Final answer:
Axons from the primary motor cortex descend to the dorsal spinal cord primarily via the corticospinal tract, crossing to the opposite side to control contralateral muscles. The corticospinal and corticobulbar tracts originate from upper motor neurons in the cortex and synapse with lower motor neurons in the spinal cord or brain stem.
Step-by-step explanation:
Axons descend from the primary motor cortex to the dorsolateral spinal cord tracts through the corticospinal tract and the corticobulbar tract. These tracts involve two types of neurons: the upper motor neuron and the lower motor neuron.
The axons of Betz cells in the primary motor cortex form the corticospinal tract which sends motor commands to activate upper motor neurons. For movements of the trunk, the anterior corticospinal tract is utilized, which does not decussate in the medulla but does so after descending into the spinal cord, where they control axial muscles through synaptic connections with lower motor neurons in the ventral horn.
The corticospinal tract has a significant contralateral arrangement, meaning that axons cross over to the opposite side of the nervous system to control muscles on the contralateral side. For the muscles of the body trunk, the descending pathway is not entirely contralateral, as the anterior corticospinal tract has some ipsilateral connections to properly coordinate postural muscles and broad movements.