Summary: A new study overturns the traditional view that the brain solely directs male sexual behavior while the spinal cord executes ejaculation. Researchers identified Galanin-expressing spinal neurons that both drive ejaculation and regulate arousal, integrating sensory input, brain feedback, and internal states.<\/p>\n
In mice, these neurons influenced pacing, coordination, and even the refractory period, suggesting a more active role for the spinal cord in sexual behavior. The findings highlight the spinal cord as a collaborator, not just a messenger, with implications for understanding sexual health and dysfunction.<\/p>\n
Key Facts<\/p>\n
Gal\u207a Neurons\u2019 Role: They link sensory input to motor output, shaping arousal and ejaculation.Integration Hub: The spinal cord actively coordinates timing, pacing, and state during sex.Clinical Potential: Offers new insights for treating sexual dysfunctions like premature ejaculation.<\/p>\n
Source: Champalimaud Centre for the Unknown<\/p>\n
For decades, it was thought that while the brain orchestrated male sexual behaviour \u2013 arousal, courtship, and copulation \u2013 the spinal cord merely executed the final act: ejaculation. <\/p>\n
But a study from the Champalimaud Foundation (CF) challenges that tidy division.<\/p>\n
It reveals that a key spinal circuit is not only involved in ejaculation but also in arousal and shaping the choreography of sex, adding a surprising new dimension to our understanding of sexual behaviour in mammals.<\/p>\n
\u201cThe spinal cord isn\u2019t just a passive relay station executing brain commands\u201d, says Susana Lima, Principal Investigator of CF\u2019s Neuroethology Lab and senior author of the study. \u201cIt integrates sensory inputs, responds to arousal, and adjusts its output based on the animal\u2019s internal state. It\u2019s much more sophisticated than we imagined\u201d.<\/p>\n
The Neurons That Drive the Drivers<\/p>\n
\u201cWe were initially interested in female sexual behaviour\u201d, recalls Lima, \u201cbut it is difficult to pinpoint the moment of orgasm. In males, ejaculation is a clear and observable marker \u2013 you can literally see it in the muscle activity\u201d.<\/p>\n
The team began with a deceptively simple question: which neurons control the muscle responsible for ejaculation?<\/p>\n
\u201cThe muscle in question is the bulbospongiosus, or BSM\u201d, explains Constanze Lenschow, co-lead author and now Group Leader at the INCIA Institute at the University of Bordeaux.<\/p>\n
\u201cIt sits just below the penis, and is critical for sperm expulsion. When a male ejaculates, the BSM fires in a characteristic burst pattern. It\u2019s like the signature of ejaculation\u201d.<\/p>\n
To trace the origins of this signal, the team used anatomical tracing techniques to map the pathway from the BSM back to its motor neurons, the cells that directly command it to contract.<\/p>\n
They then looked one step further: which neurons control those motor neurons? Initial attempts to map connections using a rabies virus tracer hit a wall.<\/p>\n
\u201cIt was frustrating\u201d, says co-first author Ana Rita Mendes, who joined the project during her MSc. \u201cSo we switched tactics\u201d.<\/p>\n
Earlier work in rats had identified a group of spinal neurons expressing a molecule called galanin (Gal) as key to ejaculation. Building on this, the team used genetically modified mice in which Gal-expressing (Gal\u207a) neurons fluoresced red.<\/p>\n
Under the microscope, they saw that the axons \u2013 long projections that transmit signals \u2013 of these Gal\u207a neurons overlapped with the BSM motor neurons, suggesting a direct link.<\/p>\n
To test this, Lenschow performed patch-clamp electrophysiology in spinal cord slices.<\/p>\n
\u201cWhen we activated the far ends of Gal\u207a neurons \u2013 where they pass on signals \u2013 we recorded a burst of activity in the BSM motor neurons. And when we blocked glutamate \u2013 the chemical these neurons use to excite others \u2013 the signal disappeared, confirming a direct, excitatory connection\u201d.<\/p>\n
This was the first time a functional, one-to-one connection between Gal\u207a spinal neurons and ejaculation-controlling motor neurons had been demonstrated in any species.<\/p>\n
\u201cAnd interestingly\u201d, notes Mendes, \u201cGal\u207a neurons didn\u2019t just project to the ejaculation muscle, they also connected to other areas involved in erection and the autonomic control of ejaculation\u201d.<\/p>\n
Importantly, the team showed that Gal\u207a neurons receive sensory input from the penis. In spinalised mice \u2013 where the spinal cord is severed from the brain \u2013 a light puff of air to the penis activated both Gal\u207a neurons and BSM motor neurons, confirming that the circuit is sensitive to genital stimulation.<\/p>\n
Turning On the Sex Circuit<\/p>\n
To test whether these Gal\u207a neurons could actually drive ejaculation, the team used either electrical stimulation or a more precise method called optogenetics, which enabled them to\u00a0 selectively activate Gal\u207a neurons in genetically modified mice using light.<\/p>\n
In rats, stimulating these neurons reliably triggers ejaculation. But in mice, things didn\u2019t go as expected.<\/p>\n
