Responses of slowly adapting type II afferent fibres in cat hairy skin to vibrotactile stimuli.

1. Slowly adapting type II (SAII) afferent fibres that supply the forelimb were isolated from the medial cutaneous nerve of anaesthetized cats and examined for their capacity to signal information about vibrotactile events in the hairy skin. 2. The SAII fibres had a single spot-like receptive field focus where they were highly sensitive to steady indentation and vibration applied with probes normal to the skin surface. However, their sensitivity was affected profoundly by the size of the stimulus probe, its position in relation to the receptive field focus and, to a lesser extent, the magnitude of any pre-indentation on which vibration was superimposed. Small stimulus probes (e.g. 250 microns diameter) were much more effective than larger (> or = 1-2 mm) ones, and small shifts in the position of the perpendicularly applied probe away from the receptive field focus led to a marked decline in responsiveness. 3. With appropriate choice of stimulus parameters for vibratory stimuli applied at the receptive field focus, the SAII fibres could respond at low threshold (< 100 microns), with a tightly phase-locked, regular 1:1 impulse pattern (one impulse per vibration cycle) that accurately signalled the vibration frequency over a bandwidth that extended to 600 Hz. Furthermore, their responses remained phase-locked up to 1000 Hz. Phase-locking in SAII fibres was marginally tighter than that in SAI fibres and comparable to that of Pacinian corpuscle fibres. 4. The sensitivity of forelimb SAII fibres to tangential skin stretch was directionally selective; stretch across the forelimb was much more effective than along its long axis. Vibration associated with tangential skin stretch led to a marked spatial expansion of the field of vibration sensitivity. SAII fibres could therefore signal information about natural stimuli that contain elements of skin stretch and vibration, as may be encountered when the forelimb brushes against textured surfaces. Should the SAII fibres fail to contribute to the sensory experience of vibrotactile stimuli, the explanation may be related to limitations imposed centrally on the processing of their signals. Nevertheless, the present results demonstrate that, with appropriate stimulus conditions, the SAII afferent fibres have much greater vibrotactile sensitivity than has been suggested by past studies.