Body parts are represented in proportion to innervation density, not size
Expansion
The postcentral gyrus (sensory) and precentral gyrus (motor) each carry an inverted map of the body.
- The leg and foot lie on the medial surface, in the anterior cerebral artery territory
- The trunk, arm, hand and face lie on the lateral convexity, in the middle cerebral artery territory
Representation is proportional to receptor density and functional importance, not to body size, so the hand, lips and tongue occupy disproportionately large areas. This correlates directly with two-point discrimination, which is a few millimetres on the fingertip and several centimetres on the back.
The map is plastic. After amputation, adjacent cortical areas invade the deafferented zone, which is one explanation for phantom limb sensation and for referred sensation when the face is touched in an arm amputee. Plasticity also underlies recovery after stroke and the cortical expansion seen with skilled practice.
Discriminative touch and proprioception reach the cortex through the dorsal column pathway, which is why cortical lesions impair fine discrimination and stereognosis more than crude sensation.