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.