Genital innervation is both somatic and autonomic. Somatic sensory afferents deliver information on tactile sexual stimuli to the sacral spinal cord, and can induce local sexual responses (vascular-erectile and glandular). Sensory information projects to suprasacral regions which are also important in sexual awareness and excitation. The erectile response is initiated by parasympathetic efferents travelling through the pelvic plexus and the greater and lesser cavernosal nerves. Blood flow in the penile artery increases. Smooth muscles lining the cavernosal sinuses in the penis relax. Helicine arterioles branching from cavernosal arteries selectively shunt blood flow to the lacunar spaces of the cavernosal bodies which fill with blood; subtunical venules become compressed. Intracorporeal pressure increases and then stabilises at a level approximating systolic blood pressure causing penile tumescence and rigidity. Continued parasympathetic activity maintains this erection (Smith & Bodner 1993). This parasympathetic pathway is, however, not the only proerectile pathway: erections are observed in humans and experimental animals after lesions of sacral cord segments and pelvic nerves, probably througha pathway involving the hypogastric nerves. The role of the hypogastric parasympathetic nerves in human penile ersction is not fully understood.
Continued stimulation eventually induces orgasm with seminal emission, rhythmic phasic contractions of perineal and pelvic floor muscles (ejaculation). Actually emission begins during arousal (Mitsuya et al 1960). Ejaculation requires sympathetic outflow from T11-L2 segments travelling through the hypogastric plexus, and along pelvic and pudendal nerves (Giuliano et al 1995). Animal experiments have shown extensive cross-innervation of the sympathetic nervous system (Kihara & Degroat 1997). Sympathetic activity causes smooth muscle contraction: in seminal vesicles, vas deferens, and prostate – to deliver seminal fluid to posterior urethra; in the bladder neck – to prevent retrograde ejaculation; in the corpora cavernosa – to cause detumescence. The latter "antierectile" activity is probably inhibited during erection through spinal coordination of reflex action. In the periphery the main proerectile transmitter is nitric oxide, which is co-localized with VIP (vasoactive intestinal peptide) and acetylcholine. The main anti-erectile neurotransmitter is probably noradrenaline (Giuliano et al 1995). Although the predominant neural control of the male accessory sexual organs is sympathetic (adrenergic and purinergic), the secretion of seminal fluid is under parasympathetic control (Hoyle et al 1994). Appropriate sensory stimulation leading to erection and orgasm is not necessarily purely genital, and erections caused by stimuli delivered through cranial nerves might also be reflexive, although this is usually subsumed under "psychogenic" (Sachs 1995). Mental imagery is the "real" psychogenic descending activating stimulus of these spinal cord-integrated reflex responses, but very little is known about these evidently important mechanisms.
The pattern of genital activation is probably similar in women, in whom parasympathetic activity causes clitoral erection, engorgement of labia, vaginal lubrication and secretion from a series of glands, the paraurethral gland – often called the female prostate – and Bartholin’s glands (Zaviacic 2000, 2001). Clitoral stimulation, involving pudendal nerve afferents, and vaginal stimulation, especially of the G-spot area, can both result in an orgasmic response. Vaginal stimulation also produces local analgesia, important during childbirth (Komisaruk & Whipple 2000). Orgasmic sympathetic activity results in contractions of uterus and fallopian tubes (Bérard 1989). Somatic motor activation causes rhythmic contractions of pelvic floor muscles sometimes noticed as expulsions from the paraurethral glands through the urethra, called female ejaculation (Whipple 2000).
The spinal cord organisation of the reflex coordination involved in human sexual responses remains to be elucidated. Spinal cord sites concerned with cavernosal smooth muscle control have been localised to preganglionic autonomic nuclei in thoracolumbar and lumbosacral spinal segments in the rat (Marson et al 1993). Genital afferents probably synapse – via interneurons – both with somatic and autonomic motoneurons; those projected to supraspinal structures travel in the anterolateral funiculus. Both thalamic and cortical areas receive sensory input from the genitalia, and sexual feelings may be elicited when such areas are stimulated. In the primary sensory cortex the genitalia are represented in the parasagittal area (Penfield & Jasper 1954). In studies using retrograde labelling in the rat, as revealed by the transneuronal transport of pseudorabies virus, most of the labelling from corpus cavernosum at the level of the brainstem was in the pons and medulla (Marson et al 1993). Descending projections from the brainstem raphe nuclei travel in the lateral funiculus. The nucleus paragigantocellularis contains serotoninergic neurons that projected to the spinal cord and provide tonic inhibition of sexual reflexes in the rat (McKenna et al 1991).
Injection of pseudorabies virus into the corpus cavernosum, revealed hypothalamic neuronal localisation, especially in the paraventricular nucleus, the tuberal region, the medial preoptic area, and the dorsal hypothalamic area (Marson et al 1993). Neurons from the paraventricular nucleus project to the thoracic and lumbosacral nuclei concerned with erection. Hypothalamo-spinal projections are situated in the dorsolateral funiculus (Giuliano et al 1995). The hypothalamus is also directly involved in the control of the gonadotropic pituitary activity and prenatal development of the genital organs, pubertal development, and the menstrual cycle. In the basal hypothalamus there is a region important for sexual desire, which is affected by tissue levels of the sex steroid hormones (testosterone, dihydrotestosterone and estradiol).
Animal experiments have delineated a dopaminergic stimulating and a serotoninergic inhibiting mechanism controlling sexual desire. Androgens are necessary but apparently not essential for normal human sexual desire (Kwan et al 1983). Sexually dimorphic nuclei are localised in the preoptic anterior hypothalamus. The medial preoptic area is important in sexual motivation and performance, and dopamine may regulate penile erection at this level. Stimulation of cortical and subcortical limbic structures– in particular, the hippocampus – can elicit erection in monkeys (Dua & MacLean 1964). Positron emission tomography (PET) studies during normal male sexual activity have shown increased regional blood flow in the ventral tegmentum but not in hypothalamus or limbic cortex (Holstege et al 2003). Important unsolved questions related to diagnostic concepts of erectile dysfunction concern the role of psychogenic and reflex erections, and whether sleep-related erections have identical neural control mechanisms to sexually elicited erections (Tables 1 and 2).
