Final answer:
The nerve impulses from the primary motor cortex to the spinal cord are transmitted via the corticospinal tract. This tract is mainly contralateral, meaning the right motor cortex controls muscles on the left side of the body. It is a critical component for the voluntary movement of skeletal muscles.
Step-by-step explanation:
Descending Somatic Motor Pathways
The somatic motor pathway responsible for carrying nerve impulses directly from the primary motor cortex to the spinal cord is the corticospinal tract. This pathway consists of two neurons: the upper motor neuron and the lower motor neuron. The upper motor neuron originates in the primary motor cortex of the frontal lobe and its axons travel through the corticospinal tract to eventually synapse on the lower motor neuron located in the ventral horn of the spinal cord. The lower motor neuron then projects to the skeletal muscle, eliciting movement.
The corticospinal tract is the principal pathway responsible for voluntary movement control, with its fibers descending from the cerebral cortex to the brain stem and spinal cord. Surprisingly, the pathway is primarily contralateral; axons from neurons in one hemisphere of the brain cross over to the opposite side of the spinal cord to control muscles on that side of the body. For example, the right motor cortex controls muscles on the left side of the body, and vice versa.
There is also a subsidiary pathway known as the corticobulbar tract, which is responsible for movements of the head and neck via cranial nerves. However, this pathway does not extend to the spinal cord. Additionally, the reticulospinal tract, which was mentioned in the question, is not the direct pathway from the primary motor cortex to the spinal cord; it is more involved in the control of posture and locomotion and originates from the reticular formation in the brainstem.