corticospinalis

The corticospinalis, or tractus corticospinalis, is a major descending motor pathway that originates in the cerebral cortex and terminates in the spinal cord. It is a key component of the pyramidal system and plays a central role in voluntary motor control.

Origin and course: The tract primarily begins in the primary motor cortex (the precentral gyrus, area 4), with contributions from the premotor and supplementary motor areas and, to a lesser extent, the primary somatosensory cortex. Fibers descend through the internal capsule, continue in the cerebral peduncles of the midbrain, pass through the basis pontis of the pons, and enter the medulla where most fibers form the pyramids. In the lower medulla, the majority of corticospinal fibers decussate to form the lateral corticospinal tract, while a smaller bundle remains uncrossed to form the anterior (ventral) corticospinal tract. Some anterior tract fibers cross at their spinal segment of termination.

Termination and function: Corticospinal fibers terminate on interneurons and on alpha motor neurons in the ventral horn of the spinal cord, at various levels, enabling precise, voluntary control of limb and trunk muscles. The lateral corticospinal tract predominantly controls distal limb movements and fine, fractionated movements, while the anterior tract contributes to axial and proximal muscles and may influence bilateral control.

Clinical relevance: Damage to the corticospinal tract leads to upper motor neuron signs, including weakness, spasticity, hyperreflexia, and a positive Babinski sign. Lesions above the decussation typically produce contralateral deficits, whereas those below produce ipsilateral weakness.

Summary: The corticospinalis is a top-tier pathway for voluntary, finely graded movements, transmitting motor commands from the cortex to the spinal motor circuitry.