Final answer:
Motor neurons in the somatic nervous system extend from the CNS to skeletal muscles to control contraction, releasing acetylcholine at the neuromuscular junction. Muscle fibers form motor units innervated by individual motor neurons, with the size of each unit varying by required precision and strength of muscle contractions.
Step-by-step explanation:
Motor Neurons and Skeletal Muscle Contraction
The motor neurons responsible for controlling skeletal muscle contraction have their cell bodies in the central nervous system (CNS), specifically within the spinal cord (and a smaller number in the brainstem for facial, head, and neck muscles). These neurons' axons run directly from the CNS to their target skeletal muscles without interruption, making up the somatic nervous system. At the neuromuscular junction, the axon terminals release acetylcholine, which causes the muscle to contract. This system is unlike the two-neuron chain found in the autonomic nervous system which regulates involuntary body functions.
Each motor neuron's axon can innervate multiple skeletal muscle fibers, forming a motor unit. The strength and precision of muscle contraction are influenced by motor neuron impulse frequency and the nature of the motor unit. For example, large muscles like the quadriceps have muscle fibers controlled by single motor neurons for robust contractions, while the muscles that move the eyes have fewer fibers per motor neuron for precise control.