T-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-10 20 40 60 80 100 120 140 160 msec

FIGURE 2 The compound action potential from the sciatic nerve. The different conduction velocities of sensory fibers from a single nerve are seen in recordings from external electrodes (A). The distal nerve is crushed to prevent the action potentials from reaching the second (reference) recording electrode. Action potentials from the fastest sensory neurons (classified as type I or Ia) travel the 120 mm to the recording electrode in only 1 ms. Smaller fibers are slower, and their action potentials take longer to travel the distance.

a to their original position also changes pressure on the neuronal membrane, and action potentials may again result. During a period of rapid change in pressure, either applied or relieved, layers have less time to compensate and more action potentials will be generated. The physical configuration of the end organ determines what rates of change provide the most potent stimuli. Accordingly, Pacinian corpuscles are most sensitive to rates of change (vibrations) of 200 to 300 Hz.

These simple mechanisms permit the Pacinian corpuscle to act as a rate detector as well as a pressure detector. Rate detectors are critically important in allowing the brain to make predictions about ongoing changes in body position. Pacinian corpuscles and other receptors located in or near the joint capsules help detect rates of movement of the different parts of the body and allow the brain to predict where feet or arms will be during any precise moment during the ongoing movement. Appropriate motor signals can then be sent to make anticipatory corrections.

Ruffini's endings are mechanoreceptors with large receptive fields and sustained responses. They have pressure transduction mechanisms similar to those of Paci-nian corpuscles, but they lack a layered capsule (Fig. 3). Without sliding layers to facilitate dissipation of the pressure, they adapt relatively slowly to a sustained stimulus and thus can provide information about the degree of sustained mechanical distortion of the skin. A Pacinian corpuscle can be made to respond like a Ruffini's ending by experimentally removing its connective tissue capsule.

Two other important mechanoreceptor types are typically found in more superficial layers of glabrous skin. Meissner's corpuscles are about one tenth the size of Pacinian corpuscles and are located in the ridges of glabrous skin, such as the raised parts of the fingerprints. Merkel's disks are located within the epidermis and consist of a nerve terminal and a flattened non-neural epithelial cell. Merkel's disks are slowly adapting, whereas Meissner's corpuscles are rapidly adapting and are most sensitive to low-frequency stimuli of around 50 Hz. Both of these receptors have relatively small receptive fields compared to Pacinian corpuscles or Ruffini's endings; however, absolute receptive field size for any single class of receptor varies from one area of the body to another. This holds true for all types of mechanoreceptors in the skin. The size of the receptive field is inversely proportional to the spatial resolution of the touch sensation, so that the smaller the receptive field, the greater the ability to discriminate. A simple test of spatial resolution is the two-point discrimination test, which measures the minimum distance necessary to differentiate between two simultaneous stimuli. The value varies 20-fold across the body surface, with fingertips showing highest resolution (2 mm) and the back showing the lowest (40 mm).

Hairy skin is innervated by hair follicle receptors, which may be either slowly adapting or rapidly adapting. Each hair follicle is innervated by a single free nerve ending that varies according to hair type. Bending of the hair deforms the associated nerve ending membrane, changes its conductance, and leads to changes in generator potentials and firing rates of the neuron.

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