Lateral corticospinal tract

The lateral corticospinal tract is an example of a descending motor tract. It comprises of upper motor neurons which influence the voluntary skilled (or fine) movements of the distal muscles of our body. Within the brain, it runs in close proximity to the anterior corticospinal tract. The latter helps adjust the posture by controlling the voluntary movements of the axial or truncal body musculature.

The corticospinal tracts–both lateral and anterior–originate from three distinct regions of the cerebral cortex, namely the primary motor cortex (pre-central gyrus, area 4), the pre-motor cortex (area 6) and the primary sensory cortex (post-central gyrus, areas 3, 1, 2). The lateral corticospinal tract is indistinguishable from the anterior corticospinal tract, at this stage. The neuronal fibers of corticospinal tract descend through the corona radiata and converge to pass through the posterior limb of the internal capsule. From here, the tract passes through the crus cerebrum or the middle cerebral peduncle of the midbrain, following which it traverses downwards through the transverse pontocerebellar fibers of the ventral pons. In the upper medulla oblongata, the corticospinal tract fibers get bundled together on either side of the midline to create swellings called the ‘pyramids’ (hence a.k.a. pyramidal tract). In the lower medulla oblongata, majority (almost 90%) of the corticospinal tract fibers cross or decussate from anterior to lateral and consequently descend in the lateral white column of the spinal cord as the lateral corticospinal tract. The upper motor neurons of the lateral corticospinal tract synapse with the cell bodies of alpha and gamma lower motor neurons in the anterior horns of the spinal cord segments.

Please note that the remaining fibers which do not cross, descend ventrally in the lower medulla and in the spinal cord, forming the anterior corticospinal tract. The fibers of anterior corticospinal tract cross the midline (or decussate) in their respective spinal cord segments, prior to synapsing with the lower motor neuron cell bodies in the anterior horns of the spinal cord.

Due to the decussation of lateral corticospinal tract in the caudal medulla, a lesion in the central nervous system above the decussation (for instance, in the cerebral cortex, internal capsule, mid-brain or pons) could lead to a contralateral weakness or paralysis. Whereas, a neuronal lesion below the decussation level (such as, in the spinal cord) will lead to ipsilateral motor deficits.