Final answer:
A somatic motor neuron exiting through spinal nerve C5 travels down the corticospinal tract to the brachial plexus and innervates the thenar muscles via lower motor neurons found in the ventral horn, which connect to muscles at neuromuscular junctions.
Step-by-step explanation:
The somatic motor neuron that exits the spinal cord through spinal nerve C5 and innervates the thenar muscles responsible for opposing the thumb follows a specific pathway from its origin in the motor cortex, through the descending motor pathways, to the neuromuscular junction. Initially, commands from the primary motor cortex travel down through the axons of the upper motor neurons that form the corticospinal tract, which crosses the midline of the brainstem or spinal cord (largely contralateral) to reach the various motor nuclei or ventral horn of the spinal cord, communicating with lower motor neurons. These lower motor neurons ultimately exit the ventral horn of the spinal cord and follow the peripheral nerves, in this case, the brachial plexus formed by the ventral rami of C5 through T1 spinal nerves, to reach the thenar muscles.
Lower motor neurons located in the ventral horn are responsible for the contraction of skeletal muscles and have long axons that extend out of the spinal cord through the ventral nerve root, joining the emerging spinal nerve, and connecting to muscle fibers at neuromuscular junctions. The number of muscle fibers innervated by a single motor neuron varies based on the precision and force required for that particular muscle's function.