The decussation of pyramids is the crossing over of pyramidal fibers to the opposite side on the anterior side of medulla oblongata. The pyramidal, or corticospinal, fibers are myelinated axons that spring from pyramidal neurons that lie in layer V of the primary motor cortex, which extends alongside and parallel to the central sulcus (Roland's fissure). They comprise about 80% of the pyramidal tract corticospinal fibers. The 20% of the fibers do not cross over.
As they run down through the brainstem, those fibers originating in the left cerebral hemisphere cortex run diagonally across the medulla ventral fissure, over to the right side of anterior aspect of this portion of brainstem. Meanwhile, the corticospinal fibers arising from the right cerebral cortex cross over to the left side of medulla anterior surface.
The decussation of pyramids takes place in lower half portion of anterior aspect of medulla. This anatomical feature makes it possible for the left cerebral hemisphere to control the right side of the body, and the right cerebral hemisphere to control the left side of the body.
Below, schematic image of decussation of pyramids on anterior aspect of medulla, with pons on top.
