Final answer:
When the axon reaches muscle tissue, it branches to innervate multiple muscle fibers, forming a motor unit which is central to muscle contraction. The site of innervation is called the neuromuscular junction, where acetylcholine is released to initiate contraction.
Step-by-step explanation:
When the axon reaches the muscle, it branches and innervates multiple muscle fibers. Each of these muscle fibers is part of a larger group called a motor unit, a concept essential for understanding how muscles contract. The motor unit comprises the motor neuron and all the muscle fibers it controls, varying in size based on the muscle's function. For example, muscles that require precise control, like those controlling the eyes, will have smaller motor units, whereas muscles that produce larger, gross movements like the quadriceps may have motor units containing hundreds of fibers.
Motor neuron axons connect at a specialized neuromuscular junction (NMJ), where they synapse with the muscle fiber or sarcolemma. Here, the axon terminals release a neurotransmitter called acetylcholine (ACh), which binds to receptors on the sarcolemma, triggering muscle contraction through a series of electrochemical events.