Ejaculation
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Ejaculation is the ejecting of semen from the penis, and is usually accompanied by orgasm. It is usually the result of sexual stimulation, which may include prostate stimulation. Rarely, it is due to prostatic disease. Ejaculation may occur spontaneously during sleep (a nocturnal emission). Anejaculation is the condition of being unable to ejaculate.
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Phases
Ejaculation has two phases: emission and ejaculation proper. The emission phase of the ejaculatory reflex is under control of the sympathetic nervous system, while the ejaculatory phase is under control of a spinal reflex at the level of the spinal nerves S2-4 via the pudendal nerve. A refractory period succeeds the ejaculation, and sexual stimulation precedes it.
Emission
During emission, the two ducts known as vas deferens contract to propel sperm from the epididymis where it was stored up to the ampullae at the top end of the vas deferens. The beginning of emission is typically experienced as a "point of no return," also known as point of ejaculatory inevitability. 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 rhythmic contractions.[1]
Ejaculation proper
These rhythmic contractions are part of the male orgasm. They are generated by the bulbospongiosus muscle. The typical male orgasm lasts about 17 seconds but can vary from a few seconds up to about a minute. After the start of orgasm, pulses of semen begin to flow from the urethra, reach a peak discharge and then diminish in flow. The typical orgasm consists of 10 to 15 contractions, each bringing an extremely pleasurable sensation to the head of the penis. Once the first contraction has taken place, there is nothing the male can do to prevent ejaculation taking place. The rate of contractions gradually slows during the orgasm. Initial contractions occur at an average interval of 0.6 seconds with an increasing increment of 0.1 second per contraction. Contractions of most men proceed at regular rhythmic intervals for the duration of the orgasm. Many men also experience additional irregular contractions at the conclusion of the orgasm.[2]
Semen content
Sertoli cells, which nurture and support developing spermatocytes, secrete a fluid into seminiferous tubules that helps transport spermatozoa to the genital ducts. The Ductuli Efferentes possess cuboidal cells with microvilli and lysosomal granules that modify the semen by reabsorbing some fluid. Once the semen enters the Ductus Epididymis the principle cells, which contain pinocytotic vessels indicating fluid reabsorption, secrete glycerophosphocholine which most likely inhibits premature capacitation. The accessory genital ducts, the seminal vesicle, prostate glands, and the bulbourethral glands, produce most of the seminal fluid. The seminal vesicles produce a yellowish viscous fluid rich in fructose and other substances that makes up ~70% of human ejaculate. The prostatic secretion, influenced by dihydrotestosterone, is a whitish, thin fluid containing proteolytic enzymes, citric acid, acid phosphatase and lipids. The bulbourethral glands secrete a clear secretion into the lumen of the urethra to lubricate it. Semen begins to spurt from the penis during the first or second contraction of orgasm. For most men the first spurt occurs during the second contraction. A small sample study of seven men showed between 26 and 60 percent of the contractions during orgasm were accompanied by a spurt of semen.[3]
Refractory period
Most men experience a lag time between the ability to ejaculate consecutively, and this lag time varies among men. Age also affects the recovery time; younger men typically recover faster than older men. 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.
Stimulation
- See also: Sexual stimulation
There are wide variations in how long sexual stimulation can last before ejaculation occurs.
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.
Volume
The force and amount of ejaculate vary widely from male to male. A normal ejaculation may contain anywhere from 1.5 to 5 milliliters.[4] Adult ejaculate volume is affected by the amount of time that has passed since the previous ejaculation. Larger ejaculate volumes are seen with greater durations of abstinence. However, a recent Australian study has suggested a positive correlation between prostate cancer and infrequent ejaculation and/or prostate milking, which performs essentially the same function. That is, frequent ejaculation appears to reduce the risk of prostate cancer. Frequent ejaculation is more easily obtained and sustained over time with the aid of masturbation and it is these ejaculations which are important, not the mechanism.[5] Also, the duration of the stimulation leading up to the ejaculation can affect the volume. Abnormally low volume is known as hypospermia, though it is normal for the amount of ejaculate to diminish with age.
Quality
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[citation needed], the age, testosterone level, the nutrition and especially hydration and the total volume of seminal fluid. An unusually low sperm count, not the same as low semen volume, is known as oligospermia, and the absence of any sperm from the ejaculate is termed azoospermia.
