Deep dissection of brain-stem. Lateral view. ("pyramidal tract" visible in red, and "pyramidal decussation" labeled at lower right.)
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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. It actually consists of two separate tracts in the spinal cord: the lateral corticospinal tract and the medial corticospinal tract. An understanding of these tracts leads to an understanding of why for the most part, one side of the body is controlled by the opposite side of the brain.
Most of the cortico-spinal fibers (about 85%) cross over to the contralateral side in the medulla oblongata (pyramidal decussation). Those that do cross in the medulla oblongata travel in the lateral corticospinal tract.
Despite which of these two tracts it travels in, the axon of a neuron which is part of this tract will synapse with another neuron in the ventral horn. This ventral horn neuron is considered a second-order neuron in this pathway, but is not part of the corticospinal tract itself.
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.
These are motor pathways that lie outside the corticospinal tract and are beyond voluntary control. Their main function is to support voluntary movement and help control posture and muscle tone. See extrapyramidal motor system.
