Medullary pyramids (brainstem)

The medullary pyramids are two prominent structures located on the anterior (ventral) surface of the medulla oblongata, the lower portion of the brainstem. They are formed by the corticospinal tracts, which are major pathways carrying motor information from the cerebral cortex to the spinal cord. These tracts are crucial for controlling voluntary movements.

Specifically, the medullary pyramids contain the axons of upper motor neurons originating in the motor cortex of the cerebrum. These axons descend through the internal capsule, cerebral peduncles, and pons before reaching the medulla.

A significant event occurs within the caudal (inferior) part of the medullary pyramids: the decussation of the pyramids. This is where the majority (approximately 70-90%) of the corticospinal tract fibers cross over (decussate) to the opposite side of the brainstem. These crossed fibers then descend through the lateral corticospinal tract in the spinal cord, controlling muscles on the contralateral (opposite) side of the body. The remaining fibers that do not decussate in the pyramids descend through the anterior corticospinal tract, and they usually decussate closer to their level of innervation in the spinal cord or innervate muscles ipsilaterally (on the same side).

Damage to the medullary pyramids, or the corticospinal tracts at any point along their path, can result in weakness or paralysis of the muscles on the contralateral side of the body, depending on the extent and location of the lesion. This is due to the interruption of motor signals being transmitted from the brain to the spinal cord.