Final answer:
The axons from the primary motor cortex descend to the ventral horn of the spinal cord to synapse onto lower motor neurons. These axons are components of the corticospinal tract and are essential for controlling voluntary skeletal muscle movements.
Step-by-step explanation:
Axons from the primary motor cortex descend in tracts to the ventral horn of the spinal cord, where they synapse onto motor neurons that directly control the skeletal muscles of the body. The corticospinal tract is the major descending tract responsible for these actions, involving two types of neurons: the upper motor neuron and the lower motor neuron. The upper motor neuron originates in the primary motor cortex located in the frontal lobe, and its axons can be either contralateral or ipsilateral, depending on whether they are part of the corticospinal or corticobulbar tract, respectively. These axons then synapse on lower motor neurons that are found within the ventral horn of the spinal cord, projecting to the skeletal muscles and facilitating voluntary muscle movement.
The anterior corticospinal tract specifically controls the muscles of the body trunk. It is interesting to note that the fibers of the corticospinal tract that synapse on ventral horn motor neurons do so after most have decussated, or crossed over, in the medulla, allowing for the right motor cortex to control muscles on the left side of the body, and vice versa. This anatomical feature is essential for voluntary movements and muscle strength, requiring the coordination of both the upper and lower motor neurons.