The fibers of the corticospinal tract arise from the precentral areas 4 and 6 (motor strip) of the frontal lobe, from the postcentral areas of the parietal lobe (areas 3, 1, and 2), and from the second sensorimotor area (area 40). Roughly two-thirds stem from the frontal lobe, and one-third from the parietal lobe. Only about 60% of the fibers are myelinated; the other 40% are unmyelinated. The thick fibers of Betz’s giant pyramidal cells in area 4 account for only 2 – 3% of the myelinated fibers. All other fibers stem from smaller pyramidal cells. The fibers of the pyramidal tract pass through the internal capsule. At the transition to the midbrain, they approach the base of the brain and together with the corticopontine tracts form the cerebral peduncles.
In the medulla oblongata, the corticonuclear fibers terminate on the cranial nerve nuclei. Between 80 and 90% of the fibers cross to the opposite side in the pyramidal decussation and form the lateral corticospinal tract. The uncrossed fibers continue in the anterior corticospinal tract, going down straight to end up in the anterior or ventral part of the spinal cord along the central medial fissure. The majority of corticospinal tract fibers terminate on interneurons in the intermediate zone between anterior and posterior horns. Only a small portion reaches the motor neurons of the anterior horn, predominantly those supplying the distal segments of the limbs, which are under the special control of the corticospinal tract. Impulses from the corticospinal tract activate neurons that innervate the flexor muscles but inhibit neurons that innervate the extensor muscles.
The corticospinal tract (pyramidal tract) and the corticonuclear fibers are regarded as pathways of voluntary movements. It is through them that the cortex controls the subcortical motor centers. The cortex can have a reducing and inhibiting effect, but it also produces a continuous tonic stimulus that promotes quick, sudden movements. The mechanical and stereotyped motions controlled by the subcortical motor centers must be modified by the influence of pyramidal impulses in order to produce specific, fine-tuned movements.