Erection

Erection

Three columns of erectile tissue make up most of the volume of the penis.

Anatomical terminology

An erection (clinically: penile erection or penile tumescence) is a physiological phenomenon in which the penis becomes firmer, engorged and enlarged. Penile erection is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is often associated with sexual arousal or sexual attraction, although erections can also be spontaneous. The shape, angle and direction of an erection varies considerably in humans.

Physiologically, erection is triggered by the parasympathetic division of the autonomic nervous system (ANS), causing nitric oxide (a vasodilator) levels to rise in the trabecular arteries and smooth muscle of the penis. The arteries dilate causing the corpora cavernosa of the penis (and to a lesser extent the corpora spongiosum) to fill with blood; simultaneously the ischiocavernosus and bulbospongiosus muscles compress the veins of the corpora cavernosa restricting the egress and circulation of this blood. Erection subsides when parasympathetic activity reduces to baseline.

As an autonomic nervous system response, an erection may result from a variety of stimuli, including sexual stimulation and sexual arousal, and is therefore not entirely under conscious control. Erections during sleep or upon waking up are known as nocturnal penile tumescence (NPT). Absence of nocturnal erection is commonly used to distinguish between physical and psychological causes of erectile dysfunction and impotence.

A penis which is partly, but not fully, erect is sometimes known as a semi-erection (clinically: partial tumescence); a penis which is not erect is typically referred to as being flaccid, or soft.

Physiology

Composite image showing the development of a penile erection

An erection occurs when two tubular structures, called the corpora cavernosa, that run the length of the penis, become engorged with venous blood. This may result from any of various physiological stimuli, also known as sexual stimulation and sexual arousal. The corpus spongiosum is a single tubular structure located just below the corpora cavernosa, which contains the urethra, through which urine and semen pass during urination and ejaculation respectively. This may also become slightly engorged with blood, but less so than the corpora cavernosa.

The scrotum may, but not always, become tightened during erection. Generally, the foreskin automatically and gradually retracts, exposing the glans, though some men may have to manually retract their foreskin.

Autonomic control

In the presence of mechanical stimulation, erection is initiated by the parasympathetic division of the autonomic nervous system (ANS) with minimal input from the central nervous system. Parasympathetic branches extend from the sacral plexus into the arteries supplying the erectile tissue; upon stimulation, these nerve branches release acetylcholine, which, in turn causes release of nitric oxide from endothelial cells in the trabecular arteries.[1] Nitric oxide diffuses to the smooth muscle of the arteries (called trabecular smooth muscle[2]), acting as a vasodilating agent.[3] The arteries dilate, filling the corpora spongiosum and cavernosa with blood. The ischiocavernosus and bulbospongiosus muscles also compress the veins of the corpora cavernosa, limiting the venous drainage of blood.[4] Erection subsides when parasympathetic stimulation is discontinued; baseline stimulation from the sympathetic division of the ANS causes constriction of the penile arteries, forcing blood out of the erectile tissue.[5]

After ejaculation or cessation of stimulation, erection usually subsides, but the time taken may vary depending on the length and thickness of the penis.[6]

Voluntary and involuntary control

The cerebral cortex can initiate erection in the absence of direct mechanical stimulation (in response to visual, auditory, olfactory, imagined, or tactile stimuli) acting through erectile centers in the lumbar and sacral regions of the spinal cord. The cortex may suppress erection, even in the presence of mechanical stimulation, as may other psychological, emotional, and environmental factors.

Nocturnal erection

The penis may erect during sleep or be erect on waking up. Such an erection is medically known as nocturnal penile tumescence (informally: morning wood or morning glory).[7][8][9][10]

Socio-sexual aspects

Social

Erection is a common indicator of sexual arousal and is required for a male to effect vaginal penetration and sexual intercourse.

After reaching puberty, erections occur much more frequently.[11] Male erections are common for children and infants, and even occur before birth.[12] The penile plethysmograph which measures erections has been used by some governments and courts of law to measure sexual orientation. An unusual aversion to the erect penis is sometimes referred to as phallophobia.[13]

Spontaneous or random erections

Spontaneous erection, also known as involuntary, random or unwanted erection, is commonplace and a normal part of male physiology. Socially, such erections can be embarrassing if they happen in public or when undesired.[11] Erections can occur spontaneously at any time of day, and if clothed may cause a bulge which (if required) can be disguised or hidden by wearing close-fitting underwear, a long shirt, or baggier clothes.[14]

