Week 11 - Muscle Physiology

Question 1

neuromuscular junction

Answer 1

when an action potential reaches the end of an axon, that signal must be transmitted across the space between the axon and the muscle (= neuromuscular junction)

Question 2

events of the neuromuscular junction

Answer 2

1. a nervous action potential is propagated down the axon (done by the opening and closing of voltage-gated ion channels) and reaches the end of the axon (synaptic end bulb) where voltage-gated Ca2+ ion channels open. because Ca2+ is more concentrated in extracellular fluid, it flows inward
2. increased Ca2+ concentration in axon serves as a signal that triggers exocytosis of vesicles filled with acetylcholine (a type of neurotransmitter)
3. acetylcholine diffuses across neuromuscular junction
4. on the muscle, acetylcholine binds ligand-gated cation channels
5. the ligand-gated cation channels open and Na+ rushes into the muscle cell, which starts a muscle action potential (MAP)
6. the muscle action potential is propagated through the transverse tubules
7. this leads to the release of Ca2+ ions from the sarcoplasmic reticulum into the sarcoplasm through calcium release channels

Question 3

physiology of muscle contraction

Answer 3

1. myosin heads hydrolyze ATP, which energizes and orients them into the “cocked” or activated position, and Ca2+ binds troponin, which pulls away tropomyosin from myosin binding sites on actin
2. myosin heads attach to actin, forming crossbridges and the previously hydrolyzed phosphate is released (ADP remains attached)
3. myosin heads swivel (rotate towards center of sarcomere); this is called a power stroke; ADP is released as the power stroke occurs
   and the thin filament slides past the thick filament towards the M-line, shortening the sarcomere
4. a new molecule of ATP binds to the myosin head causing myosin to release actin
   [the cycle begins again and will continue as long as ATP is available and Ca2+ is bound to troponin]

Question 4

mechanism of smooth muscle contraction

Answer 4

1. smooth muscle cells don’t have t-tubules and very little sarcoplasmic reticulum. Ca2+ influx occurs via channels in the caveolae in the cell’s membrane and most of the Ca2+ comes from the interstitial fluid
2. there is no troponin the the thin filament complex, Ca2+ instead binds to a regulatory protein in the cytoplasm called calmodulin
3. activated calmodulin activates an enzyme called myosin kinase
4. activated myosin kinase phosphorylates the myosin head, activating it
5. once activated, myosin head can interact with actin and contraction cycle begins