Exam 2: Muscle Structure and Function

endomysium

surrounds individual fibers in skeletal muscles

fasicles

multiple fibers bundled together in skeletal muscles

perimysium

a collagen sheath that fascicles are bound by in skeletal muscle

epimysium

surrounds all fascicles of an entire muscle in skeletal muscle

deep fascia

binds muscles into functional groups in skeletal muscle

sarcomere

basic functional and contractile unit of striated muscle tissue; repeating makes up a myofibril

g actin

monomeric form (makes up f actin); active site where actin interacts with myosin; tropomyosin wraps around

f actin

polymerized form, double helix; string of g actin

tropomyosin

covers and uncovers attachment sites for myosin on actin myofilaments; regulatory protein

troponin

provides calcium binding sites on actin filaments; causes tropomyosin to make way for myosin when calcium binds to it

rod region of myosin

backbone of thick filaments; parallel to myosin filament

globular heads of myosin

attach to actin; bind to ATP and break it down into ADP + P; when ADP is released from head it attaches to ATP and releases that from actin; involved in crossbridging; is what performs the power stroke

calcium during relaxed state of muscle sliding

calcium concentration in sarcoplasm is low and is stored in sarcoplasmic tubules

tropomyosin during relaxed state of muscle sliding

troponin-tropomyosin complex attached to actin filament, tropomyosin positioned to block myosin binding sites on actin filament

energy during relaxed state of muscle sliding

ATP and inactive ADPase bound to myosin head; low energy configuration; binding to actin is not possible

step one of events during contraction

nerve impulse (afferent signal) from motor neuron generates action potential in nerve cell; action potential propagated along sarcolemma and down t tubules

step two of events during contraction

myosin ATPase activated; ATP splits; high energy myosin-ADP complex; myosin ATPase is active at low levels even when muscle contraction is not occurring

step three of events during contraction

action potential causes release of calcium from sarcoplasmic reticulum

step four of events during contraction

calcium binds to troponin; molecular shape of troponin changes; tropomyosin is removed from binding site of myosin on the actin filament; myosin attaches to actin

step five of events during contraction

contraction (power stroke): potential energy stored in high-energy configuration is used to pivot myosin head; myosin head bends as it pulls on actin; inorganic phosphate are released from myosin

step six of events during contraction

new ATP attaches to myosin head replacing the ADP from prior power stroke; cross bridge simultaneously detaches; following death there is not ATP and muscle fibers cannot relax; rigor mortis

step seven of events during contraction

if no new impulse, calcium is pumped back into sarcoplasmic reticulum; relaxation occurs

step eight of events during contraction

if calcium is present from additional impulse, cycle repeats; myosin head “steps” to next binding site on actin

a-bands

dark bands; thick filament region where actin and myosin overlap; anisotropic; polarizes light; center of each sarcomere

what is highlighted

a-band

i-band

light bands; two regions of thin filament (actin) bisected by z-disc protein anchoring thin filaments

what is highlighted

i-band

H band / H zone

within A band; no thin filaments; myosin only, contains M line

what is highlighted

H band / H zone

M line

structural protein that runs down the middle of the sarcomere; where myosin attaches

what is highlighted

M line

Z disc

membrane; midline in i band; where sarcomere starts/ends; where actin attaches

what is highlighted

Z disc

titan filament

spring-shaped elastic protein that attaches to the thick filament and anchors it to the z disc; contributes to elasticity

what is highlighted

titan filament

actin

thin protein; made out of 2 woven strands; have active sites; has tropomyosin on it

active sites

locations where myosin and actin interact with each other and do contractions