Ejaculation is a reflex controlled by your spinal cord that happens in two distinct phases: first your body assembles semen from several internal organs, then a set of muscles rapidly pumps it out. The whole process takes only a few seconds, but it involves a surprisingly coordinated chain of nerve signals, muscle contractions, and even a valve that seals shut to keep everything moving in the right direction.
What Semen Is Made Of
Semen isn’t just sperm. Sperm cells account for only 1% to 5% of the total fluid. The rest is a mixture produced by two glands: the seminal vesicles contribute 65% to 75% of the volume, and the prostate adds another 25% to 30%. These fluids contain fructose (a sugar that fuels sperm), zinc, calcium, magnesium, and small amounts of other nutrients. A typical ejaculation produces 1.5 to 5 milliliters of semen, roughly a quarter teaspoon to a full teaspoon.
Phase One: Emission
The first phase happens entirely inside the body, and you can’t see or consciously control it. When arousal reaches a certain threshold, nerves in the lower spinal cord fire a signal through the sympathetic nervous system. This triggers a wave of contractions in the smooth muscle lining the tubes that carry sperm: the epididymis (where sperm is stored near each testicle), the vas deferens (the long tubes connecting each testicle to the pelvis), and the prostate and seminal vesicles.
These contractions squeeze sperm forward and mix it with the fluid from the prostate and seminal vesicles. The combined semen collects in the prostatic urethra, the short stretch of the urinary tube that runs through the prostate. At this point, many people feel a sense of inevitability, sometimes called the “point of no return,” because the reflex is already in motion.
How the Bladder Stays Sealed
Your urethra serves double duty for both urine and semen, so the body needs a traffic control system. During emission, the opening at the top of the bladder (the bladder neck) clamps shut. This does two things at once: it prevents semen from traveling backward into the bladder, and it keeps urine out of the semen. When this seal doesn’t close properly, semen can flow into the bladder instead of out through the penis. This is called retrograde ejaculation. It’s not harmful, but it can affect fertility.
Phase Two: Expulsion
Once semen is loaded into the prostatic urethra, the second phase kicks in. A muscle at the base of the penis called the bulbospongiosus muscle contracts in rapid, rhythmic pulses. These contractions act like a pump, propelling semen from the prostatic urethra through the length of the penis and out the tip. The pelvic floor muscles surrounding the base of the penis contract in sync, adding force.
These rhythmic contractions are what produce the physical sensation of orgasm. The first few contractions are the strongest and closest together, then they taper off in both intensity and speed. The entire expulsion phase typically lasts a few seconds.
The Nervous System Running the Show
Ejaculation is a spinal reflex, meaning the core coordination happens in the spinal cord rather than the brain. A cluster of specialized nerve cells in the lower lumbar spine (around the L3 to L5 vertebrae) acts as the control center, sometimes called the spinal ejaculation generator. These cells collect sensory input from the genitals and, when stimulation crosses a threshold, fire off a coordinated sequence of signals to multiple systems at once.
The sympathetic nervous system handles the emission phase, using the chemical messenger norepinephrine to trigger smooth muscle contractions along the reproductive tract and close the bladder neck. The somatic nervous system handles the expulsion phase, sending signals through the pudendal nerve to contract the skeletal muscles of the pelvic floor. The brain can influence timing (speeding things up through excitement or slowing them through distraction), but once the spinal reflex fires, it largely runs on autopilot.
What Happens After: The Refractory Period
After ejaculation, most men experience a refractory period where further arousal or a second ejaculation is temporarily difficult or impossible. This window can last anywhere from a few minutes to over an hour, and it tends to get longer with age.
The exact cause of the refractory period is still not fully understood. For years, a spike in the hormone prolactin after orgasm was considered the main explanation, but the scientific evidence for prolactin as the sole driver has been mixed. The current thinking is that no single molecule is responsible. Instead, it likely involves a combination of hormonal shifts, changes in nerve sensitivity, and temporary drops in the brain chemicals involved in arousal. The body essentially enters a brief recovery mode before the system resets.
Orgasm and Ejaculation Are Not the Same Thing
Although orgasm and ejaculation almost always happen together, they’re actually separate processes controlled by different nerve pathways. Orgasm is a brain event, a burst of pleasurable sensation. Ejaculation is a spinal reflex that moves fluid. It’s possible to have one without the other. Some people experience orgasm without ejaculating (called a “dry orgasm”), and in rare cases, ejaculation can occur without the subjective feeling of orgasm. Conditions like retrograde ejaculation, certain medications, and some spinal cord injuries can separate the two.