Responses in the posterior lobe of the rat cerebellum to electrical stimulation of cutaneous afferents to the snout.

1. Responses in the cortex of the posterior lobe of the cerebellum to electrical stimulation of afferent fibres from the skin of the snout have been analysed in decerebrate and pentobarbitone-anaesthetized rats by means of surface and microelectrode records. Single shocks were applied either to the exposed follicles of the mystachial vibrissae or to the infraorbital branch of the trigeminal nerve. 2. In decerebrate rats responses were mediated only via mossy fibre afferents. Stimulation of one side of the snout yielded responses with mean latency 2.4 ms throughout the uvula (largest ipsilaterally and in lobule IXa). Smaller responses with similar latency were present in both cerebellar hemispheres (largest ipsilaterally). The earliest discharges of Purkinje cells in lobule IXa occurred at latencies between 4.5 and 8.5 ms. 3. All components of the extracellular field potentials generated within the cortex by the mossy fibre input were detectable by surface recording with ball electrodes. 4. The earliest surface potentials had a latency of 0.55 ms (peak latency 0.8 ms); they arose through volume conduction from the brain stem of a potential which signalled arrival of the primary afferent volley. The short delay between this event and the arrival of the mossy fibre volley in the cerebellum suggests that only one synaptic relay occurs in the brain stem. 5. In pentobarbitone-anaesthetized rats surface responses mediated via mossy fibres persisted and were accompanied at slightly higher threshold by responses shown to be mediated via climbing fibres. The latter were present in descending order of amplitude in three sagittally directed zones, one in contralateral Crus 2 (minimum latency 13 ms), one in the vermis contralaterally near the mid line in lobule IXa (latency 16 ms) and a third in ipsilateral Crus 2 (latency 20 ms) 6. In the hemisphere the responses mediated via climbing fibres occurred within the somewhat larger zones activated via mossy fibres but in the vermis the two types of trigemino-cerebellar input influenced quite separate areas of cortex.