Motor Corticospinal
The corticospinal tract originates in several parts of the brain, including not just the motor areas, but also the primary somatosensory cortex and premotor areas.Most[clarification needed] of the
neurons originate in the primary motor cortex (precentral gyrus, Brodmann area or the premotor frontal areas. About 30% of corticospinal
neurons originate in the primary motor cortex, 30% more in the premotor cortex and supplementary motor areas, with the remaining 40% distributed between the somatosensory cortex, the parietal lobe, and cingulate gyrus. These upper motor
neurons originate in layer V pyramidal
cells of the neocortex, and travel through the posterior limb of the internal capsule in the forebrain, to enter the cerebral crus at the base of the midbrain. Then both tracts pass through the
brain stem, from the pons and then to the medulla. The corticospinal tract, along with the corticobulbar tract, form two pyramids on either side of the medulla of the brainstem—and give their name as pyramidal tracts. Corticospinal
neurons synapse directly onto alpha motor
neurons for direct muscle control.
Betz
cells are very large
cells that are very visible under a microscope, and while they account for only about 5% of
cells projecting to the spinal cord, they are often considered most crucial for communication of motor signals. These
cells are notable because of their rapid conduction rate, over 70m/sec, the fastest conduction of any signals from the
brain to the spinal cord.
There are two divisions of the corticospinal tract, the lateral corticospinal tract and the anterior corticospinal tract. The lateral corticospinal tract
neurons cross the midline in the spinal cord, and controls the limbs and digits. The lateral tract forms about 90% of connections in the corticospinal tract the vast majority cross over in the medulla, while the rest (about 2-3%) remain ipsilateral. The anterior corticospinal tract neurons, the remaining 10%, stay ipsilateral in the
spinal cord but decussate at the level of the spinal nerve in which they exit, and control the trunk, shoulder and neck muscles
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