Corticospinal Tract (Pyramidal Tract)
The corticospinal tracts begin in the cerebral cortex, from which they receive a range of inputs:
Primary motor cortex
Premotor cortex
Supplementary motor area
They also receive nerve fibres from the somatosensory area, which play a role in regulating the activity of the ascending tracts.
After originating from the cortex, the neurones converge, and descend through the internal capsule (a white matter pathway, located between the thalamus and the basal ganglia). This is clinically important, as the internal capsule is particularly susceptible to compression from haemorrhagic bleeds, known as a ‘capsular stroke‘. Such an event could cause a lesion of the descending tracts.
After the internal capsule, the neurones pass through the crus cerebri of the midbrain, the pons and into the medulla.
In the most inferior (caudal) part of the medulla, the tract divides into two:
The fibres within the lateral corticospinal tract decussate (cross over to the other side of the CNS). They then descend into the spinal cord, terminating in the ventral horn (at all segmental levels). From the ventral horn, the lower motor neurones go on to supply the muscles of the body.
The anterior corticospinal tract remains ipsilateral, descending into the spinal cord. They then decussate and terminate in the ventral horn of the cervical and upper thoracic segmental levels.