\u201cWe could get the BSM to fire, but stimulation of Gal\u207a neurons never led to a real ejaculation\u201d, says Lenschow. \u201cAnd unlike in rats, when we repeated Gal\u207a stimulation, BSM responses weakened. It was as if the system had entered a refractory state after that initial activation\u201d.<\/p>\n
Notably, robust BSM activity only occurred in spinalised mice, where brain input was removed. This suggests that descending signals from the brain actively suppress the spinal circuit \u2013 until the right moment.<\/p>\n
\u201cOur findings support a model where descending input \u2013 likely from a brainstem region \u2013 inhibits the Gal\u207a neurons and incoming genital signals until the animal reaches the ejaculatory threshold\u201d, says Mendes.<\/p>\n
Taken together, the results suggest that Gal\u207a neurons receive sensory input, weigh internal and external signals, initiate the motor pattern that ends in ejaculation \u2013 and then step aside. But there was one more twist.<\/p>\n
\u201cIf the mouse had already ejaculated, Gal\u207a stimulation didn\u2019t work \u2013 the BSM just wouldn\u2019t respond\u201d, says Lenschow.<\/p>\n
\u201cThat told us these neurons weren\u2019t just coordinating ejaculation. They were integrating the animal\u2019s internal state\u201d.<\/p>\n
In other words, the spinal cord seemed to \u201cknow\u201d whether or not the mouse had recently ejaculated.<\/p>\n
\u201cThat\u2019s a level of context sensitivity we don\u2019t typically associate with spinal circuits\u201d, adds Mendes.<\/p>\n
Of Mice and Men: A Better Fit Than Rats?<\/p>\n
The researchers then asked: what happens if we use a targeted toxin to selectively eliminate Gal\u207a neurons in live mice?<\/p>\n
\u201cIn rats, destroying these cells completely blocks ejaculation \u2013 but leaves copulatory patterns intact\u201d, explains Mendes.<\/p>\n
\u201cIn 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\u201d.<\/p>\n
This pointed to a sensory deficit, suggesting that Gal\u207a neurons in healthy mice integrate touch or mechanical feedback, as well as influence arousal and the pacing of sexual behaviour.<\/p>\n
\u201cGal\u207a spinal neurons seem to play a different role in mice\u201d, says Lenschow. \u201cIt likely reflects species-specific strategies for how sex is structured and timed\u201d.<\/p>\n
In rats, ejaculation is more like a reflex \u2013 genital stimulation is often enough to trigger it, sometimes during the first mount. Mice, by contrast, engage in repeated mounts and thrusts before ejaculation, resembling the gradual build-up seen in humans.<\/p>\n
\u201cRats may be good models for studying premature ejaculation\u201d, notes Lenschow, \u201cbut mice might actually be better for understanding how human sexuality works \u2013 how arousal builds, and how ejaculation is regulated\u201d.<\/p>\n
A Multiway Dialogue<\/p>\n
These findings challenge the traditional top-down view of sexual control and lead to a rethinking of how ejaculation is controlled. Instead of the brain simply commanding the spinal cord to act, the two appear to be in continuous dialogue \u2013 with the Gal\u207a spinal neurons receiving sensory input, modulating motor output, and integrating signals related to arousal and internal state.<\/p>\n
This spinal integration may even contribute to the refractory period, the temporary lull in sexual responsiveness after ejaculation, suggesting that the spinal cord itself helps control when the system is ready to go again, contrary to current thinking.<\/p>\n
\u201cWe think of the spinal cord as a kind of crossroads\u201d, says Lima. \u201cIt integrates input from the genitals, the prostate, and the brain, and helps orchestrate the sequence and timing of copulation and determine whether conditions are right for ejaculation\u201d.<\/p>\n
In fact, Lima speculates that the \u201cpoint of no return\u201d \u2013 the moment after which ejaculation is inevitable \u2013 may not come from the brain at all, but from the prostate, acting like an internal status update: \u201cI\u2019m ready. Time to go\u201d.<\/p>\n
Beyond basic biology, these findings open new avenues for understanding sexual dysfunction and erectile disorders. The team\u2019s next step is to record directly from Gal\u207a neurons during sex to understand how their firing patterns relate to behaviour and interact with other organs like the brain and prostate.<\/p>\n
And while the rat has long reigned as the go-to model for ejaculation, this study may mark a changing of the guard.<\/p>\n
\u201cWe\u2019re not here to dethrone the rat\u201d, says Lenschow, \u201cbut we do think the mouse has much more to contribute to our understanding of reproduction than it\u2019s been given credit for\u201d.<\/p>\n
\u201cWe\u2019re just beginning to understand how richly the spinal cord contributes as an active player to sexual behaviour\u201d, adds Mendes. \u201cIt\u2019s not just a conduit \u2013 it\u2019s a collaborator\u201d.<\/p>\n
About this neuroscience and arousal research news<\/p>\n
Author: Hedi Young<\/a>
Source: Champalimaud Centre for the Unknown<\/a>
Contact: Hedi Young \u2013 Champalimaud Centre for the Unknown
Image: The image is credited to Neuroscience News<\/p>\n