Inhibition of withdrawal responses by pelvic nerve electrical stimulation

In urethane-anesthetized rats, the compound action potential of the pelvic nerve was found to consist of three different waves, two in the Aδ fiber and one in the C-fiber range of conduction velocity. Electrical 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 biceps 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) frequencies were ineffective in inhibiting the withdrawal response, whereas intermediate frequencies (20–80 Hz) produced a complete inhibition of the withdrawal response. Short (300 ms) trains of stimulation inhibited the withdrawal response only during the stimulation period. Longer trains of stimulation (500 ms-10 s) produced long-lasting inhibition of the response to noxious stimulation. The inhibition persisted for up to 20 s after the end of electrical stimulation of the pelvic nerve. Aδ fiber stimulation was adequate to inhibit the withdrawal response in most (15 out of 17) of the animals. However, Aδ plus C-fiber stimulation was needed to inhibit the response to noxious stimulation in two animals. In addition to inhibiting the response to noxious stimulation, pelvic nerve electrical stimulation reflexively activated abdominal muscles. On the basis of present findings using electrical stimulation, it can be suggested that, in the rat, Aδ and C-visceral afferents of the pelvic nerve mediate the analgesic effect of vaginocervical probing pelvic and Aδ afferents the contraction of abdominal muscles in the fetus-expulsion reflex.