Low threshold mechanical stimulation of the vagina depresses dorsal horn unit activity in the spinal cat

Prolonged inhibition of the flexor reflex by probing the cervix uteri in the cat

In decerebrate or spinal cats, sustained mechanical stimulation of the cervix uteri inhibited the flexor reflex elicited by electrical stimulation of the foot pad during the probing period (160 s). After probing, 3-15 min were required for reflex recovery. No additional inhibition was produced if probing was repeated before recovery, but instead the reflex was facilitated. When probing was applied 5-10 min after reflex recovery the reflex was again abolished. The recovery, however, occurred earlier and was followed by facilitation. Probing the cervix with single mechanical pulses inhibited transiently (140-200 ms) the short latency reflex components, but the components with longer latencies are unaffected or facilitated. Distension of the vaginal wall with a balloon also inhibited the flexor reflex, but a transient, mild facilitation appeared several seconds after the distension. In general, whenever the inhibition decreases, the facilitation predominates. Our findings suggest that cervical probing or vaginal distension triggers both a long-lasting inhibition and a concomitant facilitation in different intraspinal flexor reflex pathways.

Somato-motor components of the pelvic and pudendal nerves of the female rat

The efferent innervation of the pelvic and pudendal nerves was characterized in this study by identifying the muscles activated by electrical stimulation of the nerves distal to the point at which they bifurcate from the L6-S1 trunk. Pelvic nerve electrical stimulation produced EMG-monitored contraction of the ipsilateral ilio- and pubococcygeus muscles, which was abolished by cutting one ('muscular') branch of the bifurcated nerve. (This 'muscular' branch receives proprioceptive input activated by tail displacement, whereas the other, 'viscero-cutaneous' branch receives sensory innervation from the midline perineal region.) Pudendal nerve electrical stimulation produced contraction of the coccygeus, external anal sphincter, and ischiocavernosus muscles. Movements of the orifice and wall of the vagina were directly visualized during electrical stimulation of the two nerves. Intravaginal pressure measured by balloon was increased by pelvic nerve stimulation and decreased by pudendal nerve stimulation. Reflexive contraction of the ilio- en pubococcygeus muscles was produced by mechanostimulation of the perineum, clitoral sheath and distal vagina. This response was abolished by gentle cervical mechanostimulation. One implication of this finding is that passage of the fetuses through the cervix during parturition may relax the ilio- and pubococcygeus muscles, thereby facilitating delivery.

Effects of carbamylcholine on membrane potential and Na-K pump activity of cultured rat skeletal myotubes

1. We measured changes in resting membrane potential (Em) and Na-K pump activity, assayed by ouabain-sensitive 86Rb uptake, in response to carbamylcholine (CCh) and its continued presence in single rat skeletal myotubes in culture. 2. CCh caused immediate depolarization from control Em (-80 to -85 mV) to near 0 followed by repolarization of varying degrees depending on the age of the culture and temperature of the recording medium; repolarization of Em was most apparent by culture age 8-9 days in vitro (DIV), Em reaching values as high as -60 mV by 5-10 min after peak depolarization at 37 degrees C. 3. Input resistance, which decreased during CCh depolarization, increased only slightly during the initial phase of repolarization and then remained essentially unchanged during the major component of membrane repolarization in the presence of CCh. 4. Ouabain, given before CCh, prevented repolarization of Em and, when given after repolarization had begun, reversed it and caused Em to return to about -7 mV. 5. Na-K pump activity was decreased in myotubes in which Em did not repolarize or did so only slightly, and was increased by over 40-50% in myotubes whose Em repolarized by 40-60 mV, even though CCh was still present in the medium. Inhibition of pump activity in non repolarizing myotubes was related to Na influx, inhibition being reversed to stimulation when CCh was administered to myotubes in Na-free medium. 6. Repeated (three or four times) or prolonged (up to 60-min) administration of CCh to myotubes in which repolarization was hardly expressed (age 6-7 DIV) caused increases both in the amount of repolarization and in 86Rb uptake, both being related to the number or duration of CCh exposures. 7. We conclude that repolarization of Em following CCh-induced depolarization of cultured rat skeletal myotubes depends to a large extent on an increase in activity of the electrogenic Na-K pump.

Analgesia produced in women by genital self-stimulation

This study investigated the relative effectiveness of specific genital regional self-stimulation in elevating pain thresholds. Anecdotal reports in humans suggest that sexual activity and orgasm decrease a wide variety of human responses to pain and touch, but the phenomenon has not been evaluated objectively. Two types of self-stimulation, pressure and pleasurable, were applied by 10 women to the anterior vaginal wall, the posterior vaginal wall, and the clitoris. Significant increases in pain thresholds but not tactile thresholds occurred when pressure stimulation was applied to the anterior wall of the vagina or when "pleasurable" self-stimulation was applied to any of the three areas. Tactile thresholds were not significantly affected by any genital stimulation condition. However, there was a significant increase in tactile threshold but not pain thresholds in the distraction control condition. On the basis of these findings, we conclude that (a) a sensation of pleasure evoked by genital stimulation can elevate pain thresholds, (b) these pleasurable stimuli were not general "distractants" because they elevated pain thresholds but not tactile thresholds differentially, and (c) genital pleasurable stimuli activate an analgesic process that is distinct from a distraction process.

Elevation of pain threshold by vaginal stimulation in women

In 2 studies with 10 women each, vaginal self-stimulation significantly increased the threshold to detect and tolerate painful finger compression, but did not significantly affect the threshold to detect innocuous tactile stimulation. The vaginal self-stimulation was applied with a specially designed pressure transducer assembly to produce a report of pressure or pleasure. In the first study, 6 of the women perceived the vaginal stimulation as producing pleasure. During that condition, the pain tolerance threshold increased significantly by 36.8% and the pain detection threshold increased significantly by 53%. A second study utilized other types of stimuli. Vaginal self-stimulation perceived as pressure significantly increased the pain tolerance threshold by 40.3% and the pain detection threshold by 47.4%. In the second study, when the vaginal stimulation was self-applied in a manner that produced orgasm, the pain tolerance threshold and pain detection threshold increased significantly by 74.6% and 106.7% respectively, while the tactile threshold remained unaffected. A variety of control conditions, including various types of distraction, did not significantly elevate pain or tactile thresholds. We conclude that in women, vaginal self-stimulation decreases pain sensitivity, but does not affect tactile sensitivity. This effect is apparently not due to painful or non-painful distraction.

Spinal opiates affect sexual behaviour in rats

Enkephalin-containing cells in the substantia gelatinosa of the spinal cord participate in the neural processing of painful stimuli. Spinal opiates may also be involved in the perception of sensory stimuli related to reproduction, as probing of the uterine cervix stimulates sexual behaviour in rats and also induces an analgesia which is partially dependent on opiate receptor mechanisms in the spinal cord. We now report that the intrathecal injection of morphine into female ovariectomized rats pretreated with oestradiol benzoate and progesterone inhibits sexual receptivity while injection of the opiate receptor antagonist naloxone enhances it. Similarly, intrathecal injection of morphine increases while injection of naloxone decreases the number of intromissions before ejaculation in male rat. Intraperitoneal injection of morphine or naloxone has no behavioural effect. Recent studies have suggested that brain opiates are involved in the control of sexual behaviour. The results reported here indicate that sexual behaviour may also be influenced by spinal opiates.