Final answer:
The primary motor cortex controls voluntary movements through two descending pathways, the corticobulbar tract and the corticospinal tract. The corticobulbar tract connects the primary motor cortex to the brain stem, while the corticospinal tract connects the primary motor cortex to the spinal cord.
Step-by-step explanation:
The primary motor cortex is responsible for controlling voluntary movements of the skeletal muscles. Motor commands from the primary motor cortex travel through two descending pathways, the corticobulbar tract and the corticospinal tract. The corticobulbar tract connects the primary motor cortex to the motor nuclei in the brain stem, while the corticospinal tract connects the primary motor cortex to the motor neurons in the spinal cord.
The axons of the corticobulbar tract are ipsilateral, meaning they project to the motor nucleus on the same side of the nervous system. On the other hand, the axons of the corticospinal tract are largely contralateral, meaning they cross the midline of the brain stem or spinal cord and synapse on the opposite side of the body. This means that the right motor cortex controls muscles on the left side of the body and vice versa.