The Science Behind Erections : How the Body Triggers and Ends an Erection.

When a man is sexually stimulated—whether through touch, thoughts, or visual cues—the brain sends signals down the spinal cord to nerves in the penis. These nerves release nitric oxide, a molecule that triggers the production of cyclic guanosine monophosphate (CGMP). CGMP relaxes the smooth muscle cells in the walls of blood vessels inside the penis, allowing them to widen. This increases blood flow into two sponge-like chambers in the penis called the corpora cavernosa. As these chambers fill with blood, they expand and hold the blood inside which helps the penis to become firm. At the same time, the veins that normally drain blood from the penis begin to constrict, which further reduces outflow and helps sustain the erection for a longer duration.

Once sexual stimulation comes to an end, the sympathetic nervous system activates and cause nitric oxide levels to decrease. As a result, an enzyme called phosphodiesterase type 5 begins to break down CGMP, Which, in turn, causes the smooth muscles in the penile blood vessels to contract, which reduces blood flow into the corpora cavernosa. Consequently, the trapped blood gradually exits through the venous drainage system, leading to a drop in intracavernosal pressure. This process ultimately causes the penis to lose its rigidity and return to a flaccid state.

Additionally, the body releases prolactin, oxytocin, and serotonin, which promote relaxation and reduce further arousal. At the same time, the refractory period begins, during which another erection may not be immediately possible.