Corticospinal fibers

The corticospinal fibres are descending projection fibres that convey motor signals from the cerebral cortex towards motor neurons in the spinal cord. Within the brain, the fibres that will form the lateral corticospinal tract and anterior corticospinal tract initially travel together and are anatomically indistinguishable; they separate principally at the pyramidal decussation in the lower medulla.

The corticospinal fibres arise from the primary motor cortex of the precentral gyrus (area 4), premotor cortex (area 6), and primary somatosensory cortex of the postcentral gyrus (areas 3, 1 and 2). From the cerebral cortex, they descend through the corona radiata and converge in the posterior limb of the internal capsule. They then pass through the crus cerebri of the midbrain, descend through the ventral pons among the transverse pontocerebellar fibres, and enter the medulla, where they form the prominent medullary pyramids. For this reason, the corticospinal pathway is also referred to as the pyramidal tract.

At the lower medulla, approximately 90% of the corticospinal fibres cross the midline at the pyramidal decussation. These crossed fibres form the lateral corticospinal tract, which descends in the lateral white column of the spinal cord and is particularly important for voluntary skilled or fine movements of distal muscles. Its upper motor neurons ultimately synapse with alpha and gamma lower motor neurons in the anterior horns of the spinal cord.

The remaining fibres do not cross at the pyramidal decussation. They continue into the spinal cord as the anterior corticospinal tract, descending in its anterior white matter. These fibres cross at their respective spinal cord segments before synapsing with lower motor neurons in the anterior horns. The anterior corticospinal tract is particularly associated with the control of axial musculature and therefore contributes to postural adjustment.

Clinical correlates

The site of the pyramidal decussation is clinically important.

• A lesion affecting corticospinal fibres above the decussation, such as in the cerebral cortex, internal capsule, midbrain or pons, produces motor weakness or paralysis predominantly on the contralateral side.

• In contrast, a lesion of the lateral corticospinal tract below the decussation in the spinal cord produces motor deficits on the ipsilateral side.