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Pyramis medullae oblongatae

Synonym: Pyramid (brainstem), Medullary pyramid, Pyramis bulbi
German: Pyramis medullae oblongatae

1 Definition

The Pyramis medullae oblongatae (or medullary pyramid) refers to the visible cranial to caudal bulge of the pyramidal tract on the ventral surface of the Myelencephalon and having a somewhat triangular cross-section. Consequently, a pyramis has a form resembling that of a pyramid.

2 Anatomy

Between the two medullary pyramids is the anterior median fissure of the medulla oblongata. The hypoglossal nerve (the twelfth cranial nerve or CN XII) exits the medulla oblongata directly lateral to the pyramids. Further laterally to each pyramid is a distinct swelling formed by the olivary body.

Within the pyramids are the pyramidal tracts. These are nerve fibers that come from various areas of the motor cortex via the internal capsule. The fibers can be assigned more specifically to either the corticobulbar tract, which lies more deeply, or the corticospinal tract, which lies on the anterior surface of the pyramid. Immediately caudal to the pyramids is the decussation of the pyramids where approximately 90% of the corticospinal fibers decussate (cross to the contralateral side) and continue their journey in the lateral corticospinal tract. The remaining fibers that have not decussated continue down the spinal cord in the anterior corticospinal tract and eventually decussate at lower levels of the spinal cord near their targeted spinal cord segments.

Extrapyramidal tract fibers are motor fibers that do not pass through the medullary pyramids.

Posterior to the pyramids, on the dorsal side of the medulla oblongata, are the gracile and cuneate fasciculi.

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