Answer:
Part A
6 actin filament
3 muscle fiber = muscle cell
1 muscle
2 bundle of muscle fibers
4 myofibril
5 myosin filament
Part B
1 Action potential in motor neuron
11 Myosin head releases ADP
6 Troponin shifts tropomyosin, exposing myosin binding sites on actin
7 Myosin head extends
8 Myosin’s ATP is hydrolyzed to form ADP-P
15 Troponin loses calcium, so tropomyosin shifts and covers myosin binding site on actin
5 Calcium binds troponin
9 Myosin head with ADP-P binds to actin
2 Action potential in neuromuscular junction
13 Muscle excitation ends so calcium release from sarcoplasmic reticulum ends
10 Myosin head releases P, causing myosin head to flex, pulling filaments past each other = the power stroke
4 Action potential in muscle fiber cell membrane and T-tubules
12 Myosin head binds ATP and realeases actin
14 Calcium pumps of sarcoplasmic reticulum take up calcium and reduce its concentration in sarcoplasm
3 Release of calcium from sarcoplasmic reticulum into sarcoplasm surrounding myofibril