Question

Muscles that control precise and delicate movements (e.g., extraocular muscles and intrinsic hand muscles) are richly innervated. In these muscles, a single nerve fiber usually innervates a single muscle fiber. By contrast, muscles that produce large and coarse movements (e.g., back and limb muscles) are typically innervated by nerve fibers with hundreds of terminal branches (twigs). In these muscles, axon twigs (not individual axons) form synapses with myofibers. Thus, a motor unit is best described as a single nerve axon and the muscle fiber or fibers that it innervates. None of the other choices describe histologic features of a motor unit. What is the best description of a motor unit?

1) A single nerve fiber innervating multiple muscle fibers
2) A single nerve axon and the muscle fiber or fibers that it innervates
3) Nerve fibers with hundreds of terminal branches forming synapses with myofibers
4) Muscles that control precise and delicate movements

Answer 1

A motor unit is a single nerve axon and the muscle fibers it innervates, with the size varying based on the precision and force of the muscle's movement.

Step-by-step explanation:

The best description of a motor unit is a single nerve axon and the muscle fibers that it innervates. This encompasses both small and large motor units, with the small motor units allowing fine motor control in muscles like the extraocular muscles of the eye, where a single motor neuron innervates a low number of muscle fibers for precise control.

Conversely, large motor units, which enable coarse movements like extending the knee, have a single motor neuron that innervates a large number of muscle fibers in muscles such as the quadriceps.