The structure of the contractile subunit of the myofibril is called the:
a. sarcoplasmic reticulum.
b. actin.
c. motor end plate.
d. sarcomere.
ANS: D
The sarcomere is the contractile portion of the myofibril. In response to a nerve impulse, RyR1 (found in skeletal muscle cells) releases intracellular calcium and initiates muscle contraction at the sarcomere, a portion of the myofibril. The sarcoplasmic reticulum is made like the endoplas-mic reticulum in other cells. Actin is a protein of the sarcomere.
It is at the motor nerve end plate where the nerve impulse is transmitted.
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