Barry R. Komisaruk

Genital Sensory Field: Enlargement by Estrogen Treatment in Female Rats

Recordings of neuronal activity in the pudendal, genitofemoral, and pelvic nerves indicate that the sensory fields of these three nerves are the perineum, the caudal abdomen, and the vagino-cervical area and rectum, respectively. The sensory field of the pudendal nerve was significantly larger in estrogen-treated ovariectomized female rats than in uninjected controls. This effect of estrogen was not mediated by pudendal efferents.

Sensory innervation of the external and internal genitalia of the female rat

Using a whole-nerve recording method, the genitalia of the female rat were found to receive afferent innervation as follows. Pelvic nerve: vagina, cervix, and perineal skin; hypogastric nerve: cervix and proximal three fifths of the uterus; pudendal nerve: skin of perineum, inner thigh, and clitoral sheath. It is probable that the pudendal and pelvic nerves are activated during copulation, and that all 3 nerves are activated during parturition.

Antinociceptive effects of vaginal stimulation in rats: neurophysiological and behavioral studies.

The present studies extend previous findings that probing the vaginal cervix of rats blocks withdrawal reflexes and induces immobilization44. In the present studies, we report that this effect is apparently not due to an action on the final motor pathway, for limb or facial movement induced by electrical stimulation of the pyramidal tract was not suppressed by the probing. In contrast, the sensory response of neurons in the ventrobasal complex of the thalamus to noxious pinch stimulation was markedly attenuated by probing the vaginal cervix. However, the response of these neurons to gentle tactile stimulation was not attenuated, indicating a selective antinociceptive effect of the probing. The antinociceptive effect was not necessarily related to changes in arousal. These findings were supported by behavioral studies in which probing the vaginal cervix blocked vocalization in response to tail shock, and elevated the current threshold for eliciting vocalization in response to tail shock. Furthermore, during the probing, the rats were found to be capable of vocalizing in response to presumably non-noxious (lifting) stimulation, even though their vocalization response to noxious tail shock was suppresed. These studies suggest that probing the vaginal cervix rats exerts an analgesic action.

Analgesic effect of vaginal stimulation in rats: Modulation by graded stimulus intensity and hormones ☆

Abstract Stimulation of the vaginal cervix with a glass rod elevated vocalization thresholds to tail shock in ovariectomized rats. As more force (in g) was applied to the cervix, we observed a progressive increase in the threshold of vocalization in response to tail shock. In a second experiment, ovariectomized rats were treated with either estradiol benzoate, progesterone, estradiol plus progesterone, or oil, and tested for vocalization to tail shock. Estradiol treatment enhanced the effect of cervical stimulation on elevating the vocalization threshold. Progesterone had no such effect when given alone, but completely prevented the estrogen effect. These findings demonstrate that a) vocalization in response to noxious stimulation is suppressed by genital tract stimulation, b) the degree of suppression increases with increasing intensity of genital tract stimulation, and c) steroid hormones affect this phenomenon.

Elevation of pain threshold by vaginal stimulation in women.

&NA; 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.

Hyperalgesia induced by altered glycinergic activity at the spinal cord

Glycine or its receptor antagonist, strychnine, were administered perispinally to investigate their effect on nociceptive responses elicited by activation of various cutaneous receptors. Strychnine produced dose-dependent sensory and motor disturbances; 1 and 5 micrograms doses were sub-convulsive, eliciting recurrent episodes of coordinated grooming, scratching and biting at the skin, which persisted for approximately 10 minutes post-injection; higher doses (25 and 100 micrograms) increased the intensity and duration of these effects, and produced convulsive motor seizures. Motor disturbances were not elicited by glycine (5, 25, 100 and 400 micrograms). Strychnine treated rats, at all doses, vocalized consistently in response to light cutaneous stimulation; a significant proportion of glycine treated rats also vocalized, but were not as sensitive to mild stimulation. Skin hyperalgesia persisted for at least 30 minutes in both strychnine and glycine treated rats. Both strychnine and glycine significantly reduced vocalization thresholds to tail shock. However, no clear effect on tail flick latency was observed following either strychnine or glycine. These results indicate that glycinergic neurons contribute to the tonic regulation of nociceptive input at the spinal cord.

