Primary somatosensory cortex · Grey Matter

The primary somatosensory cortex (S1) is the strip of the parietal lobe just behind the central sulcus where touch arrives in the cortex: pressure, vibration, temperature, pain and the position of the limbs from the opposite half of the body each reach their own spot on a map of the body.

Primary somatosensory cortex. The primary somatosensory cortex (S1) is the strip of the parietal lobe just behind the central sulcus where touch arrives in the cortex: pressure, vibration, temperature, pain and the position of the limbs from the opposite half of the body each reach their own spot on a map of the body.

It occupies the postcentral gyrus (Brodmann areas 3, 1 and 2) and receives its input through the ventral posterior nuclei of the thalamus. Its map, the sensory homunculus, mirrors the motor map across the sulcus: legs on the midline, face low on the side, and an exaggerated share for the lips, tongue, fingers and hands, because those carry the densest receptors and need the finest discrimination. Beyond it, the secondary somatosensory cortex (S2) in the parietal operculum and the posterior parietal cortex combine touch with vision and limb position, which is how an object in the hand is recognised without looking.

The map is plastic in the adult. After an arm amputation the cortex that served the hand can be taken over by its neighbours on the map (the face lies next to the hand), and touching the face can then be felt in the missing hand.

Damage here dulls the fine aspects of touch on the opposite side (two-point discrimination, recognising shapes and textures) more than the crude ability to feel pain.

Its neighbour across the central sulcus is the motor cortex, and the two strips work as a loop: every movement is checked against the touch and position it produces.

Questions: What separates the strip that moves the body from the strip that feels it, and why are lips and hands so large on both maps? The central sulcus, a deep groove running across the top of each hemisphere, has the primary motor cortex on its front bank (the precentral gyrus) and the primary somatosensory cortex on its back bank (the postcentral gyrus). Penfield mapped both by stimulating patients' cortex during surgery and found two matching body maps, the homunculi, with the leg on the midline and the face low on the side. The size of each part on the map follows how finely it is controlled or felt, which is why lips, tongue and fingers take far more cortex than the trunk or the back. Why can touching the face of someone who lost an arm be felt in the missing hand? On the somatosensory map the hand area lies next to the face area. After an amputation the hand area stops receiving its own input, and inputs from its neighbour, the face, spread into it. The rest of the brain still reads activity in that patch as coming from the hand, so a touch on the cheek is felt both on the cheek and on the phantom hand, sometimes in a point-by-point layout, as Ramachandran and colleagues reported in 1992. It is one of the clearest demonstrations that adult cortical maps keep reorganising.