Size

An erect human penis

The length of the flaccid penis does not necessarily correspond to the length of the penis when it becomes erect; some smaller flaccid penises grow much longer, while some larger flaccid penises grow comparatively less.[15] Generally, the size of an erect penis is fixed throughout post-pubescent life. Its size may be increased by surgery,[16] although penile enlargement is controversial, and a majority of men were "not satisfied" with the results, according to one study.[17]

Direction

Although many erect penises point upwards, it is common and normal for the erect penis to point nearly vertically upwards or nearly vertically downwards or even horizontally straight forward, all depending on the tension of the suspensory ligament that holds it in position. An erect penis can also take on a number of different shapes, ranging from a straight tube to a tube with a curvature up or down or to the left or right. An increase in penile curvature can be caused by Peyronie's disease. This may cause physical and psychological effects for the affected individual, which could include erectile dysfunction or pain during an erection. Treatments include oral medication (such as colchicine) or surgery, which is most often reserved as a last resort.

The following table shows how common various erection angles are for a standing male. In the table, zero degrees (0°) is pointing straight up against the abdomen, 90 degrees is horizontal and pointing straight forward, while 180 degrees would be pointing straight down to the feet. An upward pointing angle is most common.

 Occurrence of erection angles[18]
Angle (°) Percent of population
0–30 5
30–60 30
60–85 31
85–95 10
95–120 20
120–180 5

Medical conditions

Erectile dysfunction

Erectile dysfunction (also known as ED or "(male) impotence") is a sexual dysfunction characterized by the inability to develop and/or maintain an erection.[19][20] The study of erectile dysfunction within medicine is known as andrology, a sub-field within urology.[21]

Erectile dysfunction occurs when the parasympathetic nerves to the penis do not release enough Nitrous oxide (NO). Around half of American men over 40 experience some form of erectile dysfunction.[3]

Erectile dysfunction can occur due to both physiological and psychological reasons, most of which are amenable to treatment. Common physiological reasons include diabetes, kidney disease, chronic alcoholism, multiple sclerosis, atherosclerosis, vascular disease, and neurologic disease which collectively account for about 70 percent of ED cases.[3] Some drugs used to treat other conditions, such as lithium and paroxetine, may cause erectile dysfunction.[20][22]

Erectile dysfunction, tied closely as it is to cultural notions of potency, success and masculinity, can have devastating psychological consequences including feelings of shame, loss or inadequacy;[23] There is a strong culture of silence and inability to discuss the matter. In fact, around one in ten men will experience recurring impotence problems at some point in their lives.[24]

Priapism

Priapism is a painful condition in which the penis does not return to its flaccid state, despite the absence of both physical and psychological stimulation. Priapism lasting over four hours is a medical emergency.

In non-human animals

An erect horse penis.

At the time of penetration, the canine penis is not erect, and only able to penetrate the female because it includes a narrow bone called the baculum, a feature of most placental mammals. After the male achieves penetration, he will often hold the female tighter and thrust faster, and it is during this time that the male's penis expands. Unlike human sexual intercourse, where the male penis commonly becomes erect before entering the female, canine copulation involves the male first penetrating the female, after which swelling of the penis to erection occurs.[25]

An elephant's penis is S-shaped when fully erect and has a Y-shaped orifice.[26]

Given the small amount of erectile tissue in a bull's penis, there is little enlargement after erection. The penis is quite rigid when non-erect, and becomes even more rigid during erection. Protrusion is not affected much by erection, but more by relaxation of the retractor penis muscle and straightening of the sigmoid flexure.[27][28]

A male fossa's penis reaches to between his forelegs when erect.[29]

When not erect, a horse's penis is housed within the prepuce, 50 centimetres (20 in) long and 2.5 to 6 centimetres (0.98 to 2.36 in) in diameter with the distal end 15 to 20 centimetres (5.9 to 7.9 in). The retractor muscle contracts to retract the penis into the sheath and relaxes to allow the penis to extend from the sheath.[30] When erect, the penis doubles in length[31] and thickness and the glans increases by 3 to 4 times.[30] Erection and protrusion take place gradually, by the increasing tumescence of the erectile vascular tissue in the corpus cavernosum penis.[32][33] Most stallions achieve erection within 2 minutes of contact with an estrus mare, and mount the estrus mare 5–10 seconds afterward.[34]

A bird penis is different in structure from mammal penises, being an erectile expansion of the cloacal wall and being erected by lymph, not blood.[35] The penis of the lake duck can reach about the same length as the animal himself when fully erect, but more commonly is about half the bird's length.[36][37]

Terminology

Clinically, erection is often known as "penile erection", and the state of being erect, and process of erection, are described as "tumescence" or "penile tumescence". The term for the subsiding or cessation of an erection is "detumescence".

