corticospinal tract
/ kor-tih-koh-SPY-nul /
Think of the corticospinal tract as the long-distance express lane that carries 'move now' messages from the surface of the brain all the way down to the spinal cord. It is the main route for the fast, precise, voluntary movements we control on purpose — wiggling one finger, threading a needle, mouthing a word. When this cable is cut or bruised, those skilled movements suffer the most.
The fibres start in the motor areas of the cortex, gather into a dense bundle, and travel down through the brainstem. Near the bottom of the brainstem most of them cross to the other side (this crossing is called the decussation of the pyramids), which is why the left side of the brain controls the right side of the body and vice versa. They end by signalling, often through a short relay neuron, the lower motor neurons in the spinal cord that actually drive the muscles. Because so many fibres cross, a stroke in the right brain typically weakens the left arm and leg.
In rehabilitation the corticospinal tract is centre stage after stroke and spinal cord injury, because it is the pathway most responsible for hand dexterity and fine foot control. Its health (sometimes estimated with brain imaging) helps predict how much arm recovery is realistic. Therapy after damage aims partly to coax surviving corticospinal fibres, and neighbouring pathways, to take over — but a badly damaged tract sets a real ceiling on fine motor return, which is why goals are set honestly.
After a stroke in the right side of the brain damages the corticospinal tract, a man can still move his left shoulder in rough sweeps but cannot button a shirt — the gross, postural pathways survive while the fine hand control carried by the tract is lost.
The corticospinal tract specialises in fine, fractionated movement, so its loss hits dexterity hardest.
It is the main but not the only motor pathway: brainstem tracts also reach the spinal cord and handle posture and gross movement, which is why some movement can persist even when the corticospinal tract is heavily damaged.