Sexual behavior often begins with a kind of biological ‘dance’, courtship signals, and physical changes that get both partners ready. In males, this might mean an erection; in females, it’s a sign of readiness. For species that reproduce internally, physical contact and stimulation build up to a critical moment. When the male’s penis enters the female’s reproductive tract, it triggers ejaculation, the release of sperm. After this, the male typically enters a quiet phase, where sexual activity temporarily slows down.
While the brain is the command center for the buildup, handling desire, movement, and coordination, ejaculation itself is more like a spinal reflex. It happens in two steps. First comes emission, where sperm and fluids gather in a chamber called the prostatic urethra. Then comes expulsion, a powerful muscle-driven reflex. Special spinal nerves activate a ring-like muscle at the base of the penis, which contracts to push the sperm out. It’s a tightly choreographed sequence, blending brain signals with spinal reflexes to complete the reproductive act.
Scientists are still piecing together how the body knows when to trigger ejaculation during sex. For years, scientists believed the brain was the master conductor of male sexual behavior, directing everything from desire to courtship to the act itself. At the same time, the spinal cord pulled the final lever for ejaculation. But new research from the Champalimaud Foundation flips that script. It turns out the spinal cord isn’t just a backstage technician; it’s part of the main cast.
This study reveals that a specific spinal circuit plays a much bigger role than previously thought. Not only does it trigger ejaculation, but it also helps regulate arousal and coordinate the rhythm of sexual behavior. In other words, the spinal cord contributes to the entire performance, not just the finale. This discovery adds a fascinating new layer to how we understand sexual behavior in mammals, showing that the choreography of sex is a duet between brain and spine, not a solo act.
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Susana Lima, Principal Investigator of CF’s Neuroethology Lab and senior author of the study, said, “The spinal cord isn’t just a passive relay station executing brain command. It integrates sensory inputs, responds to arousal, and adjusts its output based on the animal’s internal state. It’s much more sophisticated than we imagined.”
“We were initially interested in female sexual behaviour, but it is difficult to pinpoint the moment of orgasm. In males, ejaculation is a clear and observable marker; you can literally see it in the muscle activity.”
Constanze Lenschow, co-lead author and now Group Leader at the INCIA Institute at the University of Bordeaux, said, “The muscle in question is the bulbospongiosus, or BSM. It sits just below the penis and is critical for sperm expulsion. When a male ejaculates, the BSM fires in a characteristic burst pattern. It’s like the signature of ejaculation.”
To understand how the signal for ejaculation travels, researchers traced the path from the key muscle involved, the bulbospongiosus (BSM), back to the motor neurons that tell it to contract. Then they asked, ‘Who tells those motor neurons what to do?’ Their first method, using a virus tracer, didn’t work.
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“It was frustrating,” says Ana Rita Mendes, a co-author who joined the project during her MSc. So the team changed course.
Previous studies in rats had pointed to a group of spinal neurons that produce a molecule called galanin (Gal) as important for ejaculation. Using genetically modified mice in which these Gal⁺ neurons glowed red, the team observed under the microscope that their signal, carrying branches that overlapped with the BSM motor neurons, hinted at a direct connection.
To test this, Constanze Lenschow activated the ends of Gal⁺ neurons and recorded bursts of activity in the BSM motor neurons. When they blocked glutamate, the chemical used to send signals, the activity stopped, confirming a direct link.
This was the first time scientists had shown a clear, functional connection between Gal⁺ spinal neurons and the motor neurons that control ejaculation.
Mendes adds, “Gal⁺ neurons didn’t just connect to the ejaculation muscle, they also reached areas involved in erection and automatic control of ejaculation.”
The team also found that these neurons respond to touch. In mice with severed spinal cords, even a gentle puff of air to the penis activated both Gal⁺ neurons and the BSM motor neurons, proving the circuit reacts to genital stimulation.
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To find out if Gal⁺ neurons could actually trigger ejaculation, researchers used two methods: electrical stimulation and optogenetics. This technique uses light to activate specific neurons in genetically modified mice. In rats, this stimulation reliably caused ejaculation. But in mice, the results were surprising.
