The pudendal nerve-evoked response in axial muscle

In 39 Urethane-anesthetized rats we have recorded the afferent volley in the dorsal roots and the electrical activity of the lateral longissimus muscle and its motor nerves during electrical stimulation of a cutaneous branch of the pudendal nerve. Male and female rats were used; the females were ovariectomized and either pretreated with estradiol or left without hormonal treatment. Conduction velocities in the pudendal nerve were 54 m/s for the largest A-beta fibers and averaged 10 m/s for A-delta fibers. Excitation of pudendal nerve afferents strongly potentiated the firing of axial motoneurons, at stimulus currents below threshold for A-delta fibers. Trains of three shocks to the pudendal nerve were considerably more effective than double or single shock trains. Repetition rates as low as 1/s had a long lasting excitatory effect on the lateral longissimus muscle and the magnitude of the responses increased gradually for several seconds with continued stimulation. Recordings from the axons of the epaxial motoneurons of female rats showed a strong activation of neuronal firing with an onset latency of 5.8 ms from the last shock of a three ms, three shock train; the onset in male rats, 8.4 ms, differed significantly. Peak spike activity occurred at mean latencies of 11, 22 and 102 ms in both sexes. A period of depressed firing was usually present from 34 to 50 ms. Males differed in having a larger peak in activity at 102 ms, but the overall profile of the responses was similar in males and females. No differences were seen in the overall response patterns of the estrogen-treated and untreated females. Responses of comparable magnitude were seen with ipsilateral or contralateral pudendal nerve stimulation; these were facilitated by bilateral stimulation. In electromyographic recordings, the onset of unit firings was seen at 6.4 ms latency in response to pudendal nerve stimulation. This unit activity was consistent with the firing pattern seen in the muscle nerves.