Touch is actually a combination of various somatic senses, including the sensations of temperature, pressure, and pain; kinesthetic senses that give humans a conception of their body in space (proprioception); and visceral senses, such as stomach aches or nausea. The information from these senses is processed in the postcentral gyrus, corresponding roughly to the top middle area of the brain. The postcentral gyrus, or parts of it, are often referred to as the primary somatosensory cortex, and this area gets more direct sensory input information than any other in the brain.
Along with the sense of smell, touch is one of the most primitive and universal of sensory apparatuses in the kingdom of life. Almost all animals use it to navigate complex environments, appraise their immediate surroundings, and detect the presence of food.
In humans, much of the somatosensory cortex is devoted to processing signals from the hands and face — about 90%. The feeling in these "sensory hot spots" is correspondingly sensitive and high-resolution. A palm can detect the presence of an object only weighing a fraction of a gram. Glabrous or hairless skin contains the finest receptors, called mechanoreceptors, and they translate physical force into nerve impulses. The main four mechanoreceptors in hairless skin areas are Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini corpuscles.
Different mechanoreceptors are specialized to detect different touch sensations, and they can be found at a variety of different depths in the skin. Some mechanoreceptors, such as the Pacinian copulscule, which detects deep pressure and high frequency vibrations, are as large as 0.039 inches (1 mm). Meissner's corpuscles, responsible for light sensations, are about 20 times smaller and located much closer to the surface of the skin.
Free nerve endings, the most common type of skin receptor, are cell-sized and do most of the work of the somatosensory system. The method of using a free nerve ending for touch reception is very old evolutionarily — the basic principle has not changed since the most primitive forms of multicellular life emerged about 600 million years ago.
