The beginning of sexual behavior is often marked by a biological “dance”, courtship signals and changes in physical appearance that prepare both partners. This could be an erection in males; it can also mean a female’s readiness. In species that reproduce internaly, the physical stimulation and contact build to a crucial moment. The male penis entering the female reproductive tract triggers the release of the sperm. The male enters into a phase of quietness, in which sexual activity is temporarily slowed down.
The brain controls the building up of desire, coordination and movement. However, the ejaculation is more similar to a spinal reaction. The process is divided into two stages. The first step is emission. Sperm and liquids are gathered in the chamber known as the prostatic bladder. Expulsion is a muscular reflex that produces a strong force. A ring of nerves at the base the penis contract to force the sperm. The reproductive process is a carefully choreographed combination of brain signals and spinal reflexes.
Scientists still haven’t figured out how the body determines when to ejaculate during sexual activity. Scientists believed for years that the brain controlled male sexual behavior. It directed everything, from the desire, to courtship, to the actual act. The spinal cord also acted as the lever that triggered ejaculation. New research by the Champalimaud Foundation has flipped that script. The spinal cord turns out to be more than a mere backstage worker. It’s a part of the show.
The study shows that spinal circuits play a larger role than was previously believed. It not only triggers ejaculation but also regulates arousal, and helps coordinate sexual rhythm. The spinal cord is involved in the whole performance and not only the final act. The discovery is an important new insight into how mammals behave sexually. It shows that sex choreography involves a brain-spine duet, and not just a single act.
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Susana Lima is the Principal Investigator and Senior Author of CF Neuroethology Laboratory. The spinal cord doesn’t simply act as a relay station to execute brain commands. It responds to arousal and adapts output depending on animal state. “It’s more complex than we thought.”
It is hard to pin down the exact moment when a woman has an orgasm. Ejaculation in males is an obvious and visible marker. You can see the muscular activity.
Constanze LENSCHO, now group leader at INCIA Institute, University of Bordeaux and co-author of the paper, stated: The muscle is called the BSM, bulbospongiosus. The BSM is located just below the penis. It’s critical to sperm expulsion. The BSM is activated in a burst pattern when a man ejaculates. “It’s almost like a signature for ejaculation”.
Researchers traced the route from the muscle that is responsible for the ejaculation signal, the Bulbospongiosus, back to motor neurons. They then asked: “Who is telling those motor neuron what to do?” Their first attempt, a virus-tracer, failed.
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It was frustrating. Ana Rita Mendes is a coauthor and a MSc student who was involved in the project. The team decided to change course.
In rats, previous studies had shown that a grouping of neurons in the spinal cord which produce a substance called galanin was important to ejaculation. The team used genetically modified mouse models in which the Gal+ neurons glow red. They observed that the signal carried branches that overlapped the BSM motor neuron. This suggested a connection.
Constanze lenschow tested this by activating the ends of Gal+ neuron and recording bursts in activity of the BSM motor cells. The activity stopped when they blocked the chemical glutamate that is used to transmit signals. This confirmed a direct connection.
Scientists had never before demonstrated a functional link between the Gal+ neurons in the spinal cord and the motor neurones that control ejaculation.
Mendes adds, The Gal+ neurons did not only reach the muscle of ejaculation, but also areas that are involved with erection as well as automatic control.
This team found that the neurons also respond to touch. Even a small puff of air in the penis stimulated the BSM and Gal+ motor neurons.
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Researchers used electrical stimulation as well as optogenetics to test whether Gal+ neurons can trigger ejaculation. This technique activates specific neurons using light in genetically altered mice. This stimulation consistently caused ejaculation in rats. The results in mice were unexpected.
The BSM could fire but the stimulation of Gal+ neuron never resulted in a true ejaculation,” explains Constanze Lenschow. It is even more perplexing that repeated stimulation causes the BSM to diminish, almost as if it had temporarily shut down.
It is interesting to note that BSM was only active in mice with spinalized brains, i.e., when the cord and brain were disconnected. It is possible that the brain holds back the spinal circuit until it’s the appropriate time.
Our findings are consistent with a model in which descending input from the brainstem inhibits Gal+ neurons, and the incoming genital signal, until the animal has reached the threshold of ejaculation. says Ana Rita Mendes.
The spinal cord is the source of most movements
This study portrays Gal+ neuron as a multitasker: They receive sensory information, evaluate the internal and external condition of the animal, initiate the motor pattern to ejaculate, then stand aside. There was still one more twist. Stimulating Gal+ neurons did not work if the mouse has already ejaculated. Lenschow says that “the BSM wouldn’t even respond”.
It was then revealed that these neurons were not just tracking movement but also the internal state of an animal. Mendes says, “That is a level context-sensitivity that we do not typically associate with the spinal circuits.”
Researchers tried an experiment to understand the role Gal+ neuron in sexual behaviour. They used a toxin that was specifically designed to eliminate these neurons from live mice. This type of intervention in rats completely stops ejaculation, but the other mating behaviors are left untouched. In mice, however, it was a more complex story.
Only 3 males out of 12 failed to ejaculate explains Ana Rita Mendes, Many had a disturbed sequence. They had difficulty locating the vagina and took longer to complete.
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It was concluded that Gal+ neurons played a greater role in mice. These neurons appear to play a more important role than just in triggering ejaculation. They also seem to influence arousal and help fine-tune sexual rhythm.
The role of Gal+ neurons in the spinal cord is different between mice and humans. says Constanze Lenschow. It is likely that species-specific strategies are at play in the way sex and timing is structured.
Ejaculation in rats is more a reflex. Genital stimulation can trigger it, even during the first mount. Mice take their time and engage in multiple thrusts to reach climax. This is similar to the way humans build up.
Rats are good for studying the premature ejaculation of rats. Lenschow notes, But mice may be better at understanding human sexuality, the way arousal is built up, and how ejaculation works.
The old notion that the brain was the only boss in sexual behavior is being challenged by these discoveries. The brain does not just give orders to the spine, but they work in tandem. Gal+ neurons are not just responsible for triggering ejaculation. They also adjust muscle activity and take into account the level of body arousal. This spinal teamwork could even control recovery after ejaculation by determining when the system’s ready to move on again.
We think of the spine cord as an intersection. Lima, a researcher at Lima. It helps to time the right amount of activity by gathering signals from the brain, genitals and prostate.
Lima suggests even that the trigger to ejaculation is the The “point of No Return”It could be a message from your prostate and not your brain. “I’m ready. “I’m ready to go.”
This information could help us better understand and treat sexual dysfunction. Next, the team will record Gal+ activity during sexual contact to determine how it interacts with other organs. While rats are the traditional model to study ejaculation this new research shows that the mouse is a growing star.
We’re not going to bring down the rat. says Constanze Lenschow, But the mouse is much more than what we had thought. Ana Rita Mendes adds, We’re only beginning to understand how the spinal cord influences sexual behavior. It’s more than a messenger; it is a partner.
Journal Reference
- Lenschow, C., Mendes, A.R.P., Ferreira, L. et al. A galanin positive population of spinal cord neurons in the lumbar region modulates sexual arousal, and also copulatory behaviours among male mice. Nat Commun 16, 8282 (2025). DOI: 10.1038/s41467-025-63877-2
