RESPONSES TO ELECTRICAL STIMULATION OF SINGLE SENSORY UNITS OF SKIN

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Abstract
Skin sensory spots can be stimulated with precisely controlled shocks by inserting into the circuit a spark gap as a high resistance, and allowing the stimuli to pass to the skin from a needle too flexible to stimulate mechanically, or held above the skin out of mechanical contact. Apparatus for this is described. Pain spots, touch spots and hair touch endings can be accurately located, and the sensory responses to various types of stimulation of single spots employed as a means of characterising them. Hair touch is located at single points close to the hair shafts. General skin touch is distributed in areas differing in size in different regions, and often having one central point of higher sensitivity. Pain spots consist of minutely localised central points of high sensitivity, surrounded by areas of decrementing sensitivity. From a single pain spot can be induced a low grade contact sense at threshold, itch from repeated stimulation just above threshold, non-painful prick by stronger single or repetitive shocks, and sharp pain by single, and particularly by repeated strong shocks. Prick spots have in general a lower electrical threshold than do touch spots. These spots are the same as those located by mechanical stimulation with a sharp needle. With the information available from animal experiments where impulses in single fibers from sense organs have been recorded, the character of the nerve discharge following sense organ stimulation may be correlated with the type of sensation induced. It may also be inferred that sensory endings differing in properties from nerve fibers are being stimulated, and that the responses of these endings differ from those of nerve fibers in not being all-or-none.

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