B. DESCENDING SPINAL TRACTS

The most important descending spinal tract originates in the cerebral cortex and is called the corticospinal tract (see Figure 1-5). The other major descending spinal tracts worth mentioning are: the tectospinal tract arising from the superior colliculus, the rubrospinal tract arising from the red nucleus in the mid-brain, the vestibulospinal tract with its nuclei located in the floor of the fourth ventricle, and the reticulospinal tract arising from the reticular formation in the pons and the medulla. The cortico-bulbar tract which is associated with cranial nerves will not be described in this review of neuroanatomy as it is not prominently employed in the treatment of patients.

1. The corticospinal system (pyramidal system)

The corticospinal tract supplies impulses to most of the voluntary muscles. It originates in the precentral gyrus of the cerebral cortex (area 4). The axons pass through the internal capsule and descend to the mid-brain where they form the crus cerebri (basis pedunculi). In the medulla oblongata, 80 to 90 percent of the fibers decussate to the opposite side and descend in the spinal cord where they form the lateral corticospinal tract. In the spinal cord, the axons of the lateral corticospinal tract are located internal to the posterior spinocerebellar tract and posterior to the lateral spinothalamic tract.

The lateral corticospinal tract irradiates branches at all levels of the spinal cord. The fibers enter the gray matter where they synapse in the ventral horn with second-order neurons. The latter emerge from the spinal cord in the ventral spinal roots and supply the voluntary muscles through the peripheral nerves.

The remainder of the corticospinal tract which does not cross over in the medulla oblongata divides into two separate tracts: the anterior corticospinal tract and the anterolateral corticospinal tract. The axons of the anterior corticospinal tract descend uncrossed into the spinal cord. They occupy an antero-medial position in the anterior white commissure and are contiguous to the anterior median fissure. Most of the fibers of the anterior corticospinal tract descend to the upper cervical spine where they cross in the anterior white commissure. The fibers enter the gray matter where they synapse in the ventral horn with second-order neurons.

The anterolateral corticospinal tract is the smallest of the three descending tracts. The fibers descend in the lateral funiculus and remain uncrossed in the entire course of the tract. The axons of the anterolateral corticospinal tract synapse in the ventral horn with second-order neurons. It should be emphasized that the pyramidal or voluntary muscle system is made of a two-neuron system. The neurons of the corticospinal tracts leaving the precentral gyrus and descending in the spinal cord to terminate their course in the ventral horn are called upper motor neurons. The second-order neurons leaving the spinal cord to supply the voluntary muscles are called lower motor neurons. The distinction between upper and lower motor neurons paralysis is important in clinical neurology.