postcentral gyrus
Summary
The postcentral gyrus is the primary somatosensory cortex (Brodmann areas 3, 1, 2) in the parietal lobe, located just posterior to the central sulcus. It receives somatotopically organized sensory input (touch, vibration, proprioception, pain, temperature) from the contralateral body via the VPL/VPM nuclei of the thalamus. Lesions cause contralateral loss of discriminative sensation, astereognosis, and agraphesthesia.
Detail
**Location and anatomy** - Lies in the parietal lobe, immediately posterior to the central sulcus (Rolandic fissure). The precentral gyrus (primary motor cortex, BA 4) lies anterior to the sulcus. - Brodmann areas 3a, 3b, 1, and 2: - 3a: proprioception (muscle spindle input) - 3b: cutaneous touch (primary receiving area) - 1: texture - 2: size and shape (also proprioception) - Blood supply: the lateral (convexity) surface, which represents the face, arm, and trunk, is supplied by the MCA. The medial surface, which represents the leg and foot, is supplied by the ACA. This is why ACA strokes cause contralateral leg > arm sensory and motor deficits, and MCA strokes cause contralateral face and arm > leg deficits.
**Sensory pathway (3-neuron pathways)** - Dorsal column–medial lemniscus: fine touch, vibration, proprioception. First-order neurons synapse in the nucleus gracilis/cuneatus. Second-order neurons decussate in the medulla (internal arcuate fibers) and ascend as the medial lemniscus to the VPL of the thalamus. - Spinothalamic tract: pain and temperature. Second-order neurons decussate in the anterior white commissure and ascend to the VPL. - Face: trigeminal pathways synapse in the VPM of the thalamus. - Third-order neurons project from the thalamus through the posterior limb of the internal capsule to the postcentral gyrus.
**Somatotopy (sensory homunculus)** - Medial to lateral: genitalia/foot → leg → trunk → arm → hand → face → lips → tongue/pharynx/intra-abdominal (most lateral and inferior). - Representation is proportional to receptor density, not body size. The hands, lips, and tongue have disproportionately large areas.
**Clinical significance** - Lesion (stroke, tumor, trauma) causes contralateral loss of discriminative sensation: - Impaired fine touch, two-point discrimination, vibration, and proprioception - Astereognosis (cannot identify an object by touch) - Agraphesthesia (cannot identify a number or letter traced on the palm) - Sensory extinction/hemineglect (more with non-dominant parietal association cortex lesions) - Crude touch, pain, and temperature are often partly preserved because of thalamic and other cortical representation. - Cortical sensory loss shows intact primary modalities but impaired integration. Compare thalamic stroke (VPL/VPM, Dejerine-Roussy syndrome), which causes contralateral hemisensory loss followed by central post-stroke pain. - Sensory (Jacksonian) seizures: a focal seizure from the postcentral gyrus causes contralateral paresthesias that march along the body. - Phantom limb sensation and cortical remapping: after amputation, adjacent cortical regions (e.g., face) expand into the deafferented area (neuroplasticity). - Mnemonic: "Post = Perceive" (postcentral = sensory); "Pre = Push" (precentral = motor). - Parietal association cortex (BA 5, 7), posterior to the postcentral gyrus, integrates sensory information. Lesions there cause hemispatial neglect (non-dominant, right) or Gerstmann syndrome (dominant, left): agraphia, acalculia, finger agnosia, and left-right disorientation.
Sources
- First Aid for the USMLE Step 1 (Neuroscience: Cerebral cortex, Sensory homunculus, Somatosensory pathways)
- Kandel ER et al. Principles of Neural Science
- Purves D et al. Neuroscience
- Blumenfeld H. Neuroanatomy through Clinical Cases
- Costanzo LS. BRS Physiology (Sensory systems)
- Pathoma / Sketchy Medical / Boards and Beyond (Neuroanatomy)
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