Descending Systems
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| Define upper motor neuron (UPN). | UPN axons compose the descending or motor pathways of the spinal cord, cell bodies of the pyramidal tract are located in the cerebral cortex, cell bodies of the extrapyramidal tracts are located in various centers in the brainstem, UPN axons cross to the contralateral side and end on a motor neuron in the ventral horn either directly or through an interneuron. | |
| Define lower motor neuron (LMN). | Located in the ventral horn, the final common pathway with UMN, UMN excite (facilitate) or inhibit LMN, axons of LMN leave the spinal cord through the ventral roots of the spinal nerves and innervate voluntary skeletal muscles. | |
| Define pyramidal system. | Corticospinal/pyramidal tract ends on the ventral horn cells, corticobulbar tracts end on neurons in the motor nuclei of the cranial nerves. | |
| Define extrapyramidal system. | Includes the centers and tracts (that are not part of the pyramidal system) that have a significant influence on the motor cortex or LMN. | |
| What percentage of UMN cell bodies are in the primary motor cortex of the frontal lobe/precentral gyrus area 4? | 40% | |
| What percentage of UMN cell bodies in the premotor cortex of the frontal lobe are the giant pyramidal cells of Betz? | 3%. | |
| What percentage of UMN cell bodies are in the premotor area of the frontal lobe/area 6? | 40%. | |
| What percentage of UMN cell bodies are in the somesthetic/somatosensory cortex of the parietal lobe (postcentral gyrus 3, 1, 2)? | 20%. | |
| What is the course of UMN axons? | Corticospinal tract descends through the posterior limb of the internal capsule, crus cerebri, or basis pedunculi (cerebral peduncles) through the basilar pons/horizontal pontine fibers/longitudinal fasciculi to form the pyramids of the medulla. | |
| What is the course of 85% of the crossed UMN axons? | At the caudal end of the medulla, they cross in the decussation of the pyramids and descend in the lateral corticospinal tract. | |
| What is the course of the 15% uncrossed UMN axons? | They remain uncrossed and continue into the ventral corticospinal tract. | |
| Describe the somatotopic organization of the lateral corticospinal tract. | Cervical fibers are most medial, sacral fibers are the most lateral. | |
| Where do the corticospinal fibers terminate? | The cervical (55%), Lumbosacral (25%) and Thoracic (20%) regions of the spinal cord. | |
| Where do the corticospinal fibers from the precentral areas 4 & 6 terminate in the gray matter of the spinal cord? | On interneurons which in turn synapse with lower motor neurons in the ventral horn. | |
| Where do the corticospinal fibers from the postcentral gyrus 3, 1, 2 terminate in the gray matter of the spinal cord? | In the dorsal horn where they function to modulate sensory input to the cord. | |
| Where do the ventral corticospinal fibers terminate in the gray matter of the spinal cord? | In the contralateral ventral horn of the cervical and upper thoracic segments. The fibers that remain uncrossed account for the bilateral cortical control of the muscles on the neck and trunk. |
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