Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Cutaneous, Deep, and Visceral Sensation
Touch

As noted in Chapter 5, pressure is sustained touch. Touch receptors are present in areas lacking specialized receptors; where these receptors do exist, Meissner's and Pacinian corpuscles act as rapidly adapting touch receptors, while Merkel disks and Ruffini endings function as slowly adapting touch receptors. Touch receptors are most numerous in the Skin of the fingers and Lips, and relatively sparse in the skin of the torso. In addition, many receptors are located around Hair follicles. When a hair is moved, it acts as a lever with a fulcrum at the Base of the follicle, thereby converting slight hair movements into relatively strong stimuli for the nerve endings surrounding the follicles. The stiff vibrissae on the snouts of certain animals serve as an example of highly specialized hairs that act as levers to amplify tactile stimuli. Pacinian corpuscles are found in the subcutaneous tissue, Muscles, joints, and skin. They are also abundant in the mesentery of the Small Intestine, though their function there remains unknown.

Aß sensory fibers, which transmit impulses from touch receptors to the Central Nervous system, have a diameter of 5-12 µm and a conduction velocity of 30-70 m/s. Some tactile impulses are also conveyed via C fibers.

Tactile information is transmitted via both the dorsal Column-medial lemniscal and anterolateral pathways, which is why only very extensive lesions completely abolish tactile sensation. However, there are differences in the type of tactile information transmitted by these two systems. When the dorsal columns are destroyed, vibration sense and Proprioception are impaired, the tactile threshold is elevated, and the number of cutaneous touch-sensitive spots is reduced. Furthermore, The ability to localize touch is impaired. An elevated touch threshold and a decrease in the number of tactile spots in the skin are also observed following interruption of the spinothalamic tracts. In this case, the deficit in tactile sensation is minimal, and the localization of touch spots remains normal. Information transmitted via the dorsal column system is associated with detailed localization, spatial form, and temporal patterns of tactile stimuli. Conversely, information conveyed through the spinothalamic tracts is related to gross tactile sensations of crude localization.