Corticospinal tract

The corticospinal or pyramidal tract is a massive collection of axons that travel between the cerebral cortex of the brain, and the spinal cord.

The corticospinal tract contains exclusively motor axons. About 85% of the axons cross over in the medulla oblongata (at a point known as the pyramidal decussation). This explains why movement of one side of the body, is controlled by the opposite side of the brain.

Contents

1 The motor pathway

2 Sensory pathways

3 Corticospinal tract damage

4 Extrapyramidal motor pathways

The motor pathway

The corticospinal tract originates in the giant pyramidal neurons (Betz cells) of the primary motor cortex (located mainly in the precentral gyrus). The neuronal cell bodies in the motor cortex send long axons to the motor cranial nerve nuclei mainly of the contralateral side of the midbrain (cortico-mesencephalic tract), pons (cortico-pontine tract), medulla oblongata (cortico-bulbar tract); the bulk of these fibers, however, extend all the way down to the spinal cord (cortico-spinal tract). Most of the cortico-spinal fibers (about 80%) cross over to the contralateral side in the medulla oblongata (pyramidal decussation), while some of them (20%) cross over at the level of the spinal cord.

There is a precise somatotopic representation of the different body parts in the primary motor cortex, with the leg area located medially (close to the midline), and the head and face area located laterally on the convex side of the cerebral hemisphere (motor homunculus). The arm and hand motor area is the largest and occupies the part of precentral gyrus, located inbetween the leg and face area. The motor axons move closer together as they travel down through the cerebral white matter, and form part of the posterior limb of the internal capsule.

The motor fibers continue down into the brainstem. The bundle of corticospinal axons is visible as two column-like structures ("pyramids") on the ventral surface of medulla oblongata - this is where the name pyramidal tract comes from.

After the decussation, the axons travel down the spinal cord as the lateral corticospinal tract. Fibers that do not cross over in the medulla oblongata travel down the separate ventral corticospinal tract and most of them cross over to the contralateral side in the spinal cord, shortly before reaching the lower motor neurons.

The motor neuron cell bodies in the motor cortex, together with their axons that travel down the brain stem and spinal cord, are referred to as upper motor neuron. In the spinal cord these axons connect (most of them via interneurons, but to a lesser extent also via direct synapses), with the lower motor neurons (LMNs), located in the ventral horn of the spinal cord. In the brain stem, the lower motor neurons are located in the motor cranial nerve nuclei (occulomotor, trochlear, motor nucleus of the trigeminal nerve, abducens, facial, accessory, hypoglossal). The lower motor neuron axons leave the brain stem via motor cranial nerves and the spinal cord via anterior roots of the spinal nerves respectivelly, end-up at the neuromuscular plate and provide motor innervation for voluntary muscles.

Sensory pathways