) and it does not alter the membrane conductance of squid giant axons (Hodgkin, 1947). However, in a recent series of experiments where acetylcholine was injected close-arterially into the cat's skin to study its excitatory effect on the endings of sensory non-myelinated fibres of mammalian cutaneous nerves (Douglas & Ritchie, 1960), a striking change was often observed in the shape of an antidromic C potential recorded from the nerve near the point where it left the skin. The type of change observed suggested that the after-potential had been altered. The affected region of the nerve was close to the skin, and it was observed that often the closearterial injection not only reached the skin, but also perfused the nerve. It thus seemed likely that externally applied acetylcholine could alter the electrophysiological properties of mammalian C fibres, but with the techniques then used no definite conclusions could be reached. The present experiments use a technique more suited to this particular problem and deal with the action of acetylcholine on non-myelinated fibres in isolated mammalian nerves. METHODSAdult rabbits were anaesthetized with urethane (1.6 g/kg) given into a marginal ear vein as a 25 % (w/v) solution. A length of about 60-70 mm of a cervical vagus nerve was excised and then desheathed under the microscope (magnification x 25) by dissection with sewing needles sharpened to fine scalpels. In several experiments a similar length of the thoracic *

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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receptor fibres from the cat's pad has been investigated by C. J. Armett, J. A. B. Gray, R. W. Hunsperger and S. Lal (unpublished), who have also investigated the way in which this code is modified at the first synaptic junction.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:

). However, recent experiments have shown that it acts on mammalian non-myelinated fibres to depolarize them, reduce their spike height, and enhance their spike after-positivity and slow conduction, and that this action is reduced by eserine (Armett & Ritchie, 1960).The aim of the present experiments was to determine to what extent the action ofacetylcholine on mammalian non-myelinated fibres is common to other cholinesters, to choline itself and to some alkaloids which are pharmacologically related to acetylcholine, and also to see how this action is influenced by different types of anticholinesterase and by drugs which block some of the actions of acetylcholine, such as tubocurarine, hexamethonium and atropine. METHODSDesheathed vagus nerves were obtained from rabbits anaesthetized with urethane (1-6 g/kg given into a marginal ear vein as a 25% w/v solution) and mounted in the sucrose-gap apparatus originally described by Stdmpffi (1954) and developed by Straub (1956, 1957). The methods were similar to those described earlier (Armett & Ritchie, 1960). Resting and action potentials of a desheathed nerve were recorded through a pair of AgAgCl non-polarizable cotton-wick electrodes, fed into a low grid-current cathode-follower and amplifier, displayed on a cathode-ray oscilloscope and photographed. The amplifier was directly-coupled and there was a 20% fall in its response at a frequency of about 15 kc/s with the input impedance presented by the sucrose-gap.

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