touch

As noted in , pressure is maintained touch. Touch is present in areas that have no visible specialized receptors. However, pacinian corpuscles and possibly other putative receptors may subsume special functions related to touch. Touch receptors are most numerous in the skin of the fingers and lips and relatively scarce in the skin of the trunk. There are many receptors around hair follicles in addition to those in the subcutaneous tissues of hairless areas. When a hair is moved, it acts as a lever with its fulcrum at the edge of the follicle, so that slight movements of the hairs are magnified into relatively potent stimuli to the nerve endings around the follicles. The stiff vibrissae on the snouts of some animals are highly developed examples of hairs that act as levers to magnify tactile stimuli.

The Na⁺ channel BNC1 is closely associated with touch receptors. This channel is one of the degenerins, so called because when they are hyperexpressed they cause the neurons they are in to degenerate. However, it is not known if BNC1 is part of the receptor complex or the neural fiber at the point of initiation of the spike potential. The receptor may be opened mechanically by pressure on the skin.

The Aβ sensory fibers that transmit impulses from touch receptors to the central nervous system are 5-12 um in diameter and have conduction velocities of 30-70 m/s. Some touch impulses are also conducted via C fibers.

Touch information is transmitted in both the lemniscal and anterolateral pathways, so that only very extensive lesions completely interrupt touch sensation. However, there are differences in the type of touch information transmitted in the two systems. When the dorsal columns are destroyed, vibratory sensation and proprioception are reduced, the touch threshold is elevated, and the number of touch-sensitive areas in the skin is decreased. In addition, localization of touch sensation is impaired. An increase in touch threshold and a decrease in the number of touch spots in the skin are also observed after interrupting the spinothalamic tracts, but the touch deficit is slight and touch localization remains normal. The information carried in the lemniscal system is concerned with the detailed localization, spatial form, and temporal pattern of tactile stimuli. The information carried in the spinothalamic tracts, on the other hand, is concerned with poorly localized, gross tactile sensations.