Option D is correct. The motor unit in vertebrate skeletal muscle refers to one motor neuron and all of the muscle fibers on which it has synapses.
A motor unit is made up of a motor neuron and all of the skeletal muscle fibers innervated by using the neuron's axon terminals, which includes the neuromuscular junctions between the neuron and the fibres. Groups of motor units frequently work together as a motor pool to coordinate the contractions of a single muscle.
A motor unit consists of one alpha motor neuron and all the extrafusal muscle fibers it innervates. The variety of these muscle fibers can differ from 1 or 2 to one thousand If the range of muscle fibers innervated by means of a motor neuron is few, the movement produced by means of that muscle is greater precise.
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