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

The impulse response to vibration (0.5-400 Hz, 0.001-1.0 mm) was recorded from single mechanoreceptive afferents innervating the glabrous skin of the human hand. Needle electrodes inserted into the median nerve were used for the recording. The four types of mechanoreceptive afferents (FA I, FA II, SA I, and SA II) exhibited different response characteristics. Fast adapting units were the most easily excited at frequencies between about 5 to 50 Hz (FA I) and above about 50 Hz (FA II). The sensitivity of the slowly adapting units (SA I and SA II) was greatest at lower frequencies. Unit thresholds at 2, 20, and 200 Hz were measured before and after 2 min of powerful vibration exposure. Corresponding psychophysical thresholds were also measured during the recording of responses from single units. An acute but temporary depression in sensitivity occurred in the FA I, FA II, and SA I units as a consequence of the exposure. The magnitude and the time courses of the recovery of the encountered unit threshold shifts were approximately the same as those for the corresponding psychophysical threshold shifts. It was concluded that acute impairments of the tactile sensibility caused by vibration exposure, as observed in psychophysical studies, can probably be explained by an influence on the excitability of the tactile units.