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MCAT Bio (Physiology/Genetics)

Terms in this set (164)

1) Initiation
- Nervous system innervates muscles via efferent neurons at neuromuscular junction. Signal reaches motor end plate (the nerve terminal) to release acetylcholine, which binds to receptors of sarcolemma and depolarizes it. Within a motor unit, a nerve terminal can innervate multiple myocytes.
- Depolarization launches action potential down sarcolemma to T-tubules, which travel into muscle tissues to sarcoplasmic reticulum.
- At sarcoplasmic reticulum, Ca2+ is released, which binds to regulatory subunit in troponin
- Bound tropomin causes conformation change in tropomyosin, exposing myosin-binding sites on actin thin filament
2) Shortening of Sarcomere
- Free globular heads of myosin move toward/bind with exposed actin; myosin carries ADP and Pi
- New actin-myosin cross bridges allow myosin to pull actin, drawing thin filaments to M and shortening sarcomere. Release of ADP and Pi together provides energy for power stroke so that actin thin filament slides over myosin
- ATP is hydrolyzed to ADP/Pi, which recocks myosin head so that it is in position to initiate another cross-bridge cycle.
*Repetitive binding/release of myosing -> sarcomere shortening [sliding filament model]
3) Relaxation
- Acetylcholine is degraded in synapse by enzyme, acetylcholinesterase, terminating signal at NMJ
- Sarcolemma repolarizes when signal ends so that Ca2+ release ends/is taken up by SR
- ATP binds to myosin heads, freeing them from actin (end of cross-bridge cycle)
- With actin/myosin disconnected, sarcomere returns to original width
- Without Ca2+, myosin-binding sites covered by tropomyosin