Somatotopy

Somatotopy is the point-for-point organization by which the nervous system represents the body's surface. In the brain, this manifests as orderly maps in multiple regions that correspond to specific body parts, so that neighboring parts of the body are represented by neighboring neural tissue. The most well-known examples are in the primary somatosensory cortex (S1) and the primary motor cortex (M1).

S1, in the postcentral gyrus, contains a somatotopic map often referred to as the sensory homunculus. Areas with high receptor density and fine tactile discrimination, such as the lips, tongue, and hands, occupy disproportionately large cortical territories. M1, in the precentral gyrus, contains the motor homunculus, with representations for various muscle groups arranged to support contralateral control and fine movement.

Beyond S1 and M1, somatotopy is organized in the thalamus (ventral posterior nucleus), the cerebellum, and other somatosensory areas, though the maps differ in detail. The organization is not a rigid grid; receptive fields can overlap and may change with learning and experience.

Plasticity and clinical relevance: cortical maps can reorganize after injury, limb loss, or training, a process underlying rehabilitation and phenomena such as phantom limb sensation.

Interpretation and limitations: somatotopic maps are approximations that vary among individuals and can be influenced by attention, context, and disease. In the peripheral nervous system, a similar topographic arrangement begins with sensory receptors and is transmitted through the spinal cord to higher brain centers.