corticospinal tract

The corticospinal tract is the main express cable that carries your deliberate movement commands from the brain down to the spinal cord. When you decide to wiggle a finger, type a word, or take a step, the order starts in the motor cortex — a strip near the top of the brain that plans voluntary action — and travels along this bundle of long nerve fibres. Think of it as a dedicated highway running from the brain's command center down through the brainstem and into the spinal cord, with off-ramps along the way where the signal is handed off to the nerves that actually drive the muscles.

It works as a relay in two steps. The fibres of the tract belong to upper motor neurons — long cells whose bodies sit in the cortex and whose cables reach down toward the spine. Most of these fibres cross over to the opposite side at the lower brainstem, which is why the left half of the brain controls the right side of the body and the right half controls the left. At the correct level of the spinal cord, each fibre passes its message to a lower motor neuron, a second cell that runs out to a muscle and tells it to contract. This pathway matters most for fine, skilled, finger-and-hand movements, so damage to it — from a stroke or spinal injury — typically causes weakness, clumsiness, and loss of delicate control on the affected side.