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

Abstract

SUMMARY1. 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 Ad 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.

An intracellular study of spinocervical tract cell responses to natural stimuli and single hair afferent fibres in cats.

“…The experiments described in the present paper clarify some of the issues raised by the results of the preceding ones (Brown et al 1986(Brown et al , 1987 but they also raise many questions. Thus, it is now established that applying an air jet (with the intention of moving hairs) within the firing zone of a s.c.t.…”

“…neurones (Brown & Franz, 1969) and activity of single group III axons from within the s.c.t. cell's firing zone can also excite them and this excitation lasts for at least 25 ms (Brown et al 1987 a).…”

Excitatory actions of single impulses in single hair follicle afferent fibres on spinocervical tract neurones in the cat.

“…Laminectomies were performed to expose the C1-4 cord segments (except in the control experiment) and the lumbosacral cord and dorsal root ganglia L7 and S1 were also exposed. The ipsilateral sciatic nerve was exposed in the thigh and placed, in continuity, on a pair of Ag-AgCl (Brown et al 1987a). …”

“…neurones. When a single action potential was generated in a single dorsal root ganglion neurone at a rate of once every 1500 ms (faster rates led to a reduction in response, see Brown et al 1987a) an impulse discharge of at least one action potential could be observed in the s.c.t. neurone in twenty-one of the pairs (54 %) in at least 50 % of trials (see Fig.…”

“…neurones. In one accompanying paper (Brown, Koerber & Noble, 1987a) the effects of pairs of impulses in the afferent fibre have been studied and in another (Brown, Koerber & Noble, 1987b) the analysis of the response of the s.c.t. cell has been taken to the intracellular level.…”