INHIBITION OF WITHDRAWAL RESPONSES BY PELVIC NERVE ELECTRICAL-STIMULATION

In urethane-anesthetized rats, the compound action potential of the pe lvic nerve was found to consist of three different waves, two in the A delta fiber and one in the C-fiber range of conduction velocity. Elec trical stimulation of the pelvic nerve produced a complete inhibition of the withdrawal response to noxious foot pinch or foot compression. The electromyographic (EMG) activity of the contralateral posterior bi ceps muscle was used to record the withdrawal response. The withdrawal response inhibition was related to the duration and the frequency of electrical stimulation. Low (5-10 Hz) and high (100-300 Hz) frequencie s were ineffective in inhibiting the withdrawal response, whereas inte rmediate frequencies (20-80 Hz) produced a complete inhibition of the withdrawal response. Short (300 ms) trains of stimulation inhibited th e withdrawal response only during the stimulation period. Longer train s of stimulation (500 ms-10 s) produced long-lasting inhibition of the response to noxious stimulation. The inhibition persisted for up to 2 0 s after the end of electrical stimulation of the pelvic nerve. A del ta fiber stimulation was adequate to inhibit the withdrawal response i n most (15 out of 17) of the animals. However, A delta plus C-fiber st imulation was needed to inhibit the response to noxious stimulation in two animals. In addition to inhibiting the response to noxious stimul ation, pelvic nerve electrical stimulation reflexively activated abdom inal muscles. On the basis of present findings using electrical stimul ation, it can be suggested that, in the rat, A delta and C-visceral af ferents of the pelvic nerve mediate the analgesic effect of vaginocerv ical probing pelvic and A delta afferents the contraction of abdominal muscles in the fetus-expulsion reflex.