Interaction of Activity in Frog Skin Touch Afferent Units

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1. Single-axon discharges were recorded from a dorsal cutaneous nerve in frog (Rana pipiens). Following identification of two rap- idly adapting touch spots innervated by the same axon, these two spots-which were at least 1,500 pm apart-were mechanically stimulated sequentially or repetitively with independent trains of I-ms pulses.

2. Using the conditioning-testing technique, it was demonstrated that the initiation of an action potential from one spot caused a significant decrease in excitability in the other spot. The duration of this decrease (interspot recovery curve) usually lasted 100-200 ms.

3. The two spots were stimulated individually and simultaneously by repetitive mechanical pulses; such dual-spot periodic stimulation revealed two modes of interaction of impulse discharges from two sources.

4. By careful adjustment of both the stimulus intensity to near threshold for initiating a 1:1 response from each spot as well as the duration of the mechanical pulse train, only impulses from that spot stimulated at the higher rate were seen in the parent axon (resetting).

5. Increase in stimulus intensity resulted in mixing of impulses originating from individual spots. Such impulse mixing was the more typical mode of interaction observed in our preparations. The degree of mixing was shown to be a function of stimulus intensity.

6. Based on the above findings and with some assumptions, a simple model is presented, This model can reproduce the modes of interaction observed in our experiments,