Lateral corticospinal tract crossing and the side of weakness

The lateral corticospinal tract is the pathway for fine, skilled movement of the distal limb muscles, such as the fingers. The anterior corticospinal tract is different, because it contributes mainly to axial and proximal control.

The commands start in upper motor neurones (the cells that carry movement commands from the cortex). Their axons pass through the corona radiata (the white matter that fans out beneath the cortex) and the internal capsule (where the descending fibres come together), before entering the brainstem through the cerebral peduncles, which are paired bundles of fibres at the front of the midbrain. They then descend through the pons (between the midbrain and the medulla) and the medullary pyramids (two ridges on the front of the medulla).

In the caudal medulla (the lowest part of the medulla), most of the axons cross the midline at the decussation of the pyramids and form the lateral corticospinal tract on the opposite side of the spinal cord. There they influence lower motor neurones (whose axons supply the muscle) in the ventral horn (the grey matter of the cord that holds their cell bodies), usually through other nerve cells in the cord, with some direct connections, at the level where those neurones leave the cord.

Before the crossing, the fibres lie on the same side as the cortex they came from. So the position of a lesion (an area of damage) relative to the crossing decides which side is weak. A lesion above the crossing, for example in a cerebral peduncle, weakens the opposite side of the body, with upper motor neurone signs: weakness, brisk reflexes, increased muscle tone and an extensor plantar response (the great toe goes up when the sole is stroked). A lesion in the spinal cord, below the crossing, weakens the same side as the lesion.