Precentral gyrus

Location and anatomy

Anatomically, the gyrus is located in the frontal lobe (lobus frontalis). Posteriorly, it is bounded by the central sulcus (sulcus centralis), which separates it from the postcentral gyrus. It extends upward along the convex surface of the hemisphere and onto the medial surface, forming the anterior part of the paracentral lobule.

Functionally, the precentral and postcentral gyri are often grouped together as the sensorimotor cortex due to overlapping areas and feedback regulatory loops. However, the precentral gyrus always remains strictly a motor zone.

Histologically, it corresponds to Brodmann's area 4. Typical pyramidal Betz cells are located in layer V of the cortex. Their axons are thick, fast-conducting myelinated fibers.

Somatotopic organization (Motor homunculus)

Body part projections in the primary motor cortex have a strict somatotopic organization and resemble an inverted little man — the motor homunculus:

• Upper (parasagittal) part: toes, foot, lower leg, thigh, abdomen, chest. The diaphragm projection is also located here (at the border with area 6aα).
• Middle part: shoulder, entire arm, forearm, hand, and fingers (from V to I).
• Lower part: neck, upper and lower face, tongue, jaw, palate, pharynx, and larynx.

The size and shape of these projections are studied using intraoperative electrical stimulation (Penfield's studies), and in healthy individuals, via neuroimaging methods (fMRI, PET).

Which pathways originate from the gyrus?

The precentral gyrus is the origin of voluntary movements. Axons of Betz cells converge, pass through the internal capsule (capsula interna), and form the pyramidal tract, which divides into two parts:

• Corticospinal tract (tractus corticospinalis) — passes through the brainstem, decussates at the border with the spinal cord, and terminates on motor neurons in the anterior horns of the spinal cord.
• Corticonuclear / corticobulbar tract (tractus corticonuclearis) — the descending pathway to the motor cranial nerve nuclei in the brainstem (cranial nerves V, VII, IX, X, XII).

Physiology and connections

Unlike the frontal cortex, which generates a slow expectancy wave during movement planning, the primary motor cortex is activated immediately before the execution of a movement (forming the readiness potential) and ensures its direct implementation.

To initiate movements, the gyrus receives afferent information from:

• The ventrooral posterior nucleus of the thalamus.
• Premotor areas (6 and 8), where "motor templates" are stored.
• Somatosensory areas of the cortex.

Clinical significance

• Spastic hemiparesis: occurs with simultaneous damage to the motor area (pyramidal tract) and extrapyramidal fibers.
• Central facial nerve palsy: unilateral lesion of the corticobulbar tract causes weakness in the facial muscles restricted to the lower half of the face on the contralateral side. Forehead muscle function is preserved because the upper part of the facial nerve nucleus receives bilateral innervation from the cortex of both hemispheres.

What else to study on this topic

• Pyramidal tract (corticospinal and corticonuclear pathways)
• Postcentral gyrus and sensory homunculus
• Premotor cortex (area 6) and movement planning

Other topics in the "Anatomical Structures" section

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