The
ventral portion of the
medulla oblongata contains the medullary pyramids. These two ridge-like structures travel along the length of the medulla oblongata and are bordered medially by the
anterior median fissure. They each have an
anterolateral sulcus along their lateral borders, where the
hypoglossal nerve emerges from. Also at the side of each pyramid there is a pronounced bulge known as an
olive. Fibers of the
posterior column, which transmit
sensory and
proprioceptive information, are located behind the pyramids on the medulla oblongata. The medullary pyramids contain motor fibers that are known as the
corticobulbar and
corticospinal tracts. The corticospinal tracts are on the anterior surface of the pyramids. These tracts transport motor signals that originated in the precentral gyrus and travelled through the internal capsule to the medulla oblongata and pyramids.
Extrapyramidal tracts are those motor tracts that do not traverse the medullary pyramids. At the pyramids' most caudal end, the corticospinal axons
decussate (or cross over) the midline and continue down the spinal cord on the contralateral side. The fibers that decussated will go down the lateral corticospinal tract while the fibers that did not decussate will travel down the
anterior corticospinal tract. Nearly 90 percent of the fibers decussate and travel down the lateral corticospinal tract while the other 10 percent travels down the
anterior corticospinal tract.
Decussation The two pyramids contain the
motor fibers that pass from the
brain to the
medulla oblongata and
spinal cord. These are the
corticobulbar and
corticospinal fibers that make up the
pyramidal tracts. About 90% of these fibers leave the pyramids in successive bundles and
decussate (cross over) in the
anterior median fissure of the medulla oblongata as the
pyramidal decussation or
motor decussation. Having crossed over at the middle line, they pass down in the posterior part of the
lateral funiculus as the
lateral corticospinal tract. The other 10% of the fibers stay uncrossed in the
anterior corticospinal tract. The pyramidal decussation marks the border between the spinal cord and the medulla oblongata. ==Function==