The corticospinal tract is a descending neural pathway primarily concerned with motor function extending from the motor cortex down to synapse with motor neurones of the spinal cord.
Corticospinal fibres arise from neurones in the cerebral cortex. Most of these arise in the primary motor cortex and premotor cortices. In the primary motor cortex, fibres arise specifically from Betz cells. The somatosensory and the nearby parietal cortices supply a small number of additional fibres 1-2.
Organised somatotopically, these fibres descend through subcortical white matter. Following descent through the anterior two-thirds of the posterior limb of the internal capsule, they pass through the ventral midbrain, specifically the central and lateral portions of the inner crus, and continue through the pons. In the medulla oblongata, corticospinal fibres collect into a discrete bundle forming the pyramid 1-2.
The pyramid is a discrete triangular column on the ventral medulla oblongata next to the midline. This is why the corticospinal tract is also called the pyramidal tract. Within the pyramids approximately 90% of the corticospinal fibres decussate, forming the lateral corticospinal tract. The remaining 10% fibres remain ipsilateral (i.e. do not decussate) and form the anterior corticospinal tract 1-3. The anterior and lateral tracts are discussed separately.