Pyramid of medulla oblongata

At a glance

English

Pyramid of medulla oblongata

Latin (TA2)

Pyramis medullae oblongatae

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the pyramid of medulla oblongata

The pyramids are two elongated white ridges on the front of the medulla oblongata, one on each side of the anterior median fissure. They begin at the lower border of the pons and taper downward to the junction with the spinal cord. Lateral to each pyramid is the anterolateral sulcus, from which the rootlets of the hypoglossal nerve emerge, and beyond that sulcus the oval swelling of the olive. Behind the pyramids lie the medial lemniscus and, further back, the sensory dorsal column nuclei.

Each pyramid is a compact bundle of motor fibres that began in the cerebral cortex, mainly in the precentral gyrus, and descended through the internal capsule, the cerebral peduncle and the basilar pons. Most of these are corticospinal fibres bound for the spinal cord. Some corticobulbar fibres to the motor nuclei of the lower brainstem also run with them. Together these fibres are called the pyramidal tract, after this structure, while motor pathways that do not pass through the pyramids are grouped as extrapyramidal.

At the lower end of the medulla roughly nine tenths of the corticospinal fibres leave the pyramid in interlocking bundles and cross the midline in the depth of the anterior median fissure. This pyramidal (motor) decussation is the landmark boundary between the medulla and the spinal cord. Crossed fibres continue down in the lateral funiculus, forming the lateral corticospinal tract; the uncrossed remainder descend as the anterior corticospinal tract and mostly cross lower down, in the spinal cord, before ending. This crossing is why one hemisphere controls the opposite side of the body.

A lesion of one pyramid above the decussation weakens the opposite arm and leg, often with a positive Babinski sign. In medial medullary syndrome, from occlusion of branches of the vertebral or anterior spinal artery, the pyramid, medial lemniscus and hypoglossal fibres are damaged together, giving opposite-sided weakness and loss of position sense with the tongue deviating to the side of the lesion. Severe neck hyperextension injuries at the craniocervical junction can also damage the pyramids.

Function and supply

Function

Carries voluntary motor commands from the cerebral cortex to the spinal cord and lower brainstem; most fibres cross here to control the opposite side of the body.

Blood supply

Paramedian branches of the anterior spinal artery and of the vertebral arteries.