Ejaculation is a reflex controlled by your nervous system that happens in two distinct phases: emission and expulsion. When sexual arousal reaches a critical threshold, nerves running from your reproductive organs to your spinal cord trigger a rapid sequence of events that mix, move, and forcefully push semen out of the body. The whole process takes only a few seconds, but it involves coordination between your brain, spinal cord, reproductive glands, and several muscles.
The Two Phases of Ejaculation
Ejaculation isn’t a single event. It unfolds in two back-to-back phases, each driven by different parts of your nervous system.
In the first phase, called emission, sperm travel from the testicles through a pair of tubes called the vas deferens toward the prostate. Along the way, the sperm mix with fluids from the seminal vesicles and prostate gland to form semen. The vas deferens contract to squeeze this mixture toward the base of the penis. At the same time, the opening between your bladder and urethra clamps shut so that semen can only move in one direction: outward.
The second phase, expulsion, is the part you actually feel. Muscles at the base of the penis, particularly one that wraps around the urethra called the bulbospongiosus, contract in rapid, rhythmic pulses roughly every 0.8 seconds. These contractions act like a pump, propelling semen through the length of the urethra and out of the body in several spurts. Those rhythmic contractions are closely tied to the sensation of orgasm itself.
What Semen Is Actually Made Of
Sperm cells are only a tiny fraction of the fluid that comes out. About 65% of semen volume comes from the seminal vesicles, a pair of glands behind the bladder that produce a thick, nutrient-rich fluid designed to nourish and transport sperm. Another 30 to 35% comes from the prostate gland, which adds enzymes and minerals that help sperm survive. Only about 5% of the total volume consists of sperm and the fluid they’re carried in from the vas deferens.
A typical ejaculate measures about 1.5 to 5 milliliters, roughly a teaspoon at most. The World Health Organization considers anything above 1.4 milliliters to be within the normal range, with a normal sperm concentration of at least 16 million sperm per milliliter.
How Your Nervous System Controls the Reflex
Ejaculation is a spinal reflex, meaning your spinal cord can coordinate it even without direct input from the brain. A cluster of nerve cells in the lower spinal cord, located around the L3 to L5 vertebrae (lower back), acts as the body’s “ejaculation generator.” When stimulation from the genitals reaches these cells and crosses a certain threshold, they fire and set both phases in motion.
Different branches of the nervous system handle different parts of the job. Sympathetic nerves, originating from the mid-to-lower spine, control the contraction of the tubes and glands during emission and close the bladder neck. Parasympathetic nerves from the sacral spine (lower still) help regulate the prostate and seminal vesicles. A third nerve, the pudendal nerve, triggers the rhythmic muscle contractions of the expulsion phase.
Your brain plays a modulatory role rather than a directly controlling one. During arousal, brain activity involving the signaling chemicals dopamine and serotonin helps set the threshold for when ejaculation is triggered. Higher dopamine activity generally makes the reflex easier to trigger, while serotonin tends to raise the threshold and delay it. This is why certain antidepressants that boost serotonin levels commonly cause delayed ejaculation as a side effect.
What Pre-ejaculate Does
Before ejaculation, small glands near the base of the penis (the bulbourethral glands) release a clear, slippery fluid during arousal. This pre-ejaculate serves a practical purpose: it’s alkaline, so it neutralizes any leftover acidity in the urethra from urine. That matters because sperm are fragile and wouldn’t survive an acidic environment. The fluid also lubricates the urethra, making the eventual passage of semen smoother.
Why There’s a Waiting Period Afterward
After ejaculation, most men experience a refractory period during which another ejaculation is temporarily impossible or very difficult. This isn’t just about physical fatigue. It’s driven by changes in brain chemistry.
Immediately after ejaculation, levels of dopamine and the excitatory signaling chemical glutamate drop sharply in a key brain region involved in sexual behavior (the medial preoptic area). At the same time, inhibitory signals ramp up and suppress the spinal ejaculation generator. This essentially puts the reflex on pause. The refractory period can last anywhere from minutes to hours and tends to lengthen with age. Interestingly, the hormone prolactin, which rises after orgasm and was long blamed for the refractory period, now appears to play a much smaller role than previously thought. The current evidence suggests the temporary suppression of excitatory brain signals is the primary mechanism.
When Ejaculation Doesn’t Work as Expected
Several common variations can change how ejaculation works or prevent it from happening.
In retrograde ejaculation, the bladder neck fails to close during orgasm. Instead of traveling out through the penis, semen flows backward into the bladder. You still feel orgasm, but little or no fluid comes out. This is most often caused by prostate surgery, diabetes, or certain medications. It’s not harmful (the semen is simply passed later during urination), but it can affect fertility.
Anejaculation, the complete inability to ejaculate despite adequate stimulation, can have physical or psychological roots. Physical causes include nerve damage from surgery, spinal cord injury, diabetes, neurological conditions like Parkinson’s disease or multiple sclerosis, and medications such as antidepressants or alpha-blockers. Psychological causes include anxiety, depression, relationship stress, and fear of pregnancy. When the cause is situational, ejaculation is typically still possible under different circumstances, such as during sleep or with a different type of stimulation.
Premature ejaculation, where the reflex triggers faster than desired, and delayed ejaculation, where it takes an unusually long time, are both tied to the balance of dopamine and serotonin signaling that sets the ejaculatory threshold. These are among the most common sexual health concerns, and both are treatable.