Final answer:
Motor neurons control skeletal muscle fibers at neuromuscular junctions by releasing acetylcholine, which triggers muscle contraction. The number of muscle fibers controlled by a motor neuron varies depending on the precision and force required by the muscle. Acetylcholine is degraded after binding to its receptor to prevent continuous stimulation.
Step-by-step explanation:
Every skeletal muscle fiber is controlled by a motor neuron at a single neuromuscular junction (NMJ). The motor neuron releases the neurotransmitter acetylcholine, which binds to receptors on the muscle fiber. This binding triggers an electrical signal that leads to the contraction of the muscle fiber.
The number of muscle fibers controlled by a single motor neuron varies. For example, large motor units control many muscle fibers and are associated with powerful but less precise movements, such as those performed by the quadriceps muscles. On the other hand, small motor units control fewer muscle fibers and are associated with precise movements, such as those performed by the muscles that move the eyes.
After binding to its receptor, acetylcholine is degraded by the enzyme acetylcholinesterase. This degradation ensures that the muscle fiber only contracts in response to a specific signal and prevents continuous stimulation and contraction.
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