Touch is a combination of somatic senses such as temperature, pressure, and pain. The postcentral gyrus processes this information and is referred to as the primary somatosensory cortex. Touch is a primitive and universal sense used by animals to navigate and detect food. Humans have sensitive sensory hot spots in their hands and face, with hairless skin containing the finest mechanoreceptors. Different mechanoreceptors detect different tactile sensations and free nerve endings do most of the work in the somatosensory system.
Touch is actually a combination of various somatic senses, including sensations of temperature, pressure, and pain; kinesthetic senses which give man a conception of his own body in space (proprioception); and visceral senses, such as stomach pain or nausea. Information from these senses is processed in the postcentral gyrus, which roughly corresponds to the upper central area of the brain. The postcentral gyrus, or parts of it, are often referred to as the primary somatosensory cortex, and this area receives more direct sensory input information than any other in the brain.
Along with smell, touch is one of the most primitive and universal sensory apparatuses in the realm of life. Nearly all animals use it to navigate complex environments, assess their immediate surroundings, and detect the presence of food.
In humans, a large part of the somatosensory cortex is dedicated to processing signals from the hands and face – about 90%. Sensation in these “sensory hot spots” is correspondingly sensitive and high-resolution. A palm can detect the presence of an object weighing only a fraction of a gram. Hairless or glabrous skin contains the finest receptors, called mechanoreceptors, and they translate physical force into nerve impulses. The four major mechanoreceptors in hairless skin areas are Pacinian corpuscles, Meissner corpuscles, Merkel discs, and Ruffini corpuscles.
Different mechanoreceptors are specialized to detect different tactile sensations and can be found at a variety of different depths in the skin. Some mechanoreceptors, such as the Pacinian copulcle, which senses 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 are located much closer to the skin surface.
Free nerve endings, the most common type of cutaneous receptor, are cellular in size and do most of the work of the somatosensory system. The method of using a free nerve ending to receive touch is evolutionarily very ancient: the basic principle hasn’t changed since the most primitive forms of multicellular life emerged about 600 million years ago.
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