Anatomy and Structure
Anatomically, the gyrus is situated within the frontal lobe (lobus frontalis). Posteriorly, it is bounded by the central sulcus (sulcus centralis), which separates it from the postcentral gyrus. It courses superiorly over the convex surface of the cerebral hemisphere and extends onto the medial surface to form the anterior part of the paracentral lobule.
Functionally, the precentral and postcentral gyri are often grouped together as the sensorimotor cortex due to overlapping receptive fields and feedback loops. However, the precentral gyrus remains strictly a motor area.
Histologically, it corresponds to Brodmann area 4. Layer V of the cortex contains characteristic pyramidal Betz cells, whose thick, rapidly conducting myelinated axons project downwards.
Somatotopic Organization (Motor Homunculus)
Body parts are represented topographically within the primary motor cortex in a specific, inverted arrangement known as the motor homunculus:
- Superior (parasagittal) region: Toes, foot, lower leg, thigh, abdomen, and thorax. The projection for the diaphragm is also located here (at the border with area 6aα).
- Middle region: Shoulder, entire arm, forearm, hand, and digits (from digit V to I).
- Inferior region: Neck, upper and lower face, tongue, jaw, palate, pharynx, and larynx.
The size and layout of these cortical representations have been mapped using intraoperative electrical stimulation (Penfield's studies) and modern neuroimaging techniques in healthy individuals (fMRI, PET).
Originating Pathways
The precentral gyrus is the primary origin of voluntary motor pathways. Axons from Betz cells converge, course through the internal capsule (capsula interna), and form the pyramidal tract, which is divided into two components:
- Corticospinal tract (tractus corticospinalis) — passes through the brainstem, decussates at the spino-medullary junction, and terminates on lower motor neurons in the ventral horns of the spinal cord.
- Corticonuclear (corticobulbar) tract (tractus corticonuclearis) — a descending pathway to the motor nuclei of cranial nerves in the brainstem (cranial nerves V, VII, IX, X, and XII).
Physiology and Connections
Unlike the prefrontal cortex, which generates slow readiness potentials during movement planning, the primary motor cortex activates immediately before movement execution to directly drive muscle contraction.
To initiate movements, the gyrus receives afferent input from:
- The ventro-oral posterior (VOP) nucleus of the thalamus.
- Premotor areas (6 and 8), which store learned motor programs.
- Somatosensory cortical areas.
Clinical Significance
- Spastic hemiparesis: Occurs with combined lesions of the motor cortex (pyramidal tract) and surrounding extrapyramidal pathways.
- Central facial palsy: A unilateral lesion of the corticobulbar tract causes paralysis of the muscles of facial expression restricted to the lower half of the contralateral face. Forehead wrinkling and eye closure are preserved because the upper division of the facial motor nucleus receives bilateral cortical innervation from both hemispheres.