Info

neurotransmitters

system reticulospinal or spinothalamic ♦

and cavernous N.

Pudendal afferent pathways terminate in the dorsal commissure and medial dorsal horn. In addition to activating the sacral preganglionic neurons that initiate erection, interneu-rons in these regions presumably are involved in transmitting sensations to the brain and processing supraspinal inputs. The parasympathetic preganglionic neurons are located in the intermediolateral cell column and send dendritic projections to the same regions that receive penile afferent input. Reflexic erections can be observed in men with complete spinal cord lesions above the sacral segments. In such men, there is obviously no sensation; therefore, erection depends on an intact sacral cord mechanism in isolation from the rest of the central nervous system (CNS). Afferent impulses traveling via the dorsal nerve of the penis and pudendal nerve activate spinal interneurons, which then activate preganglionic neurons within the sacral parasympathetic center.

Nocturnal Erections

Multiple areas throughout the brain participate in the sleep-wake cycle. The waking state is maintained by a diffuse collection of neurons within the medulla, pons, midbrain, and diencephalons known as the reticular activating system. Electrical stimulation within the reticular activating system leads to a change in electroencephalogram pattern from the sleep state to that of the waking state—that is, cortical arousal. The sleep state does not result from the passive withdrawal of arousal but from two sleep centers that exist within the brain. One sleep center is responsible for producing slow-wave sleep, whereas the other produces rapid eye movement (REM) sleep.

The slow-wave sleep center is located within the medulla, in a midline area containing the raphe nuclei. Neurons within this nucleus use serotonin (5-hydroxytryptamine [5-HT]) as a neurotransmitter. Administration of 5-HT directly into the cerebral ventricles of experimental animals induces a state of slow-wave sleep, whereas lesions in this region induce a permanent state of insomnia.

The REM sleep center is located in specific nuclei of the pontine reticular formation, including the locus ceruleus, which uses NE as a neurotransmitter. Lesions within this area eliminate the electrophysiological and behavioral signs of REM sleep. In adults, REM and non-REM sleep alternate through the night. In view of the important role of 5-HT and NE in sleep, it is understandable that drugs may affect the duration and/or content of sleep. Currently, very little is known regarding the neural control of nocturnal penile erection (169). The current opinion is that reduced supraspinal inhibition of the spinal function is partly involved in this process.

Psychogenic (Central) Erection

Erection in response to sexual thoughts or auditory, visual, or olfactory stimuli, without confounding tactile stimulation, occurs in most healthy men and is known as psychogenic erection (7,33,170). Although this response is psychogenic, it appears likely that initiation of an erection also leads a reflexive component through activation of receptors in the penis. These psychogenic erections, initiated supraspinally, are believed to be mediated primarily through the sympathetic thoracolumbar path, with a minor contribution from the sacral parasympathetic system. If the sacral cord is damaged, reflexic erections are lost, but psychogenic erections persist. Although it is generally assumed that the brain exerts both excitatory and inhibitory control on spinal erectile mechanisms, these pathways remain unknown. The finding of increased erections in spinally transected animals has illustrated the inhibitory influence of the brain on spinal mechanisms. Excitatory input from the brain to the spinal cord causing erection is not fully known. It is clear that psychogenic stimuli to the brain may also inhibit erections. Therefore, the brain must exert an important modulator influence over the spinal reflex pathways mediating penile erection.

Although the precise anatomic regions are not completely known, it appears that the thalamic nuclei, the rhinencephalon, and the limbic structures play a role in modulating psychogenic penile erections. Messages from these diverse regions are integrated in the medial-pre-optic-anterior hypothalamic area. The limbic system—specifically the cor-tico-subcortical region—is implicated in the integration of inhibitory and facilitator signals connecting to the medial-pre-optic-anterior hypothalamic area. Efferent pathways from this area enter the medial forebrain bundle (MFB) and then continue caudally into the midbrain segmental region near the lateral part of the substantia nigra. From here, the efferent pathways continue to the ventrolateral part of the pons and medulla, ultimately reaching the spinal centers via the lateral funiculus of the spinal cord.

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