Final answer:
Neural signals from the primary motor cortex travel via the corticobulbar and corticospinal tracts. The corticobulbar tract controls head and neck movements through cranial nerves, while the corticospinal tract, with its lateral and anterior divisions, controls movements throughout the body. Additionally, the cerebrum's basal nuclei have pathways that modulate motor activity.
Step-by-step explanation:
The neural signals from the primary motor cortex travel through four different pathways to reach the motor neurons of the spinal cord. These pathways consist of two major tracts: the corticobulbar tract and the corticospinal tract.
The corticobulbar tract consists of the axons that project ipsilaterally from the cerebral cortex to synapse with motor neurons in the cranial motor nuclei of the brain stem. This pathway controls muscles of the head and neck through cranial nerves.
The corticospinal tract extends from the cortex to the spinal cord and is responsible for the voluntary movement of muscles through the body. This pathway has two subdivisions:
- The lateral corticospinal tract, which decussates, or crosses over, at the medullary pyramids of the brain stem, resulting in the majority of fibers innervating motor neurons on the contralateral side of the body.
- The anterior corticospinal tract, which remains ipsilateral and crosses over at the level of the spinal cord where the motor neurons are innervated.
Both pathways - corticobulbar and corticospinal - are named for their origin in the cerebral cortex and their distinct destinations: the brain stem ('bulbar') and the spinal cord, respectively. These tracts synapse with lower motor neurons in the ventral horn of the spinal cord or with cranial motor nuclei, which project through spinal nerves to muscles, allowing for precise control of movement.
Lastly, the baseline nuclei within the cerebrum have two pathways, the direct and indirect pathways, that modulate motor activity by regulating motor output from the cerebral cortex.