“We could get the BSM to fire, but stimulation of Gal⁺ neurons never led to a real ejaculation,” explains Constanze Lenschow. Even more puzzling, repeated stimulation caused the BSM response to fade, as if the system had entered a temporary shutdown.
Interestingly, vigorous BSM activity only happened in spinalised mice, those with their brain disconnected from the spinal cord. This suggests that the brain typically holds the spinal circuit in check until the right moment.
“Our findings support a model where descending input, likely from a brainstem region, inhibits the Gal⁺ neurons and incoming genital signals until the animal reaches the ejaculatory threshold,” says Ana Rita Mendes.
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The study paints Gal⁺ neurons as multitaskers: they receive sensory input, assess the animal’s internal and external state, kickstart the motor pattern for ejaculation, and then step aside. But there was one final twist. If the mouse had already ejaculated, stimulating Gal⁺ neurons didn’t work at all. “The BSM just wouldn’t respond,” says Lenschow.
This revealed something remarkable: these neurons weren’t just coordinating movement, they were tracking the animal’s internal state. “That’s a level of context sensitivity we don’t typically associate with spinal circuits,” Mendes adds.
To understand the role of Gal⁺ neurons in sexual behavior, researchers tried something bold: they used a targeted toxin to remove these neurons in live mice. In rats, this kind of intervention completely blocks ejaculation but leaves the rest of the mating behavior untouched. But in mice, the story was more nuanced.
“Only 3 out of 12 males failed to ejaculate,” explains Ana Rita Mendes, “but many showed a disrupted sequence; they had trouble locating the vagina, took longer to finish, and had more failed attempts.”
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This suggested a more significant role for Gal⁺ neurons in mice. Rather than just triggering ejaculation, these neurons seem to help process touch and mechanical feedback, influence arousal, and fine-tune the rhythm of sexual behavior.
“Gal⁺ spinal neurons seem to play a different role in mice,” says Constanze Lenschow. “It likely reflects species-specific strategies for how sex is structured and timed.”
In rats, ejaculation is more of a reflex; genital stimulation alone can set it off, sometimes during the very first mount. Mice, however, take their time, engaging in repeated thrusts before reaching climax, much like the gradual build-up seen in humans.
“Rats may be good models for studying premature ejaculation,” Lenschow notes, “but mice might actually be better for understanding how human sexuality works, how arousal builds, and how ejaculation is regulated.”
These discoveries are shaking up the old idea that the brain is the sole boss of sexual behavior. Instead of simply giving orders to the spinal cord, the brain and spine work together in a constant back-and-forth. Special spinal neurons called Gal⁺ don’t just trigger ejaculation; they also respond to touch, adjust muscle activity, and factor in the body’s level of arousal. Surprisingly, this spinal teamwork might even help control the recovery phase after ejaculation, deciding when the system is ready to go again.
“We think of the spinal cord as a kind of crossroads,” says researcher Lima. “It gathers signals from the genitals, prostate, and brain, and helps time everything just right.”
Lima even suggests that the final trigger for ejaculation, the “point of no return”, might come from the prostate, not the brain, like an internal message saying, “I’m ready. Time to go.”
These insights could lead to a better understanding and treatment of sexual dysfunction. The next step for the team is to record Gal⁺ neuron activity during sex to see how they interact with other organs. And while rats have long been the standard model for studying ejaculation, this research highlights the mouse as a rising star.
“We’re not here to dethrone the rat,” says Constanze Lenschow, “but the mouse has a lot more to offer than we thought.” Ana Rita Mendes adds, “We’re just beginning to see how actively the spinal cord shapes sexual behavior, it’s not just a messenger, it’s a partner.”
Journal Reference:
- Lenschow, C., Mendes, A.R.P., Ferreira, L. et al. A galanin-positive population of lumbar spinal cord neurons modulates sexual arousal and copulatory behavior in male mice. Nat Commun 16, 8282 (2025). DOI: 10.1038/s41467-025-63877-2
