For decades, the scientific consensus held that male sexual behavior in mammals was orchestrated largely by the brain, with the spinal cord relegated to a passive, execution-only role, simply triggering ejaculation upon command. This neat division framed our understanding of male sexuality, delineating the brain as the control center and the spinal cord as a mechanical appendage. However, groundbreaking research from the Champalimaud Foundation (CF) is now shattering this simplistic view. Their new study reveals a nuanced and active role for a specific population of spinal cord neurons in not only triggering ejaculation but also modulating sexual arousal and behavioral patterns around copulation in male mice. This discovery reveals the spinal cord as a dynamic processor and integrator in the sexual response, fundamentally rewriting decades of neuroscience dogma.
The research team, led by Susana Lima of CF’s Neuroethology Lab, embarked on this project with an eye toward understanding neural substrates of sexual behavior in females, but rapidly pivoted to male sexual control due to the clearer and quantitatively measurable event of ejaculation. In male rodents, unlike females, ejaculation is accompanied by a distinctive burst of muscle activity, particularly of the bulbospongiosus muscle (BSM), which facilitates sperm expulsion. This muscular “signature” provided the researchers a tangible endpoint to trace the neuronal circuits responsible for this critical reproductive behavior.
The initial task was mapping the neural circuitry controlling the BSM. Using a combination of anatomical tracing techniques, the scientists identified direct motor neurons innervating the BSM. Yet the bigger mystery lay one synapse upstream: what neurons command these motor neurons? Attempts to map connections retrogradely using rabies virus tracers proved unsuccessful. Instead, the team leveraged previous findings identifying lumbar spinal neurons expressing the neuropeptide galanin (Gal) in rats as pivotal in ejaculation control. Employing genetically modified mice with galanin-expressing neurons fluorescently tagged allowed precise visualization and beyond-anatomical scrutiny of these neurons’ projections.
Electrophysiological experiments using patch-clamp techniques on spinal cord slices unveiled a direct and excitatory monosynaptic connection between Gal+ neurons and the BSM motor neurons. When the researchers stimulated the axonal terminals of the Gal+ neurons, they elicited robust activation of BSM motor neurons. Importantly, blocking glutamatergic signaling abolished this effect, confirming that glutamate was the neurotransmitter mediating this connection. This marked the first functional demonstration of a direct neural pathway from galanin-expressing spinal neurons to ejaculation-controlling motoneurons in any mammalian species, highlighting a previously unappreciated spinal microcircuit dedicated to male sexual function.
Intriguingly, the Gal+ neurons also showed broader connectivity. The axons projected not only to BSM motor neurons but also targeted regions involved in penile erection and autonomic regulation of ejaculation. This suggests that these spinal neurons integrate and coordinate complex aspects of male sexual behavior, exerting influence not limited to muscle contraction but encompassing sexual arousal and autonomic functions. Furthermore, sensory inputs from the penis were traced directly to the Gal+ neurons, revealing these cells as critical hubs integrating sensory feedback from genital stimulation.
Experiments performed on spinalised mice, where the spinal cord is surgically severed from the brain, further underscored the autonomy of this spinal circuit. Sensory stimulation via a gentle air puff to the penis activated both Gal+ neurons and the downstream BSM motor neurons, confirming local processing capabilities independent of supraspinal influences. This finding dramatically upends the traditional conception that spinal neurons merely relay descending brain commands, instead showing they possess intrinsic sensorimotor integration abilities.
To probe the functional sufficiency of Gal+ neurons in driving ejaculation, the team employed optogenetics, allowing precise activation of these neurons with light in genetically modified mice. Interestingly, while electrical or optogenetic stimulation reliably evoked characteristic BSM firing patterns, it failed to trigger ejaculation in intact mice, contrasting with prior results in rats. Moreover, repeated stimulation resulted in progressively diminished responses, suggesting a refractory or inhibited state. Elevated BSM activity occurred predominantly in spinalised mice, indicating that descending brain inputs normally suppress the spinal circuit until appropriate behavioral thresholds are met.
This inhibitory top-down modulation likely arises from brainstem regions, acting as a gatekeeper that suppresses Gal+ neuron activity and sensory input until male arousal reaches a critical point ensuring ejaculation occurs only under optimal conditions. This feedback loop implies a dynamic compatibility between the spinal cord and brain, functioning in continuous dialogue rather than a simple command-execute hierarchy. In essence, the spinal circuit processes genital sensory input, internal states, and descending brain signals before orchestrating the complex motor pattern culminating in ejaculation.
