|Brain: Pyramidal (brainstem)|
Medulla oblongata and pons. Anterior surface. (Pyramid visible at center.)
|Latin||pyramis medullae oblongatae|
The ventral portion of the medulla oblongata contains 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. Also located laterally from each pyramid is a pronounced bulge known as an olive. The gracile and cuneate fasciculi are located posterior to 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 decussation. 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.
When these pyramidal fibers are traced downward, it is found that some three-fifths or more of them leave the pyramids in successive bundles, and decussate in the anterior median fissure of the medulla oblongata, forming what is termed the pyramidal decussation or motor decussation.
Having crossed the middle line, they pass down in the posterior part of the lateral funiculus as the lateral corticospinal tract.
The medullary pyramids contain the motor fibres of the corticospinal and corticobulbar tracts.
Sketch of the brainstem, with the pyramidal tract visible in red, and pyramidal decussation labeled at lower right.
Whiplash as a result of a car accident can lead to brainstem injuries that affect the pyramids at the medulla oblongata. These pyramid injuries are usually a result of a dislocation at the occiput or spinal level C1. Injuries to the pyramids of the medulla oblongata can also be caused by the quick hyperextension of the neck (cervical region of the spine). Hyperextension of the neck can pull and tear the pyramids, leading to a variety of symptoms such as weakness in all four limbs, difficulty swallowing, and difficulty speaking. A bilateral infarction in the pyramids of the medulla can result in motor quadriplegia. This is rare, however, as there have only been three cases of this reported.
This article uses anatomical terminology; for an overview, see anatomical terminology.
Section of the medulla oblongata through the lower part of the decussation of the pyramids
Saladin, Kenneth (2012). Anatomy & Physiology: The Unity of Form and Function, Sixth Edition. New York: McGraw-Hill.
|Wikimedia Commons has media related to Pyramid (brainstem).|
- Diagram at csus.edu
- Whiplash Injuries: The Cervical Acceleration/Deceleration Syndrome book
- Neurology Journal article
- Medical Neurosciences