precentral gyrus

**Location/Anatomy:** Gyrus of the frontal lobe lying immediately anterior to the central sulcus (of Rolando) and posterior to the precentral sulcus. Corresponds to Brodmann area 4 (primary motor cortex, M1). Lies directly anterior to the postcentral gyrus (primary somatosensory cortex; BA 3, 1, 2). Extends onto the medial surface as part of the paracentral lobule.

**Histology:** Agranular cortex (layer IV is thin or absent) with a prominent layer V containing giant pyramidal cells (Betz cells), which contribute to the corticospinal and corticobulbar tracts.

**Somatotopy (motor homunculus):** - Lateral convexity (MCA territory): face, mouth, tongue (most lateral and inferior), then hand, arm, and trunk moving superiorly. - Medial surface/paracentral lobule (ACA territory): leg and foot. - Hand, face, and tongue have disproportionately large representation because they need fine motor control.

**Function:** Executes voluntary movement. Corticospinal fibers descend through the corona radiata, the posterior limb of the internal capsule, the cerebral peduncles (midbrain), and the basis pontis. About 90% decussate at the caudal medulla (pyramidal decussation) and form the lateral corticospinal tract. Control is contralateral. - Inputs: premotor cortex (BA 6, movement planning/sensory-guided movement), supplementary motor area (BA 6, internally generated and sequenced movement), somatosensory cortex, basal ganglia and cerebellum (via the thalamus, VL nucleus).

**Blood supply:** - MCA (superior division): lateral surface, so contralateral face and arm weakness. - ACA: medial surface, so contralateral leg weakness.

**Clinical correlations:** - **Lesion (stroke):** contralateral spastic hemiparesis/hemiplegia with UMN signs (hyperreflexia, clonus, positive Babinski, increased tone, mild atrophy only from disuse). - **ACA stroke:** contralateral leg and foot weakness > arm, with possible urinary incontinence and abulia. - **MCA stroke:** contralateral face and arm weakness > leg, plus aphasia if the dominant hemisphere is involved, or neglect if the nondominant hemisphere is involved. - **Irritative lesion/focal seizure:** Jacksonian march, where clonic activity spreads along the homunculus (e.g., hand to arm to face). It may be followed by Todd paralysis (transient postictal weakness). - **ALS:** degeneration of Betz cells (UMN) and anterior horn cells (LMN), so mixed UMN and LMN signs. - **Internal capsule lacunar infarct:** pure motor stroke, from lesion of the descending fibers. - **Broca area** (BA 44/45, inferior frontal gyrus) lies just anterior to the inferior precentral gyrus. Lesions there cause non-fluent aphasia with intact comprehension. - **Cortical stimulation/mapping:** used in epilepsy and tumor surgery to preserve motor function (Penfield's work).

**High-yield board pearls:** - Precentral = motor, postcentral = sensory ("**P**recentral = **P**ush/motor"; the order is motor in front, sensory behind). - Face area is lateral (MCA); leg area is medial (ACA). - Betz cells are in layer V. - Corticospinal tract decussates at the caudal medulla, so a cortical lesion causes contralateral deficits (a spinal cord lesion below the decussation causes ipsilateral deficits). - Cortical lesion in M1 → contralateral weakness; the face is spared only partly (the lower face is affected, the upper face is spared due to bilateral cortical input to the upper facial nucleus).