Precentral gyrus

At a glance

English

Precentral gyrus

Latin (TA2)

Gyrus precentralis

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the precentral gyrus

The precentral gyrus forms the hindmost strip of the frontal lobe. It runs obliquely across the lateral surface of each hemisphere from the superior margin down to the lateral sulcus, bounded behind by the central sulcus, which separates it from the postcentral sensory gyrus, and in front by the precentral sulcus. On the medial surface it continues as the front part of the paracentral lobule.

Its cortex is the primary motor cortex, Brodmann area 4. This is agranular cortex, thick, with a poorly marked granular layer and a deep pyramidal layer (layer V) containing the giant Betz cells, which are among the largest neurons of the brain. Their axons, with those of many smaller pyramidal cells, descend as the corticospinal and corticobulbar fibres, often grouped as upper motor neurons.

The body is laid out along the gyrus upside down and reversed: the foot and leg lie on the medial surface, the trunk at the top of the lateral surface, then the arm and a very large hand area, and the face, lips, tongue and larynx lowest, beside the lateral sulcus. This motor homunculus gives most space to parts with the finest control. From the cortex the fibres gather in the hind limb of the internal capsule, run through the cerebral peduncle and pons, and enter the pyramids of the medulla. Most corticospinal fibres cross at the pyramidal decussation to form the lateral corticospinal tract; the uncrossed remainder descend in the anterior tract and mostly cross in the spinal cord. Corticobulbar fibres reach the motor nuclei of cranial nerves, largely on both sides.

Injury to the gyrus, usually by stroke, causes weakness of the opposite side, with a pattern that follows the map: a middle cerebral stroke weakens the face and arm more than the leg, while an anterior cerebral stroke chiefly affects the leg. Upper motor neuron signs, increased tone and brisk reflexes, develop over the following weeks. Seizures here produce jerking that can march along the body map.

Function and supply

Function

Starts voluntary movement on the opposite side of the body by sending corticospinal and corticobulbar fibres to spinal and cranial motor neurons.

Blood supply

The middle cerebral artery, through its cortical branches, supplies most of the lateral surface; the medial leg area is supplied by the anterior cerebral artery.