Touch. The somatosensory system is a part of the sensory nervous system. The somatosensory system is a complex system of sensory neurons and neural pathways that responds to changes at the surface or inside the body. The axons of sensory neurons connect with, or respond to, various receptor cells. These sensory receptor cells are activated by different stimuli such as heat and nociception, giving a functional name to the responding sensory neuron, such as a thermoreceptor which carries information about temperature changes. Other types include mechanoreceptors, chemoreceptors, and nociceptors which send signals along a sensory nerve to the spinal cord where they may be processed by other sensory neurons and then relayed to the brain for further processing. Sensory receptors are found all over the body including the skin, epithelial tissues, muscles, bones and joints, internal organs, and the cardiovascular system. Somatic senses are sometimes referred to as somesthetic senses, with the understanding that somesthesis includes the sense of touch, proprioception, and haptic perception. The mapping of the body surfaces in the brain is called somatotopy. In the cortex, it is also referred to as the cortical homunculus. This brain-surface map is not immutable, however. Dramatic shifts can occur in response to stroke or injury. The four mechanoreceptors in the skin each respond to different stimuli for short or long periods. Merkel cell nerve endings are found in the basal epidermis and hair follicles; they react to low vibrations and deep static touch such as shapes and edges. Due to having a small receptive field, they are used in areas like fingertips the most; they are not covered and thus respond to pressures over long periods. Tactile corpuscles react to moderate vibration and light touch. They are located in the dermal papillae; due to their reactivity, they are primarily located in fingertips and lips. They respond in quick action potentials, unlike Merkel nerve endings. They are responsible for the ability to read Braille and feel gentle stimuli. Lamellar corpuscles determine gross touch and distinguish rough and soft substances. They react in quick action potentials, especially to vibrations around 250 Hz. They are the most sensitive to vibrations and have large receptor fields. Pacinian reacts only to sudden stimuli so pressures like clothes that are always compressing their shape are quickly ignored. Bulbous corpuscles react slowly and respond to sustained skin stretch. They are responsible for the feeling of object slippage and play a major role in the kinesthetic sense and control of finger position and movement. Merkel and bulbous cells-slow-response-are myelinated; the rest-fast-response-are not. All of these receptors are activated upon pressures that squish their shape causing an action potential. The postcentral gyrus includes the primary somatosensory cortex collectively referred to as S1. BA3 receives the densest projections from the thalamus. BA3a is involved with the sense of relative position of neighboring body parts and amount of effort being used during movement. BA3b is responsible for distributing somatosensory information, it projects texture information to BA1 and shape and size information to BA2. Region S2 divides into Area S2 and parietal ventral area. Area S2 is involved with specific touch perception and is thus integrally linked with the amygdala and hippocampus to encode and reinforce memories. Parietal ventral area is the somatosensory relay to the premotor cortex and somatosensory memory hub, BA5. BA5 is the topographically organized somato memory field and association area. BA1 processes texture info while BA2 processes size + shape info. Area S2 processes light touch, pain, visceral sensation, and tactile attention. S1 processes the remaining info. BA7 integrates visual and proprioceptive info to locate objects in space. The insular cortex plays a role in the sense of bodily-ownership, bodily self-awareness, and perception. Insula also plays a role in conveying info about sensual touch, pain, temperature, itch, and local oxygen status. Insula is a highly connected relay and thus is involved in numerous functions. The somatosensory system is spread through all major parts of the vertebrate body. It consists both of sensory receptors and afferent neurons in the periphery, to deeper neurons within the central nervous system. All afferent touch/vibration info ascends the spinal cord via the posterior column-medial lemniscus pathway via gracilis or cuneatus.