Spatial spread of in‐field afferent inhibition in the cat's spinocervical tract.

Abstract

SUMMARY1. Extracellular microelectrode recordings were made from twenty-three spinocervical tract (SCT) cells in the lumbar spinal cord of cats anaesthetized with chloralose and paralysed with gallamine triethiodide. Excitation and inhibition of the cells were elicited by applying small brief (4 mN, 60 ms) localized jets of air to the clipped hair in and around the receptive fields.2. Receptive field extents ranged from 40 to 180 mm. Excitation occurred in the period 30-130 ms after the start of the stimulus, and in-field afferent inhibition from 130 ms up to 700 ms or more. The inhibition was manifest as a reduction in background discharge and as a reduction in responsiveness to a test stimulus which followed a conditioning stimulus.3. When the conditioning stimulus was spatially separated from the test stimulus, the degrees of in-field afferent inhibition depended on the spatial separation, even when both were within the excitatory receptive field. The spatial spread of in-field afferent inhibition was limited to 100 mm or less.4. In two units only, afferent inhibition was produced from a narrow strip just outside the excitatory receptive field. In the other units, it could only be produced from within the excitatory receptive field.5. The results suggest that the inhibitory input to SCT cells is organized in subdomains no more than 100 mm across, which may correspond to the receptive fields of interneurones between the primary afferent fibres and the SCT cells.

Receptive fields and in‐field afferent inhibition of neurones in the cat's lateral cervical nucleus.

“…Janig, Schoultz & Spencer (1977), Janig, Spencer & Younkin (1979) and Laskin & Spencer (1979) have carried out somewhat similar experiments to ours but recording from the dorsal column nuclei (cuneate), the thalamus and the somaesthetic cortex respectively. Their results establish the presence of in-field afferent inhibition at these sites but it appears to have a different spatial organization in comparison to that shown in this and the preceding paper (Noble & Short, 1989). Thus: (1) the inhibitory areas tended to extend beyond the excitatory ones; (2) inhibition was most effective when pitted against excitation from near the borders of the excitatory field (Jiinig et al 1977) even though the maximal inhibition was evoked from the receptive field centre (that is, the positions of maximal excitation and inhibition coincided); (3) no evidence for the presence of inhibitory subdomains within the excitatory fields were observed with the experimental protocol used.…”

“…6). This experimental protocol was the same as that used in the accompanying paper (Noble & Short, 1989) and the results give the impression that the degree of inhibition produced is dependent on the strength of the response evoked by the conditioning jet. Such a conclusion is incorrect as is shown when units with larger fields are examined and the locations of conditioning and testing stimuli are moved about.…”

“…When two air-jet stimuli were used, within the excitatory receptive field, one to condition the response from the other location, in-field afferent inhibition similar to that seen in SCT cells (Noble & Short, 1989) was observed. Its organization was similar to that in the SCT with a maximum spatial spread of about 120 mm even in LCN neurones with large or very large fields that included parts of the trunk and all of a limb or more.…”

“…Thermal stimulation was not used. Quantitative, timed, mechanical stimulation was provided with air-jet stimuli, usually of 4 mN force, as described in the accompanying paper (Noble & Short, 1989). In order to examine for in-field afferent inhibition, pairs of air-jet stimuli, with one location fixed and the other selected sequentially from the array of air nozzles, were used and the data analysed as described by Noble & Short (1989).…”

“…Brown et al (1987 b,c) showed that transmission between a single hair follicle afferent fibre and a SCT cell was suppressed following activation, and this suppression could be evoked from throughout the excitatory receptive field of the SCT neurone and was not accompanied by signs of postsynaptic inhibition in the SCT cell. This phenomenon of in-field afferent inhibition has been analysed using air-jet stimuli (see the accompanying paper, Noble & Short, 1989) and it was shown that SCT excitatory receptive fields contain suppressive subdomains likely to correspond with the receptive fields of neurones intercalated between the hair follicle afferent fibres and the SCT cells. One of the objectives of the present series of experiments was to examine the target neurones of the SCT (the neurones of the lateral cervical nucleus (LCN)) for evidence of this in-field afferent inhibition and its organization after another series of synaptic relays.…”