Functional MRI of the Brain during Orgasm in Women

Abstract Women diagnosed with complete spinal cord injury (SCI) at T10 or higher report sensations generated by vaginal-cervical mechanical self-stimulation (CSS). In this paper we review brain responses to sexual arousal and orgasm in such women, and further hypothesize that the afferent pathway for this unexpected perception is provided by the Vagus nerves, which bypass the spinal cord. Using functional magnetic resonance imaging (fMRI), we ascertained that the region of the medulla oblongata to which the Vagus nerves project (the Nucleus of the Solitary Tract or NTS) is activated by CSS. We also used an objective measure, CSS-induced analgesia response to experimentally induced finger pain, to ascertain the functionality of this pathway. During CSS, several women experienced orgasms. Brain regions activated during orgasm included the hypothalamic paraventricular nucleus, amygdala, accumbens-bed nucleus of the stria terminalis-preoptic area, hippocampus, basal ganglia (especially putamen), cerebellum, and anterior cingulate, insular, parietal and frontal cortices, and lower brainstem (central gray, mesencephalic reticular formation, and NTS). We conclude that the Vagus nerves provide a spinal cord-bypass pathway for vaginal-cervical sensibility and that activation of this pathway can produce analgesia and orgasm.

Nociceptive responses to altered gabaergic activity at the spinal cord

GABA agonists and antagonists were injected intrathecally at the spinal cord, to determine their effect on nociceptive thresholds. Tactile stimulation, applied against the flank by a medium diameter von Frey fiber (5.5 g force), elicited distress vocalizations after, but not before injection of the GABA antagonists, bicuculline MI or picrotoxin (0.25 and 1 microgram dosages). Vocalization threshold to tail shock was significantly reduced by bicuculline MI or picrotoxin. Tail flick withdrawal latency from radiant heat was not altered by GABA antagonists. The GABA agonist, muscimol, significantly elevated vocalization threshold to tail shock at a 5 micrograms dose. At a lower dose level (1 microgram), muscimol significantly reduced vocalization threshold to tail shock. Tail flick latency was significantly prolonged by the 5 micrograms dose of muscimol; however, flaccid paralysis of the hind limbs was also evident. Nociceptive thresholds were not altered by GABA or saline injection. These findings indicate that GABAergic activity contributes to the tonic modulation of nociception at the spinal cord.

Women's clitoris, vagina, and cervix mapped on the sensory cortex: fMRI evidence.

INTRODUCTION The projection of vagina, uterine cervix, and nipple to the sensory cortex in humans has not been reported. AIMS The aim of this study was to map the sensory cortical fields of the clitoris, vagina, cervix, and nipple, toward an elucidation of the neural systems underlying sexual response. METHODS Using functional magnetic resonance imaging (fMRI), we mapped sensory cortical responses to clitoral, vaginal, cervical, and nipple self-stimulation. For points of reference on the homunculus, we also mapped responses to the thumb and great toe (hallux) stimulation. MAIN OUTCOME MEASURES The main outcome measures used for this study were the fMRI of brain regions activated by the various sensory stimuli. RESULTS Clitoral, vaginal, and cervical self-stimulation activated differentiable sensory cortical regions, all clustered in the medial cortex (medial paracentral lobule). Nipple self-stimulation activated the genital sensory cortex (as well as the thoracic) region of the homuncular map. CONCLUSION The genital sensory cortex, identified in the classical Penfield homunculus based on electrical stimulation of the brain only in men, was confirmed for the first time in the literature by the present study in women applying clitoral, vaginal, and cervical self-stimulation, and observing their regional brain responses using fMRI. Vaginal, clitoral, and cervical regions of activation were differentiable, consistent with innervation by different afferent nerves and different behavioral correlates. Activation of the genital sensory cortex by nipple self-stimulation was unexpected, but suggests a neurological basis for womens reports of its erotogenic quality.

fMRI study of acupuncture-induced periaqueductal gray activity in humans

BOLD fMRI was used to study acupuncture-induced activation (increase in the BOLD signal from undetectable) of the periaqueductal gray (PAG) and two somatosensory cortical areas in seven healthy human subjects. Mechanical stimulation (push-pull) was given to the LI4 (Hoku) acupoint or to a non-acupoint. The stimulation paradigm consisted of 5 runs, each consisting of four 30 s On/30 s OFF periods over 30 min. The scan for each ON period was analyzed individually. The PAG and cortical areas showed different activity patterns. PAG activity was episodic and reliably demonstrated after 20–25 min of stimulation; both cortical areas, however, were active >90% of the time. Stimulation of a non-acupoint (leg) resulted in reduced levels of PAG and cortical activity.