|TouchThe somatosensory system is a diverse sensory system composed of the receptors and processing centres to produce the sensory modalities such as touch, temperature, proprioception (body position), and nociception (pain). The sensory receptors cover the skin and epithelia, skeletal muscles, bones and joints, internal organs, and the cardiovascular system. While touch (also, more formally, tactition ; adjectival form: "tactile" or "somatosensory") is considered one of the five traditional senses, the impression of touch is formed from several modalities. In medicine, the colloquial term touch is usually replaced with somatic senses to better reflect the variety of mechanisms involved.
The system reacts to diverse stimuli using different receptors: thermoreceptors, nociceptors, mechanoreceptors and chemoreceptors. Transmission of information from the receptors passes via sensory nerves through tracts in the spinal cord and into the brain. Processing primarily occurs in the primary somatosensory area in the parietal lobe of the cerebral cortex.
At its simplest, the system works when activity in a sensory neuron is triggered by a specific stimulus such as heat; this signal eventually passes to an area in the brain uniquely attributed to that area on the body-this allows the processed stimulus to be felt at the correct location. The point-to-point mapping of the body surfaces in the brain is called a homunculus and is essential in the creation of a body image. This brain-surface ("cortical") map is not immutable, however. Dramatic shifts can occur in response to stroke or injury.