The cortex directs motor activity through 2 pathways, the lateral corticospinal tract and the anterior corticospinal tract.
The lateral corticospinal tract is the larger of the 2 pathways. The fibers descend from the cortex and go through the brainstem, and are found within the pyramids of the medulla. In the lowermost medulla, the fibers cross, and descend as the lateral corticospinal tract. The tract is located in the lateral aspect of the white matter of the spinal cord. Other fibers remain ipsilateral and form the anterior corticospinal tract, which is located in the anterior portion of the white matter of the spinal cord; most of these fibers cross at the local spinal level.
Functionally, these tracts have a strong influence on voluntary motor activity. Some of the fibers of the lateral corticospinal tract terminate directly on the motor neurons (anterior horn cells) of the spinal cord, particularly those involved in fine motor control of the fingers and hand. Most others act via the interneurons of the anterior horn, which then influence the motor neurons.
Damage to the corticospinal tract results in changes in modulation of the deep tendon reflexes to cause hyperreflexia and spasticity at segments below the level of corticospinal damage. The three clinical characteristics of hyperreflexia are amplitude of reflex, speed of action and spread of reflex.