The somatosensory system provides information about the body and its interaction with the physical environment through multiple submodalities: touch (mechanoreception), temperature (thermoreception), pain (nociception), and proprioception (body position and movement). Together these systems enable us to manipulate objects with precision, avoid tissue damage, maintain posture, and experience the rich tactile qualities of the world.
Mechanoreception and Touch
The skin contains four types of mechanoreceptors, each tuned to different aspects of tactile stimulation. Merkel cells (slowly adapting, small receptive fields) signal pressure and fine spatial detail. Meissner corpuscles (rapidly adapting, small receptive fields) detect light touch and texture during movement. Pacinian corpuscles (rapidly adapting, large receptive fields) respond to vibration. Ruffini endings (slowly adapting, large receptive fields) signal skin stretch and sustained pressure.
Two-point discrimination — the minimum distance at which two points of contact can be distinguished — varies dramatically across the body, from approximately 1 mm on the fingertip to over 40 mm on the back, reflecting the density of mechanoreceptors and the cortical magnification of body regions important for fine touch.
Wilder Penfield's cortical homunculus, mapped by electrically stimulating the somatosensory cortex during neurosurgery, reveals that body representation in the cortex is not proportional to body size but to sensory acuity. The hands, lips, and tongue have disproportionately large cortical representations, reflecting their importance for manipulation, speech, and feeding. This map is not fixed but can reorganize following amputation or extensive practice.
Pain Perception
Pain involves both sensory and emotional dimensions, processed by partially distinct neural pathways. The lateral pain system (somatosensory cortex) encodes the location and intensity of pain, while the medial pain system (anterior cingulate cortex, insula) processes its unpleasantness and motivational significance. Gate control theory, proposed by Ronald Melzack and Patrick Wall (1965), explained how non-painful input can reduce pain perception by activating inhibitory interneurons in the spinal cord — the mechanism behind rubbing a sore spot to reduce pain.
Chronic pain involves central sensitization — changes in spinal cord and brain processing that amplify pain signals and can maintain pain even after tissue healing. Phantom limb pain, experienced by many amputees, demonstrates that pain can be generated entirely by central nervous system activity without peripheral input.
Proprioception
Proprioception — the sense of body position and movement — relies on receptors in muscles (muscle spindles), tendons (Golgi tendon organs), and joints. Without proprioception, as documented in the remarkable case of Ian Waterman (who lost large-fiber afferents from the neck down due to illness), even simple movements require conscious visual monitoring and become exhausting, demonstrating how essential this normally unconscious sense is for motor control.
Active Touch and Haptic Perception
When we actively explore objects with our hands — squeezing, stroking, lifting — we engage haptic perception, which integrates tactile and proprioceptive information to extract object properties such as shape, texture, hardness, weight, and temperature. Susan Lederman and Roberta Klatzky identified systematic exploratory procedures: lateral motion for texture, pressure for hardness, static contact for temperature, enclosure for global shape, and unsupported holding for weight. This active, purposeful exploration is far more effective than passive touch for object identification.
Cortical Processing
Somatosensory information is processed in the primary somatosensory cortex (S1) in the postcentral gyrus, which contains the body map, and the secondary somatosensory cortex (S2) in the lateral sulcus, which integrates information from both body sides. Higher-order processing in the posterior parietal cortex and insula contributes to object recognition by touch, body schema maintenance, and the emotional dimension of touch.