The Sensory Neurons of Touch

Although previous physiological and anatomical experiments have identified four afferent fiber types (PC, RA, SA II, and SA I) in glabrous (nonhairy) skin of the human somatosensory periphery, only three have been shown to mediate tactile (mechanoreceptive) sensation. Psychophysical evidence that four channels (P, NP I, NP II, and NP III) do, indeed, participate in the perceptual process is presented. In a series of experiments involving selective masking of the various channels, modification of the skin‐surface temperature, and testing cutaneous sensitivity down to very low‐vibratory frequencies, the fourth psychophysical channel (NP III) is defined. Based on these experiments and previous work from our laboratory, it is concluded that the four channels work in conjunction at threshold to create an operating range for the perception of vibration that extends from at least 0.4 to greater than 500 Hz. Each of the four channels appears to mediate specific portions of the overall threshold‐frequency characteristic. Selection of appropriate neural‐response criteria from previously published physiological data and correlation of their derived frequency characteristics with the four psychophysical channels indicates that each channel has its own physiological substrate: P channel and PC fibers, NP I channel and RA fibers, NP II channel and SA II fibers, and NP III channel and SA I fibers. These channels partially overlap in their absolute sensitivities, making it likely that suprathreshold stimuli may activate two or more of the channels at the same time. Thus the perceptual qualities of touch may be determined by the combined inputs from four channels.

Four channels mediate the mechanical aspects of touch

Recordings from single peripheral nerve fibres made it possible to analyse the functional properties of tactile afferent units supplying the glabrous skin of the human hand and to assess directly the relation between impulse discharge and perceptive experiences. The 17,000 tactile units in this skin area of the human hand are of four different types: two fast adapting types, FA I and FA II (formerly RA and PC), and two slowly adapting types, SA I and SA II. The receptive field characteristics and the densities in the skin of the type I units (FA I and SA I) indicate that these account for the detailed spatial resolution that is of paramount importance for the motor skill and the explorative role of the hand. The relationship between the stimulus amplitude and perceived intensity during sustained skin indentations did not match the corresponding stimulus response functions of SA units suggesting non-linear transformations within the central nervous system. These transformations, in turn, appear to vary between subjects. A single impulse in a single FA I unit may be felt when originating from the most important tactile regions of the hand, indicating that the psychophysical detection may be set by the threshold of the sense organs. Moreover, no significant noise seems to be superimposed in the respective central sensory pathways.

Properties of cutaneous mechanoreceptors in the human hand related to touch sensation.

Although information from sense organs is used by all animals, only man can verbally report his sensory experience. A brief consideration of these experiences may be useful in providing insight into cutaneous sensory mechanisms. Nonnoxious deformation of the human skin with a stimulator having a small surface area (2–3 mm2) evokes sensations usually referred to as “touch”. If the stimulator remains stationary, the sensation fades. Movement of the stimulator perpendicular to the skin produces changes in the sensory process which allow even rapidly repeated stimuli (e.g. 500 Hz) to be distinguished from stationary ones. When larger stimulators are used, some appreciation of texture can be obtained, and this is enhanced by movement of the stimulating surface across the skin. A smooth stimulator or an insect moving slowly and gently over the skin evokes distinctive sensations usually referred to as “tickle”. A sharp object such as a pin lightly pressed against certain points in the skin leads to an unpleasant experience which differs from touch but is equivocally painful; when the pin is pressed against the skin more firmly it usually elicits pain. Thus, sensations induced by mechanical events can be diverse. Moreover, common experience indicates that they may occur with greater or lesser intensity. On the other hand, neurophysiological experiments defining the properties of cutaneous sense organs have most often been done on animals other than man.

Cutaneous Mechanoreceptors and Nociceptors

Responses of mechanoreceptive afferent units in the glabrous skin of the human hand to sinusoidal skin displacements.

The human capacity to scale the intensity of brief mechanical indentations of the hairy skin was measured by the method of subjective magnitude estimation. The afferent discharges evoked by nearly identical stimuli delivered to comparable locations on the hairy skin in monkeys were recorded in a number of types of first-order mechanoreceptive afferents. It was shown that of these only those which adapt slowly to mechanical stimuli possess sufficient dynamic range of response to account for the range of human pressure sensation. Two such afferents innervating the hairy skin, previously identified by others as the Type I and Type II afferents, were studied in detail as regards their responses to brief mechanical stimuli of different intensities. The relation of the human subject's estimate of intensity to stimulus magnitude is described by a power function with an exponent of about 0.4. Using impulse frequency in the “early steady state” as a measure, the responses of both types of slowly adapting mechanoreceptive afferents are related to stimulus magnitudes by power functions with exponents averaging about 0.5. On the assumption that the stimuli used elicit similar patterns of discharge in the first-order afferents innervating the hairy skin of men and monkeys, the results support the general proposition, now shown to hold for a number of senses, that the intensity of human sensation is determined by the functional properties of the first-order sensory fibers, and that the final behavioral output in the form of a subjective estimate of magnitude is a linear function of the frequency of discharge in those first-order fibers.

Neural coding in the sense of touch: human sensations of skin indentation compared with the responses of slowly adapting mechanoreceptive afferents innvervating the hairy skin of monkeys.

Thomas Harrington

Papers

Neural coding in the sense of touch: human sensations of skin indentation compared with the responses of slowly adapting mechanoreceptive afferents innvervating the hairy skin of monkeys.