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. 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. 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.