This topographic distribution is maintained in the cerebral peduncle, where the corticospinal fibers occupy approximately the middle of the peduncle, the fibers destined to innervate the facial nuclei lying most medially. More caudally, in the basis pontis (base, or ventral part of the pons), the descending motor tracts separate into bundles that are interspersed with masses of pontocerebellar neurons and their cerebellipetal fibers. A lesser degree of somatotopic organization can be recognized here as well, exemplified by selective weakness of the face and hand with dysarthria, or of the leg, which may occur with pontine lacunar infarctions. Anatomic studies in nonhuman primates indicate that arm–leg distribution of fibers in the rostral pons is much the same as in the cerebral peduncle; in the caudal pons, this distinction is less-well defined. In humans, a lack of systematic anatomic study leaves the precise somatotopic organization of corticospinal fibers in the pons less certain. Restricted pontine lesions may cause a pure motor hemiplegia that is indistinguishable from the syndrome of the internal capsule. However, a study conducted by Marx and colleagues using MRI mapping techniques of patients with hemiplegia due to brainstem lesions suggests that the usual somatotopic organization breaks down in the base of the pons, and there is a concentration of fibers innervating proximal muscles lying more dorsally and those exciting distal parts of the limbs, more ventrally.
Another point of uncertainty has been the existence and course of fibers that descend through the lower pons and upper medulla and then ascend again to innervate the facial motor nucleus on the opposite side. Such a connection must exist to explain occasional instances of facial palsy from brainstem lesions caudal to the midpons. A discussion of the various hypothesized sites of this pathway, including a recurrent tract (Pick bundle), can be found in the report by Terao and colleagues. They conclude from imaging studies that corticobulbar fibers destined for the facial nucleus descend in the ventromedial pons to the level of the upper medulla, where they decussate and then ascend again; but there is considerable variation between individuals in this configuration.
The descending pontine bundles, now devoid of their corticopontine fibers, reunite to form the medullary pyramid. The brachial–crural pattern may persist in the pyramids and is certainly reconstituted in the lateral columns of the spinal cord (see Fig. 7-3), but it should be emphasized that the topographic separation of motor fibers at cervical, thoracic, lumbar, and sacral levels is not as discrete as usually shown in schematic diagrams of the spinal cord.
The corticospinal tracts and other upper motor neurons terminate mainly in relation to nerve cells in the intermediate zone of spinal gray matter (internuncial neurons), from which motor impulses are then transmitted to the anterior horn cells. Only 10 to 20 percent of corticospinal fibers (presumably the thick, rapidly conducting axons derived from Betz cells) establish direct synaptic connections with the large motor neurons of the anterior horns.
Motor, Premotor, and Supplementary Motor Cortices and Cerebral Control of Movement
The motor area of the cerebral cortex is defined physiologically as the region of electrically excitable cortex from which isolated movements can be evoked by stimuli of minimal intensity. The muscle groups of the contralateral face, arm, trunk, and leg are represented in the primary motor cortex (area 4 in Fig. 3-3), those of the face being in the most inferior part of the precentral gyrus on the lateral surface of the cerebral hemisphere and those of the leg in the paracentral lobule on the medial surface of the cerebral hemisphere. The parts of the body capable of the most delicate movements have, in general, the largest cortical representation, as displayed in the motor homunculus (“little man,” a term first suggested by Wilder Penfield) shown in Fig. 3-4.