Beyond Reflexes: The Spinal Cord’s Role in Shaping Sexual Behavior

For many years, the scientific view was that the brain was the primary controller of male sexual activity, overseeing arousal, courtship, and climax, with the spinal cord simply carrying out the final step—ejaculation. However, groundbreaking research from the Champalimaud Foundation is challenging this traditional hierarchy. The study demonstrates that the spinal cord itself plays an active role, not only in ejaculation but also in modulating arousal and influencing the overall sequence of sexual behavior in mammals.

According to lead researcher Susana Lima, the spinal cord is more than a passive relay station. It integrates sensory information, responds to arousal signals, and adjusts its output based on the internal state of the animal, revealing a level of complexity previously unacknowledged.

The investigation initially aimed to understand female sexual responses, but it became clear that in males, the process is more straightforward due to the observable nature of ejaculation. The researchers focused on the muscles responsible for ejaculation, specifically the bulbospongiosus muscle (BSM). Using anatomical tracing techniques, they mapped the pathway from this muscle back to the spinal motor neurons, then further identified neurons that control these motor neurons. To their surprise, they found a specific group of neurons expressing galanin (Gal)—a neuropeptide—playing a crucial role in controlling ejaculation.

Electrophysiological experiments revealed that these Gal-expressing neurons directly excite motor neurons controlling the BSM, with sensory inputs from genital stimulation activating the circuit. Remarkably, activating these neurons in spinalized mice (whose brain input was severed) could trigger muscle responses associated with ejaculation, confirming their integral role in the process.

Further, the study found that descending signals from the brain usually inhibit these Gal neurons until a certain threshold of arousal or internal readiness is reached, explaining why stimulation often did not produce ejaculation in intact mice. The neuronal activity also depended on whether the animal had recently ejaculated, suggesting the spinal circuit’s capacity to monitor internal state and readiness for subsequent rounds of activity.

Interestingly, when researchers used targeted toxins to eliminate Gal neurons in mice, the effects were species-specific. In rats, this blockade prevented ejaculation entirely. In mice, the results were more nuanced: many struggled with mounting, and the sequence was disrupted, indicating that these neurons also contribute to processing sensory feedback and arousal regulation.

These findings fundamentally alter our understanding of sexual control, revealing a dynamic dialogue between the brain and spinal cord. Rather than a one-way command from the brain, the spinal cord is an active participant, receiving sensory input, integrating internal signals, and modulating motor outputs that influence sexual behavior.

Lima suggests that the spinal cord may even contribute to the refractory period—the period of sexual unresponsiveness following ejaculation—by controlling the state of readiness for subsequent activity. Furthermore, the study raises intriguing possibilities that internal organs like the prostate could signal the probability of ejaculation, adding depth to the biological control mechanisms.

This research opens new avenues for understanding sexual dysfunctions and disorders. Future studies aim to directly record neuron activity during sexual activity to better understand how spinal circuits and other organs interact during reproduction. While rats have traditionally been used as models for studying ejaculation, mice might provide a more accurate representation of human sexual dynamics, especially regarding arousal build-up and response regulation.

Overall, the findings demonstrate that the spinal cord is not just a passive conductor but a crucial, active partner in controlling sexual behavior, continually integrating sensory, internal, and contextual signals to orchestrate the complex process of reproduction.