What is ejaculation?
Ejaculation is the process of ejecting semen from the penis, and is usually accompanied by orgasm as a result of sexual stimulation. It may also occur spontaneously during sleep (called a nocturnal emission), due to stimulating of the prostate or, rarely, due to prostatic disease.
Ejaculation is a reflex which usually cannot be stopped once it has started, without painful cramping. It has two phases: emission and ejaculation proper. During emission, the two ducts known as vas deferens contract to propel sperm from the epididymis (where it was stored) up to the ampullas at the top end of the vas deferens. The beginning of emission is typically experienced as a "point of no return". The sperm then passes through the ejaculatory ducts and is mixed with fluids from the seminal vesicles, the prostate, and the bulbourethral glands to form the semen or ejaculate. During ejaculation proper, the semen is ejected through the urethra with rhythmical contractions.
The force and amount of ejaculate vary widely from male to male. A normal ejaculation may contain anywhere from 2 to 15 cubic centimetres (from half a teaspoon to a tablespoon), although 5 to 7 cm3 is typical. The amount of ejaculate is highly dependent on how recent the last previous ejaculation was within a range of the first two days. In other words, an ejaculation soon after a previous ejaculation will be very small, but will be much larger if ejaculation has not occurred for two days or longer. In most men, the more semen ejaculated the more pleasurable an orgasm they experience. Many also experience a higher amount of semen by having foreplay before sex.
Some men may be able to shoot their ejaculate a distance of several feet, while the semen may simply ooze out of some men's penises. Again, many say the force is dependent on several factors such as the last previous ejaculation or simply how sexually aroused the male is to begin with.
The number of sperm in an ejaculation also varies widely, depending on many factors, including the recentness of last ejaculation, the average warmth of the testicles, the degree and length of time of sexual excitement prior to ejaculation, the age, testosterone level, and general fertility of the subject, and the total volume of seminal fluid. Also, the use of marijuana and other substances may kill or damage sperm.
The ejaculation reflex is caused by the sympathetic nervous system, while erection of the penis is caused by the parasympathetic nervous system. Most men experience a lag time of some half-hour or so between the ability to ejaculate consecutively. During this refractory period it is difficult or impossible to attain an erection, because the sympathetic nervous system counteracts the effects of the parasympathetic nervous system.
There are wide variations in how long sexual intercourse can last before ejaculation occurs. Studies have shown that most men can only avoid ejaculation during active thrusting for five minutes or less. A minority can ejaculate more or less at will, and delay ejaculation for an hour or longer during sexual intercourse.
When a man ejaculates before he wants to it is called premature ejaculation. If a man is unable to ejaculate in a timely manner after prolonged sexual stimulation, in spite of his desire to do so, it is called delayed ejaculation or anorgasmia. An orgasm that is not accompanied by ejaculation is known as a dry orgasm.
(Extract from http://en.wikipedia.org/wiki/Ejaculation)
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