[Excitable actions of counterirritants on cutaneous sensation]

Studies were conducted on the actions of counterirritants on sensory receptors in the skin and spinal monosynaptic reflex. Nerve discharges in the cat's saphenous nerve were promoted by the external application of menthol and nonylic vanillyl amide ( NVA ) dissolved in ethanol on the receptive field. When plasters containing counterirritants were externally applied on the receptive field, marked activation was recognized in touch-pressure-pin prick fibers immediately after the application as seen with plaster without drugs, but significantly high activity was maintained thereafter, and the increased firing rate in touch-pressure-pin prick-cold fibers lasted for 60 min. The activity of pressure-pin prick fibers was increased with the lapse of time and warm fibers was not influenced. Effects on activities of spinal dorsal horn cells were almost similar to the changes in activities of primary afferent from the skin. Monosynaptic reflex elicited by stimulation on the medial gastrocnemius nerve was weekly inhibited (significantly different from control at P less than 0.05), by the external application of plasters, and motoneuron receiving IPSP from the skin lightly touched on was observed. These findings suggest that the receptors activated with counterirritants were thought to be as follows: A delta and C polymodal nociceptors, A delta and C cold nociceptors, and slow adapting cutaneous mechanoreceptors. The cutaneous mechanoreceptors were also activated by the application of plasters. The volley from sensory nerve excited with counterirritants was thought to cause a suppression on the spinal monosynaptic reflex resulting from an inhibition on the motoneuron.