Corticospinal+Tract

The corticospinal/pyramidal tract consists of all axonal fibers which descend from the neocortex, through the white matter, into the brainstem where some of which make contact with cranial nerve nuclei, and which then cross over longitudinally in the pyramids of the lower medulla, and then descend into the spinal cord where they make contact with sensory and spinal motor neurons.

Source of corticospinal tracts: -~31% from the pyramidal cells of agranular primary motor area 4 - 30% from areas 6, 8, and the SMA - 30% from the primary and secondary/association somesthetic areas in the parietal lobe - 9% from the deep layers of the temporal, occipital, and orbital frontal lobes, pyramidal cells throughout the amygdala, cingulate, and striatum. .

Presumably, those corticospinal axons arising from the parietal lobe project to the dorsal horns of the spinal cord so as to regulate sensory inflow rather than directly controlling movements. Hence, as two thirds of the corticospinal tract arise in the frontal motor area as these project to motor neurons, it is the frontal motor areas which dominate and exert controlling influences over most aspects of purposeful motor behavior. Because the motor cortex is specialized for controlling movement, the functional organization of the motor cortex differs from that of other brain areas.



Within the frontal motor areas, it is layers III and V which gives rise to the c

 orticospinal tract, and which consists of large and medium size pyramidal cells. Layer V also gives rise to the corticbulbar, corticopontine, and corticorubral fibers. About 60% of the axons of the corticospinal tract are myelinated.

Specifically, the axons of the corticospinal (and related) tracts arise from large pyramidal neurons located in layers III and V of the neocortex and descend via the internal capsule to the midbrain, some of which make contact and terminate in the red nucleus, the periaqueductal gray, and a variety of other brainstem motor nuclei. Collectively these latter fibers are actually referred to as the corticobulbar tract and many of its axons arise in the orbital and medial frontal lobe as well as the inferior lateral convexity.

Corticospinal axons: -Descend through the pons where they separate into tiny nerve bundles before regrouping within the medulla to form the medullary pyramid. _At the spinal-medulla border, about three fourths of these axons cross over at the midline of the medulla to form the pyramidal decussation, with the crossed fibers forming the lateral corticospinal tracts and the remaining uncrossed axons forming the ventral corticospinal tract.

Consists of two divisions: Lateral: projects to the lateral motor nuclei of the ventral horn and to intermediate zone interneurons (originates in areas 4, 6, parietal somesthetic areas 3,1,2,5,7, throughout the neocortex as well as in the limbic system) <span style="font-family: Times New Roman,serif;">Ventral (uncrossed) : projects bilaterally to the medial cell column which is concerned with the axial muscles. (originates in Broadmans areas 4, 6 and 8) <span style="font-family: Times New Roman,serif;">The vast majority of descending motor fibers penetrate the brainstem and establish direct synaptic interconnections with spinal motor nuclei. These latter fibers are referred to as the corticospinal tracts.

<span style="font-family: Times New Roman,serif;">By these long descending pyramidal axons, neocortical motor control is gained via neocortical pyramidal axons which innervate the motor nuclei of the brainstem and spinal cord at all levels.