CHAPTER 4. THE PYRAMIDAL SYSTEM

OBJECTIVE:
    BE ABLE TO IDENTIFY THE ORIGIN OF THE PYRAMIDAL SYSTEM, THE LOCATION AND RELATIONSHIPS OF THE CORTICOSPINAL TRACT AT ALL BRAIN AND SPINAL CORD LEVELS, AND THE LOCATION OF THE CORTICOBULBAR TRACT IN THE INTERNAL CAPSULE AND MIDBRAIN.

The corticospinal (pyramidal) and corticobulbar (corticonuclear) tracts arise from upper motor neurons mostly in the motor cortex, which is located in the precentral gyrus and anterior part of the paracentral lobule (Pls. 4, 5, 26, 27, 28).

Identify the corticospinal and corticobulbar tracts as they descend through the posterior limb of the internal capsule (Sls. 49, 48, 47; Pls. 26, 33; MRI 5, 9).

Observe the corticospinal and corticobulbar tracts in the cerebral crus (Sls. 36, 35, 34, 33, 32, 31; Pls. 10, 28; MRI 13), where they are located in about the middle tenth of the crus. Note the close proximity of these upper motor neuron fibers to the lower motor neuron fibers of the oculomotor nerve.

The corticospinal and corticobulbar tracts leave the cerebral crus as numerous bundles which enter the basilar part of the pons (Sl. 29). These bundles are separated from one another by the pontine nuclei and the transversely-directed pontine fibers that enter the middle cerebellar peduncle. Observe that as the bundles of corticospinal fibers descend through the pons (Sls. 27, 25, 23, 21), they gradually move closer together again, so that upon entering the medulla, they form one bundle, the medullary pyramid (Sls. 19, 17, 14, 12, 10; Pls. 6, 8, 10).

Note the close proximity of the pyramid to the abducens nerve as it emerges at the pontomedullary sulcus and to the hypoglossal nerve rootlets as they emerge in the preolivary sulcus. The fact that the upper motor neuron fibers of the pyramidal tract are close to the lower motor neuron fibers of the III, VI, and XII nerves is the anatomical basis for the superior, middle, and inferior alternating hemiplegia syndromes.

Identify in the caudal part of the medulla the pyramidal decussation (Sl. 9; Pl. 10). Here, the decussating fibers, ordinarily comprising 90% of the pyramidal tract, pass dorsolaterally and form the lateral corticospinal tract which descends through all spinal cord levels in the dorsal part of the lateral funiculus (Sls. 8, 7, 6, 5, 4, 3, 2, 1) The uncrossed pyramidal fibers pass directly into the anterior funiculus of the spinal cord as the ventral corticospinal tract (Sls. 8, 7, 6, 5) which is limited to the cervical segments. Most fibers of the ventral corticospinal tract decussate at the level at which they terminate.

The corticobulbar tract is formed from upper motoneurons located primarily in the ventral part of the precentral gyrus, the head region. The fibers accompany the corticospinal tract through the corona radiata and the internal capsule. Below this level the corticobulbar fibers are difficult to trace. Some descend in relation to the corticospinal fibers, others in the tegmentum of the midbrain, pons and medulla.