Final answer:
A motor neuron communicates with a muscle fiber at the neuromuscular junction through the release of acetylcholine, which triggers muscle contraction. The strength and number of contracting muscle fibers can be influenced by the frequency of motor neuron impulses.
Step-by-step explanation:
A motor neuron communicates with a muscle fiber at the neuromuscular junction through a series of steps. First, the synaptic end bulbs of the motor neuron release a neurotransmitter called acetylcholine. This neurotransmitter binds to receptors on the muscle fiber's sarcolemma, which triggers the opening of ligand-gated ion channels. This leads to depolarization of the sarcolemma, initiating muscle contraction. The strength and number of muscle fibers contracting can be influenced by the frequency of motor neuron impulses.