The formal definition of male ejaculation is “the release of semen from the penis”. During sexual stimulation, semen collects in the ejaculatory ducts located where the vas deferentia join seminal vesicles within the prostate gland. When stimulation reaches climax, a spinal reflex causes contractions to the muscles within the penis, urethra, and the prostate gland. These contractions propel the semen through the urethra and out of the tip of the penis. Although male ejaculation and orgasm occur simultaneously in most cases, they are not the same process. An orgasm, specifically, is the rhythmic muscular contractions that are a release of accumulated sexual tension and result in an intensely pleasurable sensation.
The rhythmic contractions occur initially at 0.8-second intervals, just as in women, and account for the spurting action of the semen during ejaculation. The intervals between contractions become longer and the intensity of the contractions tapers off after the first three or four contractions. Semen usually takes a few seconds to appear … ejaculatory inevitability (the point at which stopping is impossible) … because of the distance the fluid has to travel. During ejaculation, the sphincter of the urinary bladder is tightly sealed to keep urine from mixing with the semen, as well as making sure that the fluid travels forward, or out.
There are a surprising 20 million sperm/ml of seminal fluid, and average ejaculation consists of 1.5-5ml. Semen contains aboutonia, ascorbic acid, blood-group antigens, calcium, chlorine, cholesterol, choline, citric acid, creatine, deoxyribonucleic acid, fructose, glutathione, hyaluronidase, inositol, lactic acid, magnesium, nitrogen, phosphorus, potassium, purine, pyrimidine, pyruvic acid, sodium, sorbitol, spermidine, spermine, urea, uric acid, vitamin B12, and zinc -- but all in very modest quantities. Some of this is designed to nourish the semen enroute, while other ingredients are designed to neutralize the acidic environment of the vagina (which would normally be inhospitable to Sperm). In case you are wondering, most of these ingredients are indeed very healthy, and an average ejaculation contains only about 15 calories.
Since orgasms and ejaculation are separate, it is possible to have either without the other.
In situations where ejaculation occurs on its own, there are two main causes. In the first, involuntary ejaculation occurs during sleep, and is known as nocturnal emission (formally) or a wet dream (in slang). The second main cause is known as Premature Ejaculation), which is a sexual condition in which a man consistently feels he has little or no control over the timing of his build up to ejaculation.
Orgasm can occur without ejaculation. Retrograde ejaculation is a condition where the bladder's sphincter does not close off properly during ejaculation, so semen flows into the bladder. It is commonly found in men who have multiple sclerosis, diabetes, or after some types of prostate surgery. Before you get worried, it can also occur occasionally in men who do not have any serious problems. It is not physically harmful, but it can render the man infertile and he may have a different sensation during ejaculation. This condition is also known as a 'dry come' because the man may experience orgasm, but no semen is released from the penis.
There is also a condition known as retarded ejaculation, ejaculatory incompetence, or delayed ejaculation. It is a sexual condition in which a man is unable to ejaculate even though he is highly sexually aroused.