Acetylcholine Receptors in Muscle Fiber (Cells)
Motor Units in Skeletal Muscle
Each skeletal muscle fiber is stimulated to contract by chemicals released from a somatic motor neuron. Most of these elongated nerve cells originate in the gray matter of the spinal cord. Their thread-like axon processes run inside nerves and carry electrochemical impulses (= action potentials) to target muscles.
A motor neuron axon branches many times after entering a target muscle, and each branch makes its way to a different muscle fiber. As it approaches the midpoint of a muscle fiber, the axon splits again, forming a small cluster of terminal branches. The tips of the terminal branches expand into small synaptic bulbs, which fit into grooves along the surface of the muscle fiber.
Together, the expanded axon tips and the nearby muscle fiber membrane make up a neuromuscular junction.
A motor unit is the term applied to a single motor neuron and all of the muscle fibers that it stimulates.
When a motor neuron fires, all the muscle fibers in the motor unit contract at once. The size of a motor unit varies from just a few fibers in the eye muscles (precise movements) to over a thousand fibers in the large leg muscles (powerful movements).
Micrograph demonstrating a neuromuscular junction: