A motor unit is
A)the functional unit of smooth muscle.
B)the whole muscle that is supplied by a motor nerve.
C)only the muscle fibers it innervates.
D)a motor neuron and all the muscle cells that it innervates.
E)the part of a muscle cell that acts independently during muscle contraction.

Question

Quizplus · Accepted Answer

A motor unit includes a motor neuron and all the muscle cells that it innervates. Therefore, option D is the correct choice. Option A is incorrect because the functional unit of smooth muscle is the smooth muscle cell itself, not a motor unit. Option B is incorrect because a whole muscle can be supplied by multiple motor nerves, not just one. Option C is incorrect because a motor unit includes not just the muscle fibers it innervates, but also the motor neuron that innervates them. Option E is incorrect because there is no such thing as a "part of a muscle cell that acts independently" during muscle contraction; instead, all muscle cells contract as a single unit as a result of the coordinated activity of their myofibrils.

A Motor Unit Is