Excitation of receptors in the pad of the cat by single and double mechanical pulses

Abstract

Information about the strength and position of one or more points of mechanical contact with the skin may be transmitted in a number of nerve fibres acting as an organized group. The coding of such information in a group of receptor fibres from the cat's pad has been investigated by C. J. Armett, J. A. B. Gray, R. W. Hunsperger and S. Lal (unpublished), who have also investigated the way in which this code is modified at the first synaptic junction. A single stimulus, having a particular position and strength, sets up in the primary fibres a pattern of activity which is, within limits, uniquely related to that stimulus. The mechanism which determines this pattern is naturally of interest, because an understanding of it would enable the patterns of activity set up by more complicated combinations of stimuli to be predicted. Basic points in this problem are the size of the receptive fields and the amount of overlap between them. Such receptive fields might be determined by a branching of the receptor nerve fibre to cover the area; on the other hand a single receptor in the centre of the field would be all that would be required if the stimulus set up a mechanical wave that travelled across the pad. The experiments described in this paper are concerned with this problem, and the results indicate that a mechanical wave does travel across the pad as a result of each stimulus and that this wave is important in determining the pattern of activity that results in the primary fibres.The first section of the results deals with experiments in which single stimuli were used to excite single receptor units. The second section deals with the effects of a mechanical pulse on the excitability of the units to a second pulse. METHODSThe experiments were performed on cats anaesthetized with chloralose, 005 g/kg, and urethane, 0-5 g/kg. The lumbar cord was exposed to give access to the dorsal roots, and the animal was then mounted rigidly in a frame (cf. Fernandez de Molina & Gray, 1957); steel * Present address:

The neural signal for skin indentation depth. I. Changing indentations

“…It is well known that the indentation thresholds of receptors increase as a stimulator is moved away from the focus of the receptive field (Armett and Hunsperger, 1961;Janig, 1971;Tuckett et al, 1978), and such shifts were seen in the present study. Therefore, as stimulator depth increases, there will be a progressive recruitment of receptors at greater and greater distances from the stimulator (lateral recruitment).…”

Responses of the rat dorsal column system to mechanical stimulation of the hind paw

“…The results did not allow an accurate estimate of the conduction velocity of first-order axons to be made, since the time occupied by the initiation of impulses was not measured; for this reason the conduction velocity of the fastest axons must have been greater than 50 m/sec, as the distance to C4 level was roughly 20 cm (19-21 cm in four animals) and the earliest activity in this region had a latency of 4 msec. This value of conduction velocity may be compared with that found by Armett & Hunsperger (1961) for phasic mechanoreceptor units in the pad of the cat (46-85 m/sec).…”

“…The results do indicate that some cells in the middle section might be monosynaptically connected to fast-conducting primary units and others could have monosynaptic inputs if they were driven from slower conducting fibres which might have been missed by the extracellular techniques employed. However, there is no certain evidence that slower conducting fibres are excited by mechanical pulses of the sort used in these experiments (see also cat's pad, Armett & Hunsperger, 1961;Armett et al 1962). The view taken by Perl et al (1962) is that primary receptor units responsive to hair movement are connected to cells with small receptive fields and that these cells are found predominantly in the middle section of the nucleus.…”

“…These inputs consisted of activity evoked in a population of primary receptor units which had axons running in the dorsal column; each unit could be excited by mechanical stimulation of a certain area of skin to fire most commonly a single impulse which arrived in the vicinity of the nucleus after a measured delay. A more detailed analysis of the patterns of activity evoked in a population of primary receptor units has been made in the cat (Armett & Hunsperger, 1961;Armett et at. 1962).…”

Projections from Pacinian corpuscles and rapidly adapting mechanoreceptors of glabrous skin to the cat's spinal cord.

“…In addition, according to Janig et al (1968 a) the receptors activated in the studies of Gray and his collaborators (Armett & Hunsperger, 1961;Armett et al 1962) may be positively identified as Pacinian corpuscles. It may be concluded, therefore, that the population of interneurones in the medial part of the dorsal horn studied by Armett et al (1961,1962) …”