Review—Erectile Dysfunction/AndrologyPhysiology of Ejaculation: Emphasis on Serotonergic Control
Introduction
Amongst the different physiological functions, those related to reproduction are probably the unique essential for survival of the human species without being vital for the individual. Thus, from an evolutionist perspective, the rewarding sensory expression represented by orgasm occurring concomitantly with ejaculation is an advantage for the perpetuation of the human race.
In recent years, male sexual dysfunctions have become a topic of interest in biological science. However, animal and clinical studies have focused on mechanisms and pathophysiology of penile erection whereas the ejaculatory process and its dysfunctions remained poorly investigated. Epidemiological studies performed to date in European countries and the USA have reported high prevalence for ejaculatory disorders, mainly premature but also delayed and painful ejaculation [1], [2], [3], [4], [5]. Since decades, ejaculatory disorders were assumed to be psychological or urological issues but recently a proposal has emerged that considers these disorders as possibly having a neurobiological substratum [6]. Owing to the fact that aetiology of ejaculatory dysfunction is multiple (including psychological or organic origins) [7], it can be claimed that it is a multifactorial disorder.
The organisation of anatomical structures implicated in ejaculation fundamentally shows common features in mammalians including humans. Therefore, the development of an experimental approach is useful for understanding the human physiology of ejaculation and may allow the discovery of potential pharmacological therapies applicable for the treatment of ejaculatory dysfunctions. Ejaculation, usually accompanied by orgasm, is constituted by two successive phases, emission and expulsion, which involve different pelvi-perineal anatomical structures. A tight coordination of sympathetic, parasympathetic as well as somatic divisions of the nervous system is necessary for normal antegrade ejaculation to occur.
Section snippets
Physiology of ejaculation
The importance of the autonomic nervous system in regulating ejaculatory process is well documented [8], [9], [10]. Two groups of anatomical structures involved in ejaculation can be distinguished depending on the phase they participate in (i.e. emission or expulsion). The organs involved in the emission phase comprise epididymis, vas deferens, seminal vesicles, prostate gland, prostatic urethra and bladder neck. The organs participating to the expulsion phase comprise bladder neck and urethra
Neuroanatomical organisation of the circuitry of ejaculation
The sympathetic and parasympathetic nervous systems, closely interconnected into the pelvic plexus which represents an integrative peripheral crossroad site, act in synergy to command physiological events occurring during ejaculation. Both sympathetic and parasympathetic tones are under the influence of sensory genital and/or cerebral erotic stimuli integrated and processed at the spinal cord level (Fig. 1).
Serotonergic control of ejaculation
The neurochemistry of ejaculation is a complex field calling for additional studies. A number of neurotransmitters, including serotonin (5-HT), dopamine, oxytocin, GABA, adrenaline, acetylcholine and nitric oxide, have been shown to be involved in the regulation of ejaculation [36]. Amongst them, 5-HT intervening at different levels of the neuraxis plays a primary role. The data presented below have been obtained in rats.
Conclusion
Even if there are still many details which need to be clarified, the recent progress that the neurobiological approach has provided on our understanding of the ejaculatory process open new avenues for the treatment of ejaculatory disorders in human. In particular, targeting of CNS serotonergic components seems promising. In this sense, the use of selective serotonin reuptake inhibitors (SSRIs) which basically increase the amount of central 5-HT and delay ejaculation in human has given
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