

the contraction cycle continues as long as Ca2+ remains elevated and ATP is available

sliding filament model

explains how thin and thick filaments slide past one another during contraction

sarcomere shortening

the distance between Z lines decreases as myosin heads pull actin toward the M line

tension production

caused by myosin pulling on actin; tension can be generation without visible shortening (isometric)

filament lengths

neither actin nor myosin filaments change length; they only slide past each other

length-tension relationship

predicts that the amount of muscle tension depends on sarcomere length at the time of stimulation

optimal length

maximum tension is produced when sarcomeres start at an intermediate length, allowing maximum overlap of actin and myosin

too short

filaments overlap too much, reducing crossbridge formation

too long

not enough overlap for crossbridges to form, reducing tension

muscle twitch

the contractile response of a muscle fibre to a single action potential

latent period

time between stimulation and contraction; excitation occurs

contraction phase

Ca2+ levels rise, crossbirdges form, and tension increases

relaxation phase

Ca2+ is pumped back into the SR and tension decreases

summation

repeated stimulation before complete muscle relaxation increases overall tension due to residual Ca2+ in the sarcoplasm

tetanus

sustained, maximum tension caused by overlapping twitches with continous stimulation

concentric contraction

muscle shortens as internal tension exceeds external load (produces movement)

isometric contraction

tension equals load; muscle produces force without changing length (stabilization)

eccentric contraction

muscle lengthens while contracting because external load is greater than internal tension (acts as a brake).

load-velocity relationship

heavier loads slow the rate of shortening; no shortening occurs when internal tension equals the load