Pyramid (brainstem)

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Brain: Pyramid (brainstem)

Medulla oblongata and pons. Anterior surface. (Pyramid visible at center.)

Section of the medulla oblongata at about the middle of the olive. (Pyramid visible at bottom center.)
Latin pyramis medullae oblongatae
Gray's p.768
NeuroNames hier-702

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.

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.

See also

  • Corticospinal tract
  • Decussation of the pyramids

Additional images

  • The formatio reticularis of the medulla oblongata, shown by a transverse section passing through the middle of the olive.

  • Dissection showing the projection fibers of the cerebellum.

  • Human brainstem anterior view

References

Saladin, Kenneth (2012). Anatomy & Physiology: The Unity of Form and Function, Sixth Edition. New York: McGraw-Hill.

External links

This article incorporates text from a public domain edition of Gray's Anatomy.

Dorsal
Surface
Grey: Cranial nuclei
afferent:
efferent:
Grey: other
White: Sensory/ascending
White: Motor/descending
Ventral
White: Motor/descending
Grey matter
Surface
Grey: Raphe/
reticular

M: CNS

anat (n/s/m/p/4/e/b/d/c/a/f/l/g)/phys/devp

noco (m/d/e/h/v/s)/cong/tumr, sysi/epon, injr

proc, drug (N1A/2AB/C/3/4/7A/B/C/D)

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