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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.

SUMMARY1. Extracellular microelectrode recordings were made from projection neurones of the lateral cervical nucleus (LCN) in cats anaesthetized with chloralose and paralysed with gallamine triethiodide.2. The receptive fields of eighty-five units were analysed. Most units had excitatory receptive fields similar in size and shape to those of spinocervical tract (SCT) cells. A few (14 %) had either very large fields or 'stocking-like' fields. The majority of the LCN neurones (fifty-five, 65 %) were excited by hair movement and, in addition, by noxious rnechanical stimulation within the skin area responding to hair movement. Twenty-five units (29 %) were excited by hair movement alone. For seven of these twenty-five neurones, noxious mechanical stimulation within the excitatory receptive field produced inhibition of the background discharge. One unit was excited by noxious mechanical stimulation and for the remaining four units no receptive field could be found. In six units inhibitory receptive fields outside the excitatory field were found.3. Air-jet stimuli were used to define the excitatory profiles of the units' receptive fields to hair movement. In general, receptive fields had single regions of greatest sensitivity usually at or near the centre of the field, where that was oval in shape, with the sensitivity declining towards the field's circumference. In some units with very large fields that included parts of one or two limbs and the trunk there could be more than one highly sensitive region.4. Pairs of air-jet stimuli were used to investigate in-field afferent inhibition in LCN cells. One jet was used to condition the responses to another jet located at a different position within the excitatory receptive field and occurring 200 ms later. Sixteen units were tested and significant in-field inhibition was observed in all sixteen.5. The in-field afferent inhibition was organized spatially in the sense that inhibition was generally strongest when the conditioning and testing stimuli were close together and became weaker as they were moved apart. The afferent inhibition was not simply a function of the response produced by the conditioning stimulus.Furthermore, increasing the strength of the stimuli did not in general lead to larger areas from which the inhibition could be produced. The inhibitory areas defined in these experiments were generally less than 120 mm in length in units with receptive fields much longer than 100 mm.A. G. BROWrN, D. J. AIAXIVELLANDA. D. SHORT 6. We conclude that: (1) neurones of the LCN that project through the medial lemniscus have the same classes of excitatory inputs as neurones of the SCT that excite them -there is no convergence from different classes of SC(T cells onto LCN projection neurones; (2) the major, and perhaps the only. operation carried out at the level of the LCN is a considerable degree of spatial summation for a small subpopulation of neurones, although most LCN neurones have receptive fields no larger than those of SCT cells; (3) the in-field afferent inhibitio...

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