Stimulus-response functions of slowly adapting mechanoreceptors in the human glabrous skin area

1. Single unit impluses were recorded from the ulnar and median nerves of awake human subjects with tungsten electrodes inserted percutaneously in the upper arm. 2. One hundred and one slowly adapting receptors with receptive fields in the glabrous skin area were studied. The units were classified as type SA-I and type SA-II largely on the basis of their responses to lateral stretching of the skin. Eighty-eight receptors did not respond to this type of stimulus (type SA-I), whereas thirteen receptors readily responded to stretching (type SA-II), AND OFTEN EXHIBITED DIRECTIONAL SENSITIVITY. 3. The SA-I receptors showed no spontaneous discharge, and the discharge pattern was mostly rather irregular, whereas most of the SA-II receptors had a spontaneous discharge, and a very regular discharge pattern. 4. The conduction velocities of the afferent were all in a A alpha range. The mean value for the SA-I receptors was 58-7 plus or minus 2-3m/sec, and for the SA-II receptors 45.3 plus or minus 3.6 m/sec. 5. The neural response to stimuli of varying skin indentation amplitudes was analyzed. The threshold for a dynamic response ranged for the SA-I receptors from 0.15 to 1.35 mm and for the SA-II receptors from 0.25 to 0.95 mm. The threshold for a static discharge ranged for the SA-I receptors from 0.25 to more than 2.0 mm and for the SA-II receptors from 0.55 to 1.65 mm. 6. The stimulus-response functions were analysed for 25 SA-I receptors and 2 SA-II receptors. A hyperbolic log tangent function was the best description when the neural response was defined as the total number of impluses evoked by a stimulus of 1 sec duration. When only the static part of this type of plot was analyzed, a power function was a very good description for many units, but other functions (linear, logarithmic exponential, log tanh) were equally good or better for many units. This was also the dase when the mean impulse frequency of the sustained discharge was defined as a measure of the neural response. These two latter types of plots were clearly negatively accelerating, the exponent of the power function being 0.66 (mean).