Final answer:
Somatic motor neurons that innervate skeletal muscle fibers are called alpha motor neurons, which are responsible for the voluntary contraction of the muscles. These neurons form part of the motor units, varying in size based on the muscle's function, with the quadriceps having large units for forceful contractions and extraocular muscles having small units for precise control.
Step-by-step explanation:
Somatic motor neurons that innervate skeletal muscle fibers are known as alpha motor neurons. These neurons are part of the somatic nervous system and are responsible for the voluntary contraction of skeletal muscles. Gamma motor neurons control the muscle spindle sensitivity and are part of the proprioceptive mechanism. The intrafusal fibers are those fibers within the muscle spindle that are innervated by gamma motor neurons, whereas the extrafusal fibers are the regular contractile fibers of the muscle and are stimulated by alpha motor neurons.
Every skeletal muscle fiber is innervated by the axon terminal of a motor neuron, and each muscle fiber typically receives input from only one motor neuron. The entirety of muscle fibers supplied by a single motor neuron is known as a motor unit, and the size of this unit can vary depending on the muscle's requirements for precision and force. For instance, muscles such as the quadriceps contain larger motor units for powerful contractions, while muscles that control the eyes possess considerably smaller motor units to allow for precise movements.
The lower motor neurons in question are predominantly located in the ventral horn of the spinal cord and project through spinal nerves to neuromuscular junctions, where they communicate with skeletal muscle fibers to trigger their contraction.