SYNAPTIC TRANSMISSION BETWEEN SINGLE SLOWLY ADAPTING TYPE-I FIBERS AND THEIR CUNEATE TARGET NEURONS IN CAT

1. The synaptic linkage between single, identified slowly adapting typ e I(SAI) fibres and their central target neurones of the cuneate nucle us was examined in pentobarbitone-anaesthetized cats. Simultaneous ext racellular recordings were made from individual cuneate neurones and f rom fine, intact fascicles of the lateral branch of the superficial ra dial nerve in which it was possible to identify and monitor the activi ty of each group II fibre. Individual SAI fibres were activated by sta tic displacement and by vibration delivered with a fine probe (0.25-2 mm diameter) to their associated touch domes in the hairy skin of the forelimb. 2. Transmission properties across the synapse were analysed for nine SAI-cuneate pairs in which the single SAI fibre of each pair provided a suprathreshold input to the cuneate neurone. Neither spatia l nor temporal summation was required for effective impulse transmissi on, and often more than 80 % of BAI impulses led to a response in the cuneate neurone. Responses of the cuneate neurones to single SAI impul ses occurred at a short, fixed latency (S.D. often < 0.1 ms), and freq uently consisted of a burst of two or three impulses, at low SAT input rates in particular. 3. The tight phase-locking in the responses to v ibration of single SAI fibres was preserved in the cuneate responses f or frequencies up to similar to 400 Hz. However, as the impulse rates of the cuneate neurones were less than 150 impulses s(-1), their impul se patterns could not directly signal the vibration periodicity at fre quencies > 100-150 Hz despite 1:1 responses in their single SAI input fibres up to similar to 500 Hz. 4. The reliable transmission of touch dome-associated SAI input across the cuneate nucleus indicates that tr ansmission failure at this first relay is unlikely to be responsible f or the reported failure of touch dome-SAI inputs to contribute to tact ile perception. 5. The transmission characteristics for the SAI fibres were very similar to those demonstrated previously for fibres associa ted with Pacinian corpuscles, which argues against any marked differen tial specialization in transmission characteristics for dorsal column nuclei neurones that receive input from different tactile fibre classe s.