Pleasure
Each contraction is associated with a wave of sexual pleasure, especially in the penis and loins. The first and second convulsions are usually the most intense in sensation, and produce the greatest quantity of semen. Thereafter, each contraction is associated with a diminishing volume of semen and a milder wave of pleasure.
Ejaculate development during puberty
The first ejaculation in males occurs about 12 months after the onset of puberty. This first ejaculate volume is small. The typical ejaculation over the following three months produces less than 1 ml of semen. The semen produced during early puberty is also typically clear. After ejaculation this early semen remains jellylike and unlike semen from mature males fails to liquify. Most first ejaculations (90 percent) lack sperm. Of the few early ejaculations that do contain sperm, the majority of sperm (97%) lack motion. The remaining sperm (3%) have abnormal motion. [6]
As the male proceeds through puberty, the semen develops mature characteristics with increasing quantities of normal sperm. Semen produced 12 to 14 months after the first ejaculation liquifies after a short period of time. Within 24 months of the first ejaculation, the semen volume and the quantity and characteristics of the sperm match that of adult male semen. [6]
Central nervous system control
To map the neuronal activation of the brain during the ejaculatory response, researchers have studied the expression of c-fos, a proto-oncogene expressed in neurons in response to stimulation by hormones and neurotransmitters [7] Expression of c-fos in the following areas have been observed: [8],[9]
- medial preoptic area (MPOA)
- lateral septum, bed nucleus of the stria terminalis
- paraventricular nucleus of the hypothalamus (PVN)
- ventromedial hypothalamus, medial amygdala
- ventral premammillary nuclei
- ventral tegmentum
- central tegmental field
- mesencephalic central gray
- peripeduncular nuclei
- parvocellular subparafascicular nucleus (SPF) within the posterior thalamus
Fertilization
During heterosexual intercourse, the vagina provides sexual stimulation to the penis, typically resulting in orgasm and ejaculation. Normally, ejaculation is required for emission of sperm; if ejaculation happens while the penis is either near or within the woman's vagina, sperm can then travel into the uterus and fertilize an egg if present, impregnating the woman. However, almost all men produce a small amount of pre-ejaculate fluid when their penis is erect and they are sexually stimulated, and this pre-ejaculate may contain some sperm which can also lead to pregnancy. For this reason, coitus interruptus may still lead to unwanted pregnancies for couples engaging in vaginal intercourse if other forms of birth control are not used as well. See also artificial insemination.
See also
- Aspermia
- Azoospermia
- Coitus interruptus
- Coitus reservatus
- Delayed ejaculation
- Female ejaculation
- Retrograde ejaculation
- Keri - ejaculation in Jewish law
References
- ^ Walter F. Boron, Emile L. Boulpaep, (2005). Medical Physiology: A Cellular and Molecular Approach. Philadelphia, PA: Elsevier/Saunders. ISBN 1-4160-2328-3.
- ^ Bolen, J. G., (1980-Dec-9). "The male orgasm: pelvic contractions measured by anal probe". Archives of Sexual Behavior (6): 503-21. doi:. PMID 7458658.
- ^ Gerstenburg, T. C. (1990). "Erection and ejaculation in man. Assessment of the electromyographic activity of the bulbocavernosus and ischiocavernosus muscles". British Journal of Urology 65 (4): 395-402. PMID 2340374.
- ^ Swimming Toward Conception: The Semen Analysis. Focus on Fertility, American Infertility Association and Organon Pharmaceuticals USA Inc.
- ^ Masturbation Cuts Cancer Risk. BBC News Online (16 July 2003).
- ^ a b Janczewski, Z. and Bablok, L. (1985). "Semen Characteristics in Pubertal Boys". Archives of Andrology 15: 199-205. PMID 3833078.
- ^ Sagar SM, "et al.". "Expression of c-fos protein in brain: metabolic mapping at the cellular level". Science: 1328-1332. doi:. PMID 3131879.
- ^ Pfaus JG and Heeb MM, (1997). "Implications of immediate-early gene induction in the brain following sexual stimulation of female and male rodents". Brain Res Bull 44: 397-407. doi:. PMID 9370204.
- ^ Veening JG and Coolen LM, (1998). "Neural activation following sexual behavior in the male and female rat brain". Behav Brain Res 92: 181-193. doi:. PMID 9638960.
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