“ For many years, scientists thought that the brain 'directed the show' of sexual behavior – managing arousal, courtship, and copulation – while the spinal cord merely 'pulled the trigger' for ejaculation.", summarizes with Science and Future neurobiologist Constanze Lenschow. Recent work published in Nature Communications and of which she is the co-first author directly challenge this model in favor of a much more complex dialogue between brain and sexual organs, with an intermediary much more decisive than previously thought: the spinal cord.
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Incomplete ejaculation despite stimulation
The researchers are surprised. After stimulating the spinal cord neurons that trigger ejaculation in rats, the mice do not ejaculate. These neurons produce galanin, a neuropeptide through which nerve signals are transmitted. These spinal neurons (i.e., those located in the spinal cord) are what trigger ejaculation in rats. But when they are stimulated in mice, only the bulbospongiosus muscles of the penis are activated—those responsible for the final ejection of semen—without any ejaculation occurring. Other muscles are involved upstream of ejaculation, primarily smooth muscles of the vas deferens, seminal vesicles, and prostate.“, explains Constanze Lenschow.
These neurons do not appear to respond to direct stimulation of galanin neurons in mice as they do in rats. And indeed, in mice modified so that their galanin neurons are fluorescent, researchers observe their direct connection to the bulbospongiosus muscles. Although the literature on rats suggests a link between galanin neurons and motor neurons in the bulbospongiosus muscles, this link had never before been proven anatomically and physiologically.“,” Constanze Lenschow rejoices.
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Neurons that exhibit a refractory period after ejaculation
Furthermore, repeated stimulation of these neurons drastically weakened its effects. Unlike the rats, when we repeated the stimulation, the responses of The bulbospongiosus muscles were weakened. This means that the galanin neurons were somehow depressed after the male mouse ejaculated. It was as if the system entered a refractory state after this initial activation."The neurobiologist interprets this. While researchers cannot yet explain the mechanisms, these spinal neurons appear to be involved in determining the refractory period following ejaculation in males.
Ejaculation, an ongoing dialogue arbitrated in the spinal cord
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In addition to the refractory period, selective deactivation of spinal neurons altered the sexual behavior of mice. We observed more frequent missed matings and disrupted sexual rhythms.“,” reports Constanze Lenschow. And indeed, researchers observed that these neurons, far from only coming into play at the moment of ejaculation, were already active from the stage of sexual arousal and the first intromissions (penetrations). The contribution of this circuit therefore appears to go beyond the simple mechanics of ejaculation to play an active role in overall sexual behavior", concludes the researcher.
Ejaculation is therefore far from being a simple reflex triggered by spinal neurons on the brain's orders. On the contrary, the course of sexual activity appears to be shaped by a continuous dialogue between sensory signals, internal state (including whether or not ejaculation has already occurred), and spinal circuits. At the heart of this process, galanin neurons appear to be true integrators, capable of determining when to activate the motor pattern, depending on the signals received and the physiological state of the animal.“, explains Constanze Lenschow.
Rising sexual arousal, repeated penetrations, and a refractory period: the process leading to ejaculation in mice seems much closer to that of humans than in rats, which are capable of reflexive and repeated ejaculation. Rats can be good models for studying premature ejaculation, but mice might be better suited for understanding how human sexuality works, how arousal develops, and how ejaculation is regulated."The team hopes in future work to further investigate the involvement of these spinal neurons during the entire sexual behavior, particularly in females and throughout their reproductive cycle, and to trace them back to the brain neurons with which they communicate.
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