corticospinal

The corticospinal tract is the major descending motor pathway that conveys voluntary movement commands from the cerebral cortex to the spinal cord. It plays a central role in initiating and shaping precise, intentional muscle contractions, especially in the limbs and in fine motor control.

Origin and course: Corticospinal fibers originate primarily from neurons in the primary motor cortex (the precentral gyrus) with contributions from the premotor and supplementary motor areas. The fibers travel through the internal capsule (posterior limb), descend via the midbrain in the cerebral peduncles, pass through the pons, and reach the medullary pyramids of the medulla.

Decussation and tracts: In the caudal medulla, about the majority of corticospinal fibers cross to the opposite side, forming the lateral corticospinal tract, which innervates motor neurons in the ventral horn for limb and trunk muscles. A minority of fibers do not cross in the medulla and descend as the anterior corticospinal tract; these fibers typically decussate at the level where they terminate in the spinal cord. Some segments then influence axial muscles through interneurons and motor neurons.

Function: The tract supports voluntary, skilled movements and fractionated motor control, enabling precise finger and hand actions, as well as coordinated activity of limb and trunk muscles. It also modulates muscle tone through corticospinal interactions with spinal circuits.

Clinical relevance: Lesions of the corticospinal tract produce upper motor neuron signs such as weakness, increased tone (spasticity), hyperreflexia, and pathological reflexes like the Babinski sign. Damage can result from stroke, trauma, tumors, or demyelinating diseases and may affect recovery and motor learning through neural plasticity.