Ejaculation is a two-phase reflex that moves sperm out of the body through a precisely timed sequence of muscle contractions, nerve signals, and fluid mixing. The entire process, from internal buildup to external release, takes only a few seconds but involves coordination across multiple organs, three branches of the nervous system, and a chain of events that begins well before anything exits the body.
The Two Phases: Emission and Expulsion
Ejaculation happens in two distinct stages. The first, called emission, is the loading phase. Sperm travels from the testicles through a pair of tubes called the vas deferens toward the prostate, where it mixes with fluids from several glands to form semen. At the same time, the opening between the bladder and the urethra clamps shut. This prevents semen from traveling backward into the bladder and keeps urine out of the mix. That closure is the very first step in the sequence, and it’s driven by the sympathetic nervous system, the same branch responsible for fight-or-flight responses.
The second phase, expulsion, is what most people think of as ejaculation itself. A group of muscles at the base of the penis contracts rhythmically, about once every 0.8 seconds, forcing semen through the urethra and out of the body in several pulses. The contractions involve multiple muscles working in concert: the bulbospongiosus muscle wraps around the base of the urethra and acts like a pump, while surrounding pelvic floor muscles (including the ischiocavernosus and levator ani) squeeze in a coordinated wave. These clonic contractions continue until the urethra is fully emptied.
What Semen Is Actually Made Of
Sperm cells make up a surprisingly small fraction of the fluid that comes out. The bulk of semen, roughly 65% to 75% of its volume, is a fructose-rich fluid produced by the seminal vesicles, two glands that sit behind the bladder. This fluid provides energy for the sperm to swim. Another 25% to 30% comes from the prostate gland, which adds enzymes and minerals that help semen liquefy after it’s been deposited. A small additional contribution comes from the bulbourethral glands (sometimes called Cowper’s glands), which release a clear, slippery secretion before ejaculation that lubricates the urethra and clears out any residual urine that could harm sperm.
All of these fluids mix together in the prostatic urethra during the emission phase. The final product is what we call semen. A typical ejaculate measures about 1.5 to 5 milliliters, with the World Health Organization setting 1.4 ml as the lower reference limit for healthy men.
How the Nervous System Coordinates It
No single nerve controls ejaculation. It requires all three branches of the peripheral nervous system working together: sympathetic, parasympathetic, and somatic. The sympathetic nerves, originating from the thoracolumbar spine (roughly the mid-to-lower back region), control the emission phase. They trigger the contractions of the vas deferens, seminal vesicles, and prostate that push their contents into the urethra, and they close the bladder neck.
The parasympathetic nerves, originating from the lower sacral spine, contribute to the wavelike contractions of the accessory glands that help propel fluid forward. The somatic nervous system handles the voluntary-type muscle contractions of the pelvic floor through the pudendal nerve, which is what drives the rhythmic pumping action during expulsion.
There’s also a “spinal ejaculation generator,” a cluster of neurons in the lower spinal cord that coordinates the entire reflex. This is why ejaculation can still occur in some people with spinal cord injuries above that level. The brain influences the process, particularly through arousal and sensory input from the genitals, but the core machinery runs through the spinal cord.
What Triggers the Reflex
Sensory nerves in the penis, particularly along the dorsal nerve that runs along the top, send signals to the spinal cord as stimulation builds. Once those signals reach a threshold, the emission phase begins involuntarily. This is often described as the “point of no return,” the moment where ejaculation will proceed regardless of whether stimulation continues. Sensory feedback from the urethra filling with semen during emission then triggers the expulsion reflex, initiating and maintaining the rhythmic pelvic floor contractions until the urethra is cleared.
The Role of Pre-Ejaculate
Before ejaculation, the bulbourethral glands release a small amount of clear fluid, commonly called pre-cum. This secretion serves two purposes: it lubricates the urethra to reduce friction during ejaculation, and it flushes out any acidic urine residue that could damage sperm as they pass through. These glands also produce immune-protective compounds, including antibodies that help defend the reproductive tract from infection.
What Happens After: The Refractory Period
Following ejaculation, most men enter a refractory period during which another erection or orgasm is temporarily impossible. This window can last anywhere from minutes to hours, and it tends to lengthen with age. Despite decades of study, the exact mechanism behind the refractory period is still not well understood. A common explanation points to a surge of prolactin (a hormone released after orgasm), but the scientific evidence for this is mixed. Studies comparing prolactin levels across different groups have failed to confirm it as the primary driver. The refractory period is also notably resistant to being shortened with medication, which suggests the underlying mechanism is more complex than a single hormone switch.
When the Process Doesn’t Work as Expected
Because ejaculation depends on so many systems working in sync, several things can go wrong. Retrograde ejaculation happens when the bladder neck fails to close properly during emission, sending semen backward into the bladder instead of out through the penis. This is painless and harmless but can affect fertility. It’s most commonly caused by nerve damage from diabetes, certain surgeries, or medications that relax smooth muscle.
Premature ejaculation, where the reflex triggers faster than desired, is the most common ejaculatory concern and relates to the threshold sensitivity of the spinal reflex arc. Delayed or absent ejaculation can result from nerve damage, medications (particularly antidepressants that affect serotonin), or psychological factors that interrupt the signaling chain between arousal and the spinal ejaculation center.
In each case, the underlying issue maps back to a specific part of the process: the nerve signals, the muscle contractions, the sphincter closure, or the coordination between them.