Primary somatosensory cortex
Also known as: S1, postcentral gyrus
The primary somatosensory cortex is the brain region in the postcentral gyrus of the parietal lobe that processes touch, pressure, pain, temperature, vibration, and proprioception from the opposite side of the body.
The primary somatosensory cortex (S1) is the brain's first cortical stop for bodily sensation. It occupies the postcentral gyrus — the ridge of parietal lobe tissue immediately behind the central sulcus — directly across from the primary motor cortex on the precentral gyrus. It corresponds to Brodmann areas 3, 1, and 2.
Its function is to receive and process somatic sensory input: touch, pressure, vibration, temperature, pain, and proprioception (the sense of body position). Sensory signals ascend from receptors through the spinal cord and brainstem, cross to the opposite side, and relay through the thalamus (the VPL and VPM nuclei) before reaching S1. Because the pathways decussate, the left hemisphere's S1 represents the right side of the body and vice versa.
The body is mapped onto S1 as the sensory homunculus — a distorted body map in which regions with fine sensory discrimination, like the fingertips and lips, claim disproportionately large territory, while the trunk gets relatively little. The leg and foot representation curls over the top of the hemisphere onto its medial surface in the paracentral lobule. Damage to S1 produces contralateral sensory deficits — numbness or impaired discrimination on the opposite side of the body.
The MCAT tests the primary somatosensory cortex within its sensing-the-environment psychology and biology content, and USMLE Step 1 tests it in neuroanatomy — including localizing lesions, the homunculus, and its blood supply (lateral S1 by the middle cerebral artery, medial leg area by the anterior cerebral artery).
Key takeaways
- The primary somatosensory cortex sits in the postcentral gyrus of the parietal lobe (Brodmann areas 3, 1, 2).
- It processes touch, pressure, vibration, temperature, pain, and proprioception from the contralateral body.
- Sensory input relays through the thalamus before reaching S1.
- The sensory homunculus devotes the most cortex to high-acuity regions like the hands and lips.
- Lesions of S1 cause contralateral sensory loss, a common exam localization question.