Neuroanatomy

Ejaculation By Command

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In recent years, much progress has been made in neuroanatomical research of eja-culatory processes. Most knowledge about the functional neuroanatomy of ejaculation is derived from male rat studies. With regard to male rat copulatory behavior, one has to distinguish among brain, brainstem, and spinal cord regions that become activated before and following ejaculation, when sensory information returns from the genitals (Fig. 9.1). The medial preoptic area (MPOA) in the rostral hypothalamus and the nucleus paragigantocellularis (nPGi) in the ventral medulla (6,7) are suggested as being important players in the process leading towards ejaculation. Electrical stimulation of the MPOA promotes ejaculation (8). It is hypothesized that ejaculation is tonically inhibited by serotonergic pathways descending from the nPGi to the lumbosacral motor nuclei. The present hypothesis is that the nPGi itself is inhibited by inhibitory stimuli from the MPOA. Disinhibition of the nPGi is supposed to lead to an ejaculation. The discovery of serotonergic neurons in the nPGi and the well-known ejaculation delay induced by serotonergic antidepressants suggests an action of the SSRIs on the nPGi. However, the precise location in the CNS on which SSRIs act to inhibit ejaculation has not yet been demonstrated.

On the other hand, brain areas activated as a result of the occurrence of one or more ejaculations have been observed in several mammals (9). Using expression of the immediate early gene, c-fos, as a marker for neural activity

Figure 9.1 Cerebral areas and neuronal pathways mediating ejaculation according to animal research data.

in male rats, Coolen and co-workers (9-13) demonstrated the presence of distinct ejaculation-related neural activation in several brain regions following ejaculation: the posteromedial part of the bed nucleus of the stria terminalis (BNSTpm), a lateral subarea in the posterodorsal part of the medial amygdala (MEApd), the posterodorsal preoptic nucleus (PD), and the medial part of the parvicellular subparafascicular nucleus (SPFp) of the thalamus. These brain regions containing ejaculation-induced activation are extensively interconnected and reciprocally connected with the MPOA (12), forming an ejaculation-related subcircuit within the larger brain circuits underlying male sexual behavior (12). The functional significance of this ejaculation-subcircuit is still poorly understood but it might well be that these areas play a role in "satiety" and thus mediate the post-ejaculatory interval.

Truitt and Coolen (13) highlighted the role of the lumbar spinal cord in the processing of ejaculation. They identified a group of lumbar spinothalamic cells (LSt) that are specifically activated after ejaculation, and provide direct genital sensory inputs to the SPFp in the thalamus and the ejaculation-related subcircuit in the brain. The LSt cells also project to sympathetic and parasympathetic neurons related to the genitals. It is suggested that the LSt cells contribute to triggering the ejaculatory reflex and the sensation of ejaculation, that is, orgasm. These and other animal studies have clearly shown the existence of a neural circuitry for ejaculation in mammals.

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