Actions of trains and pairs of impulses from single primary afferent fibres on single spinocervical tract cells in cat.

1. In cats under chloralose anaesthesia single lumbosacral dorsal root ganglion cells of hair follicle afferent fibres were stimulated intracellularly to produce trains or pairs of impulses. At the same time, single spinocervical tract (s.c.t.) neurones were recorded extracellularly, from their axons in the upper lumbar spinal cord. Afferent fibre-neurone pairs were chosen in which the receptive field of the fibre was contained within the excitatory receptive field (firing zone) of the neurone. 2. Trains of impulses of 2.0 Hz were less effective in increasing the probability of s.c.t. cell firing than trains at 0.67 Hz, and this latter rate was usually less effective than trains at 0.33 Hz. 3. Successive responses to individual members of a train of hair follicle afferent impulses were variable. In some pairs of units succeeding responses declined until a fairly consistent plateau was reached. In others there was no decline and the responses remained irregular. 4. Pairs or short trains of impulses revealed two phenomena: over the first 5 ms or so following an impulse in a group II hair follicle afferent fibre, a second or small group of impulses produced a greater response from the s.c.t. neurone but at intervals of 25-200 ms there was a profound depression of the responses evoked by the second member of a pair of impulses. For A delta afferent fibres the early facilitation lasted for at least 25 ms. 5. It is concluded that a single impulse in a single hair follicle afferent fibre from within the excitatory receptive field of a s.c.t. neurone has complex actions on transmission through that neurone. An initial excitatory influence is followed by a long-lasting depression that influences transmission through the system for at least 1500 ms. Possible mechanisms underlying this depression are discussed.