Ejaculation happens in two rapid phases: emission, where semen is assembled and moved into position, and expulsion, where muscular contractions force it out of the body. The entire process is controlled by a spinal reflex that coordinates signals from the brain, the lower spinal cord, and the autonomic nervous system. Here’s how each part works.
The Two Phases: Emission and Expulsion
During the emission phase, sperm travel from the testicles to the prostate, where they mix with fluids to form semen. The vas deferens, a pair of muscular tubes that store and transport sperm, contract in coordinated waves to push sperm toward the base of the penis. This movement happens sequentially along the entire reproductive tract, with smooth muscle contractions rippling through each structure in order. The walls of the vas deferens are remarkably thick for their size, with three layers of muscle arranged in spiral and lengthwise patterns that generate powerful peristaltic force.
At the same time, the fluid components of semen come together. Sperm cells make up only a small fraction of the total volume. About 65 to 75 percent of semen comes from the seminal vesicles, which contribute a sugar-rich fluid that nourishes sperm. Another 25 to 30 percent comes from the prostate gland, which adds enzymes and minerals. A small additional amount comes from the bulbourethral glands near the base of the penis.
The expulsion phase follows almost immediately. Muscles at the base of the penis contract rhythmically, about once every 0.8 seconds, forcing semen out in several spurts. These contractions are driven by the bulbocavernosus muscle, which compresses the urethra in repeated pulses to propel fluid forward.
How the Bladder Stays Sealed
One critical detail during emission: the internal sphincter at the bladder neck clamps shut. This prevents semen from flowing backward into the bladder and also keeps urine from mixing with semen. The closure is involuntary and happens automatically as part of the ejaculatory reflex. When this sphincter doesn’t close properly, semen can travel into the bladder instead of out of the body, a condition called retrograde ejaculation.
The Spinal Reflex That Runs the Process
Ejaculation is fundamentally a spinal reflex, not a conscious decision. A cluster of specialized nerve cells in the L3 to L5 segment of the lumbar spinal cord acts as the “ejaculation generator.” Once these cells are activated, they trigger the entire sequence of emission and expulsion automatically. This is why ejaculation can occur even in people with certain types of brain injuries, as long as the spinal cord circuitry remains intact.
Three different branches of the nervous system coordinate the process. Sympathetic nerves originating from the T10 to L2 spinal segments trigger peristaltic contractions in the prostate, seminal vesicles, vas deferens, and the bladder neck. Their primary chemical messenger is norepinephrine. Parasympathetic nerves from the S2 to S4 segments control the prostate and seminal vesicles. And the pudendal nerve, also originating at S2 to S4, drives the rhythmic muscular contractions of the expulsion phase.
The sympathetic activation during ejaculation has whole-body effects. Heart rate can increase by as much as 100 percent at the moment of ejaculation due to the surge of epinephrine (adrenaline) that accompanies it.
How Stimulation Reaches the Threshold
The sensory input that builds toward ejaculation travels primarily through the pudendal nerve, which carries touch and pressure signals from the skin of the penis to the spinal cord. One of its branches, the dorsal nerve of the penis, supplies both the erectile tissue and the skin along the shaft. This nerve carries mostly sensory fibers, making it the main pathway through which physical stimulation accumulates toward the ejaculatory threshold.
As stimulation continues, sensory signals build in the spinal ejaculation generator until they cross a point of no return. Once that threshold is reached, the emission phase begins involuntarily. The brain can influence this buildup (erotic thoughts or visual stimulation can contribute), but the emission phase itself is not under conscious control. This is why the sensation of ejaculatory inevitability feels so distinct: once the reflex fires, you can’t stop it.
Sensitivity of the dorsal nerve varies between individuals. Premature ejaculation has been linked to hyperexcitability of this nerve and a lower sensory threshold, meaning the reflex fires with less stimulation than typical.
The Brain Chemistry Behind Timing
Two neurotransmitters play opposing roles in controlling when ejaculation happens. Dopamine acts as an accelerator. It facilitates sexual motivation, physical performance, and genital reflexes. Serotonin, on the other hand, generally acts as a brake. It is released in the hypothalamus at the moment of ejaculation and has an overall inhibitory effect on the process.
The balance between these two chemicals largely determines ejaculatory timing. Drugs that increase serotonin activity in the brain (the same class of antidepressants known as SSRIs) delay the onset of sexual activity and delay ejaculation after it begins. They do this partly by reducing dopamine release in reward pathways. This is why delayed ejaculation is one of the most common sexual side effects of antidepressant medications, and why SSRIs are sometimes prescribed specifically to treat premature ejaculation.
What Happens After: The Refractory Period
After ejaculation, most men enter a refractory period during which further ejaculation is temporarily impossible and arousal drops sharply. This window can last anywhere from minutes to hours, and it tends to lengthen with age.
For decades, the hormone prolactin was assumed to be responsible. Prolactin surges around the time of ejaculation, and the logic seemed straightforward: higher prolactin, lower sexual drive. But research published in Communications Biology tested this directly by pharmacologically raising and lowering prolactin levels during sexual activity. Neither manipulation affected the refractory period. The current thinking is that prolactin release during sex is more of a byproduct of other neurochemical shifts, particularly the drop in dopamine and the rise in serotonin and oxytocin, rather than the cause of the refractory period itself. The actual mechanism likely involves those broader changes in brain chemistry, though the precise trigger remains an open question.