Upper Motor Neuron Pool

There are many neurons in the cerebral cortex and brain stem that initiate and modify movement through their connections with the lower motor neurons in the anterior spinal gray. These projections also synapse on the internuncial pool of interneurons. which in turn make contact with the alpha and gamma motor neurons. These are known as the upper motor neurons. These supraspinal motor neurons in the cerebral cortex and brain stem are interconnected with each other and participate to various degrees in all automatic and voluntary movements. Descending tracts from the motor centers in the brain stem are primarily involved with posture and automatic movements, whereas the motor areas of the cerebral cortex are required for voluntary movement.

CORTICOSPINAL TRACT

The corticospinal tract is composed of axons of pyramidal neurons located in lamina 5 of the cerebral cortex. Most of the axons of the corticospinal tract come from neurons in the precentral gyrus (Brodmann area 4), '17' although significant contributions come from many other areas of the cortex, especially premotor area 6 and supplementary motor area. This is the only motor tract that passes directly from the cerebral cortex to the spinal cord without making any intermediate synaptic connections. The cytoarchitecture of the primary motor area, supplementary motor area, and premotor cortex is distinct from that of the adjacent sensory and prefrontal regions. Layer 4, the major afferent layer for the sensory cortex, is absent in the motor areas, and hence these motor regions are often termed agranular.

Layer 5 in the primary motor cortex contains the distinctive giant pyramidal neurons known as Betz cells. The axons of these cells become the corticospinal or pyramidal tract and represent one of several descending influences on the motor neurons of the brain stem and spinal cord. The remaining cells mostly come from the supplementary motor and premotor cortex. Most of the axons terminate on interneurons in the internuncial pool of the ventral gray of the spinal cord. Axons that connect monosynaptically with the lower motor neurons are involved in control of the distal muscles of the extremities, especially the intrinsic muscles of the hand that control finger movements.y Upper motor neurons in the cerebral cortex also send axons to the motor nuclei of the cranial nerves and form the corticobulbar tract. The motor neurons in the primary motor area have a somatotopic organization. The homunculus was mapped by Penfield and colleagues^ by electrically stimulating the surface of the cortex during epilepsy surgery in humans. Stimulation of most of the lateral surface of the primary motor area produced movement in the mouth, tongue, face, and hand muscles, which is consistent with the importance of fine voluntary control of these structures (Fig. 15-15 (Figure Not Available) A).

The axons of the corticospinal tract gather together, forming the corona radiata, and descend through the posterior limb of the internal capsule into the middle two-thirds of the cerebral peduncles of the midbrain. In this descent, the fibers shift position, and those representing the face become most medial, while those of the legs become most lateral. The corticospinal fibers become less recognizable as a tract as they course through the ventral two-thirds of the pons. They form bundles interspersed with a variety of other descending and crossing white matter tracts. When the fibers enter the medulla, they form a very discrete, easily recognizable bundle on the ventral aspect of the medulla known as the pyramids. Because it is such an easily recognizable bundle of nerve fibers, the corticospinal tract is often referred to as the pyramidal tract, although the pyramids also contain fibers other than those in the corticospinal system. y Half to three-fourths of the fibers of the pyramidal tract originate in areas 4 and 6 anterior to the central sulcus, and the rest come from areas posterior to the central sulcus in the parietal lobe.

At the caudalmost level of the medulla, most of the corticospinal fibers cross the midline in the decussation of the cervical medullary junction and continue in the lateral funiculus of the spinal cord as the lateral corticospinal tract. A much smaller number of fibers continue uncrossed in the ventral funiculus as the ventral corticospinal tract. The lateral corticospinal axons terminate in internuncial pools and motor neurons involved in movement of the contralateral extremities, whereas the ventral corticospinal axons terminate in the medial and ventral portions of the anterior gray in which are found neurons innervating the paravertebral muscles involved in posture (see Fig. 15-15 (Figure Not Available) B). The corticospinal tracts and their relationship to other descending pathways in the spinal cord are shown in Figure...15116 .

Corticobulbar fibers destined for the motor nuclei of the cranial nerves leave the corticospinal tract in the brain stem. Muscles of the head, except for the lower facial muscles, receive both crossed and uncrossed corticobulbar fibers. Therefore, as a rule, in a patient with a lesion of the corticobulbar tract on one side, one seldom sees significant weakness of the jaw, tongue, pharynx, or larynx.

Because the corticospinal tract extends from the cerebral cortex to the spinal cord, its considerable length makes it vulnerable to lesions at many different sites along its path. One of the most common lesions is ischemia resulting from occlusion of an artery. Therefore, it is helpful to know the

Figure 15-15 (Figure Not Available) The corticospinal tracA, The course of the corticospinal tract from primary motor cortex in Brodmann, area 4 to the spinal coB, The fibers destined for the limbs decussate at the cervical medullary junction and become the lateral corticospinal tract. The fiber destined for axial musculature continues uncrossed in the anterior corticospinal trfFcom Snell RS: Clinical Neuroanatomy for Medical Students. Boston, Little, Brown & Co. Inc, 1987.)

Figure 15-16 Several motor tracts of the spinal cord, showing the relationship between the lateral corticospinal tract and the otifFrom Curtis BA, Jacobson S, Marcus EM: Introduction to the Neurosciences. Philadelphia, W.B. Saunders, 1972.)

vascular supply of the corticospinal tract as it courses through the brain and spinal cord (see Chapter.22 ). BRAIN STEM MOTOR TRACTS

Whereas the corticospinal tracts direct voluntary movements of the distal extremities, the brain stem motor tracts are responsible for the automatic reflexes involved in posture.y The one exception is the rubrospinal tract, which forms a lateral pathway that terminates in the same part of the ventral gray as the pyramidal tract and is involved in distal limb movements.1?! The medial pathways include the reticulospinal, vestibulospinal, and tectospinal tracts. The medial pathways travel in the ventral columns of the spinal white matter and terminate in medial ventral motor neurons innervating the axial and proximal limb muscles.

The reticulospinal tract originates from several nuclear groups in the reticular formation in the pons and medulla (Fig. 15-17 (Figure Not Available) ). The reticulospinal system also receives input from a variety of other sources, including the cerebral cortex and the vestibular nuclei, and integrates this information in maintaining posture. Crude voluntary proximal movements of the extremities seen monkeys with a pyramidal tract lesion probably result from the corticoreticular portion of the reticulospinal input.

The vestibulospinal tracts originate in the vestibular nuclei and convey information from the vestibular labyrinth ( .Fig 15.-1.8 ). The vestibulospinal tract has two parts.

The largest is the lateral vestibulospinal tract, which originates in the lateral vestibular (Deiters') nucleus and reaches all levels of the spinal cord. Its input to both alpha and gamma motor neurons is excitatory, and maintains contraction of antigravity muscles to maintain posture. The medial vestibulospinal tract reaches only the cervical and upper thoracic segments of the cord; it mediates reflex head movements in response to vestibular stimuli.

The tectospinal tract arises in the superior colliculus and is involved in coordination of head and eye movements.

The rubrospinal tract that forms the lateral pathway arises in the magnocellular portion of the red nucleus (Fig. 15-19 (Figure Not Available) ). In the cat it is an important pathway for controlling movements of the distal extremities, but in humans it has become a vestigial pathway whose function has been taken over by the corticospinal tract. y Most of the red nucleus in the human, the parvicellular portion, contains smaller neurons whose major connections are with the cerebellum and inferior olive.

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