The Spinal Circuit Is Not Only Involved in Ejaculation but Also in Arousal

Until recently, researchers thought the brain managed arousal and courtship, leaving the spinal cord to handle ejaculation alone.

A new study from the Champalimaud Foundation in Lisbon and the University of Bordeaux challenges that view, showing spinal circuits help regulate arousal, timing and the refractory period in male mice.

“We’re just beginning to understand how richly the spinal cord contributes as an active player to sexual behavior,” said co-first author Dr. Ana Rita Mendes, who joined the project during her master of science degree. “It’s not just a conduit – it’s a collaborator”.

How sexual behavior was thought to be controlled

For a long time, scientists believed the brain drives sexual behavior, in particular arousal, courtship and copulation, while the spinal cord acts only at the end of the line, executing ejaculation.

However, this sharp division misses some questions. How are sensory signals from the genitals, arousal and the ejaculatory reflex tied together? How much does the spinal cord contribute beyond being a highway for commands from the brain?

Disorders such as premature ejaculation or erectile dysfunction reduce quality of life. Further understanding of how sexual function works is needed to inform treatments, and yet much of what we know comes from studies in rats, where ejaculation is often a reflex triggered early in copulation.

That pattern does not match how humans typically experience sex, which involves a buildup of arousal, repeated stimulation and then ejaculation.

In male mice, ejaculation follows repeated mounting and thrusting, resembling the gradual sexual buildup seen in many mammals, including humans. Mice may therefore offer a more relevant model for studying how arousal, sensory feedback and motor output are integrated.

The new study aims to map the spinal circuit involving neurons expressing galanin (Gal⁺ neurons) in the lumbar spinal cord in male mice. It asks whether these neurons do only the final motor job of ejaculation, or also play a role in arousal, sensory input and pacing of sexual behavior.

Mapping spinal circuits that coordinate sexual behavior

“We were initially interested in female sexual behavior,” said corresponding author Dr. Susana Q. Lima, a principal investigator at the Champalimaud Foundation. “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”.

“The muscle in question is the bulbospongiosus, or BSM”, explained Dr. Constanze Lenschow, co-lead author and group leader at the Institute for Cognitive and Integrative Neuroscience of Aquitaine at the University of Bordeaux. “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”.

Ejaculation has two stages: emission of sperm into the urethra and expulsion by the BSM.

The team started by tracing the BSM back to the motor neurons that control it and then looked at which neurons connect to those.

Early attempts to map the circuit failed, so the team switched tactics.

They used genetically engineered mice in which galanin-expressing neurons fluoresced red. Under the microscope, these Gal⁺ neurons appeared to project directly to the BSM motor neurons. To test the connection, the researchers used patch-clamp recordings.

“When we activated the far ends of Gal⁺ neurons – where they pass on signals – we recorded a burst of activity in the BSM motor neurons. And when we blocked glutamate – the chemical these neurons use to excite others – the signal disappeared, confirming a direct, excitatory connection,” said Lenschow.

The Gal⁺ neurons also responded to genital touch, even when the brain was disconnected from the spinal cord. They also projected more widely than expected, linking to areas involved in erection and autonomic control.

Yet their activity depended on the animal’s state. Repeated stimulation weakened responses, and after ejaculation, the system shut down.  

“We could get the BSM to fire, but stimulation of Gal⁺ neurons never led to a real ejaculation,” said Lenschow. “And unlike in rats, when we repeated Gal⁺ stimulation, BSM responses weakened. It was as if the system had entered a refractory state after that initial activation.”

Removing Gal⁺ neurons reinforced the idea that these neurons help coordinate sex rather than just execute ejaculation.

“In rats, destroying these cells completely blocks ejaculation – but leaves copulatory patterns intact,” said Mendes. “In mice, however, the effect was more subtle. Only 3 out of 12 males failed to ejaculate, and many showed a disrupted sequence: they struggled to find the vagina and took longer to ejaculate, with more failed mounts and probing attempts”.

What the findings mean for sex research

Although it was previously believed that the brain commands and the spinal cord only reacts, the team has shown that the spinal cord appears to play an active role, weighing genital input, arousal and brain signals in real time.

“The spinal cord isn’t just a passive relay station executing brain commands. 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,” said Lima.

“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,” said Mendes.

The authors speculate that the prostate might signal the spinal cord when the body is ready, helping set the “point of no return” for ejaculation.

“If the mouse had already ejaculated, Gal⁺ stimulation didn’t work – the BSM just wouldn’t respond,” said Constanze Lenschow. “That told us these neurons weren’t just coordinating ejaculation. They were integrating the animal’s internal state.”

“That’s a level of context sensitivity we don’t typically associate with spinal circuits,” added Mendes.

The study shows the level of importance when choosing which animal model to use. “Gal⁺ spinal neurons seem to play a different role in mice,” said Lenschow. “It likely reflects species-specific strategies for how sex is structured and timed.”

“Rats may be good models for studying premature ejaculation,” said Lenschow. “But mice might actually be better for understanding how human sexuality works, how arousal builds and how ejaculation is regulated.”

 “We’re not here to dethrone the rat, but we do think the mouse has much more to contribute to our understanding of reproduction than it’s been given credit for,” he added.

Future work will focus on recording Gal⁺ activity during natural sex and mapping connections to organs like the prostate and brainstem.

 

Reference: Lenschow C, Mendes ARP, Ferreira L, et al. A galanin-positive population of lumbar spinal cord neurons modulates sexual arousal and copulatory behavior in male mice. Nat Comm. 2025. doi: 10.1038/s41467-025-63877-2

 

This article is a rework of a press release issued by the Champalimaud Centre for the Unknown. Material has been edited for length and content.