somatosensory system somatosensory system in physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position (proprioception), and pain. it is a subset of the sensory nervous system, which also represents visual, auditory, olfactory, and gustatory stimuli. touch is a crucial means of receiving information. this photo shows tactile markings identifying stairs for visually impaired people. somatosensation begins when mechano- and thermosensitive structures in the skin or internal organs sense physical stimuli such as pressure on the skin (see mechanotransduction, nociception). activation of these structures, or receptors, leads to activation of peripheral sensory neurons that convey signals to the spinal cord as patterns of action potentials. sensory information is then processed locally in the spinal cord to drive reflexes, and is also conveyed to the brain for conscious perception of touch and proprioception. note, somatosensory information from the face and head enters the brain through peripheral sensory neurons in the cranial nerves, such as the trigeminal nerve. the neural pathways that go to the brain are structured such that information about the location of the physical stimulus is preserved. in this way, neighboring neurons in the somatosensory cerebral cortex in the brain represent nearby locations on the skin or in the body, creating a map, also called a cortical homunculus. system overview this diagram linearly (unless otherwise mentioned) tracks the projections of all known structures that allow for touch to their relevant endpoints in the human brain. sensory receptors 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 (5–15 hz) and deep static touch such as shapes and edges. due to having a small receptive field (extremely detailed information), they are used in areas like fingertips the most; they are not covered (shelled) and thus respond to pressures over long periods. tactile corpuscles react to moderate vibration (10–50 hz) 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. pacinian corpuscles determine gross touch and distinguish rough and soft substances. they react in quick action potentials, especially to vibrations around 250 hz (even up to centimeters away). they are the most sensitive to vibrations and have large receptor fields. pacinian corpuscles react only to sudden stimuli so pressures like clothes that are always compressing their shape are quickly ignored. they have also been implicated in detecting the location of touch sensations on handheld tools. 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. somatosensory cortex gray's anatomy, figure 759: the sensory tract, showing the pathway (blue) up the spinal cord, through the somatosensory thalamus, to s1 (brodmann areas 3, 1, and 2), s2, and ba7 gray's anatomy, figure 717: detail showing path adjacent to the insular cortex (marked insula in this figure), adjacent to s1, s2, and ba7 the postcentral gyrus includes the primary somatosensory cortex (brodmann areas 3, 2 and 1) 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 (secondary somatosensory cortex) 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 and shape information. area s2 processes light touch, pain, visceral sensation, and tactile attention. s1 processes the remaining info (crude touch, pain, temperature). ba7 integrates visual and proprioceptive info to locate objects in space. the insular cortex (insula) 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. structure the somatosensory system is spread through all major parts of the vertebrate body. it consists both of sensory receptors and sensory neurons in the periphery (skin, muscle and organs for example), to deeper neurons within the central nervous system. general somatosensory pathway