Beyond modulating motor output, Gal+ neurons appear capable of integrating the animal’s internal physiological state. Following ejaculation, optogenetic stimulation of these neurons failed to elicit the usual BSM activity, indicating that the spinal circuit “remembers” recent sexual activity and adjusts responsiveness accordingly. This capacity for state-dependent modulation challenges long-standing assumptions that spinal neurons lack contextual sensitivity and positions the spinal cord as an active participant in behavioral state regulation, potentially underpinning phenomena such as the refractory period.
To explore the necessity of Gal+ neurons in sexual behavior, selective ablation in male mice was achieved using targeted toxins. Contrary to the complete abolition of ejaculation observed in rats, mouse males showed a more nuanced phenotype characterized by impaired sexual receptivity, delayed ejaculation, increased failed mounts, and disrupted copulatory sequences. These outcomes indicate that Gal+ neurons contribute not only to ejaculation mechanics but also to sensory processing and arousal pacing, integrating multifaceted stimuli to shape complex courtship and mating behaviors.
The differential outcomes between rats and mice suggest species-specific adaptations of the spinal sexual circuit. Rats exhibit rapid, reflexive ejaculation often triggered by minimal genital stimulation, whereas mice engage in repeated mounting and thrusting, reminiscent of the human sexual experience marked by gradual arousal buildup. These behavioral differences imply that mouse models may better parallel human sexual function, offering more nuanced insights into neural control of arousal and ejaculation relevant to human sexual health.
This study signals a paradigm shift in sexual neurobiology. Instead of viewing the spinal cord as a mere conduit for descending brain commands, the spinal cord is repositioned as an integrative hub engaged in a multi-directional dialogue with the brain and peripheral organs. Gal+ neurons exemplify this dynamic, balancing sensory input, internal states, and cortical modulation to efficiently time ejaculation and regulate sexual behavior. Their role in the refractory period also sheds light on the spinal cord’s active participation in controlling sexual readiness, a function previously attributed mostly to higher brain regions.
Susana Lima highlights this integrative role by likening the spinal cord to a crossroads where signals from genital sensory neurons, the prostate, and the brain converge to orchestrate copulatory sequences and ejaculation timing. She proposes a provocative hypothesis that the “point of no return” triggering ejaculation might emanate from the prostate’s feedback rather than the brain, suggesting an internal bodily “go-ahead” signal integrated at the spinal level. This reconceptualization emphasizes the peripheral nervous system’s influence on fundamentally central behaviors.
The broader implications of these findings extend into clinical realms. Understanding the spinal circuit’s active role may yield novel insights into sexual dysfunctions, erectile disorders, and treatment avenues targeting spinal neural populations alongside traditional brain-focused approaches. The CF team anticipates future experiments involving real-time recording from Gal+ neurons during sexual behavior to decode how their firing patterns interrelate with systemic physiological states and contribute to complex sexual experiences.
While the rat has historically dominated reproductive neuroscience, this research suggests a “changing of the guard,” with mouse models offering richer, more human-relevant insights into sexuality. The nuanced modulation of sexual arousal and copulatory behavior by spinal Gal+ neurons in mice underscores the species’ underappreciated value as a model organism for sexual neurobiology. As co-lead author Constanze Lenschow remarks, mice hold promise for unraveling the subtle neurobiological controls governing arousal buildup and ejaculation timing, potentially revolutionizing our grasp of mammalian reproduction.
Together, these findings herald a reconceptualization of the spinal cord from a simple relay to an intelligent collaborator within a complex sexual behavior network. Integrating sensory, motor, autonomic, and central signals, the lumbar spinal cord shapes sexual outcomes in a sophisticated, state-dependent manner. This study sets the stage for a profound expansion of our understanding of mammalian sexual neurobiology, revealing new players and pathways in the orchestration of one of life’s most fundamental behaviors.
Subject of Research: Animals
Article Title: A galanin-positive population of lumbar spinal cord neurons modulates sexual arousal and copulatory behavior in male mice
News Publication Date: Not specified (article published 23-Sep-2025)
Web References: http://dx.doi.org/10.1038/s41467-025-63877-2
Image Credits: Neuroethology Lab, Champalimaud Foundation
Keywords: Galanin neurons, spinal cord, ejaculation, sexual arousal, bulbospongiosus muscle, copulatory behavior, mouse model, optogenetics, sexual neurobiology, refractory period