Colloquially and in slang, erection is known by many informal terms. Commonly encountered English terms include 'stiffy', 'hard-on', 'boner' and 'woody'.[38] There are several slang words, euphemisms and synonyoms for an erection in English and in other languages.

See also

References

  1. ↑ wiley.com > Viagra function image Retrieved on Mars 11, 2010
  2. ↑ APDVS > 31. Anatomy and Physiology of Normal Erection Retrieved on Mars 11, 2010
  3. 1 2 3 Marieb, Elaine (2013). Anatomy & physiology. Benjamin-Cummings. p. 895. ISBN 9780321887603.
  4. ↑ Moore, Keith; Anne Agur (2007). Essential Clinical Anatomy, Third Edition. Lippincott Williams & Wilkins. p. 265. ISBN 0-7817-6274-X.
  5. ↑ Drake, Richard, Wayne Vogl and Adam Mitchell, Grey's Anatomy for Students. Philadelphia, 2004. (ISBN 0-443-06612-4)
  6. ↑ Harris, Robie H. (et al.), It's Perfectly Normal: Changing Bodies, Growing Up, Sex And Sexual Health. Boston, 1994. (ISBN 1-56402-199-8)
  7. ↑ Morning Erections: Sizemed retrieved 28 February 2012
  8. ↑ After Prostate Cancer: A What-Comes-Next Guide to a Safe and Informed recovery: p.48
  9. ↑ Listen To Your Hormones, Abraham Harvey Kryger - 2004. p.32
  10. ↑ Janell L. Carroll (29 January 2009). Sexuality Now: Embracing Diversity: Embracing Diversity. Cengage Learning. p. 149. ISBN 978-0-495-60274-3.
  11. 1 2 Lynda Madaras (8 June 2007). What's Happening to My Body? Book for Boys: Revised Edition. Newmarket Press. p. 119. ISBN 978-1-55704-769-4. Retrieved 22 July 2013.
  12. ↑ erections in babies retrieved 11 February 2012
  13. ↑ Goldblatt, Howard (1990). Worlds Apart: Recent Chinese Writing and Its Audiences. p. 56.
  14. ↑ Sarah Attwood (15 May 2008). Making Sense of Sex: A Forthright Guide to Puberty, Sex and Relationships for People with Asperger's Syndrome. Jessica Kingsley Publishers. p. 62. ISBN 978-1-84642-797-8. Retrieved 22 July 2013.
  15. ↑ "Penis Size FAQ & Bibliography". Kinsey Institute. 2009. Retrieved 2013-11-07.
  16. ↑ Li CY, Kayes O, Kell PD, Christopher N, Minhas S, Ralph DJ (2006). "Penile suspensory ligament division for penile augmentation: indications and results". Eur. Urol. 49 (4): 729–733. PMID 16473458. doi:10.1016/j.eururo.2006.01.020.
  17. ↑ "Most Men Unsatisfied With Penis Enlargement Results". Fox News. 2006-02-16. Retrieved 2008-08-17.
  18. ↑ Sparling J (1997). "Penile erections: shape, angle, and length". Journal of Sex & Marital Therapy. 23 (3): 195–207. PMID 9292834. doi:10.1080/00926239708403924.
  19. ↑ Milsten, Richard (et al.), The Sexual Male. Problems And Solutions. London, 2000. (ISBN 0-393-32127-4)
  20. 1 2 Sadeghipour H, Ghasemi M, Ebrahimi F, Dehpour AR (2007). "Effect of lithium on endothelium-dependent and neurogenic relaxation of rat corpus cavernosum: role of nitric oxide pathway". Nitric Oxide. 16 (1): 54–63. PMID 16828320. doi:10.1016/j.niox.2006.05.004.
  21. ↑ Williams, Warwick, It's Up To You: Overcoming Erection Problems. London, 1989. (ISBN 0-7225-1915-X)
  22. ↑ Sadeghipour H, Ghasemi M, Nobakht M, Ebrahimi F, Dehpour AR (2007). "Effect of chronic lithium administration on endothelium-dependent relaxation of rat corpus cavernosum: the role of nitric oxide and cyclooxygenase pathways". BJU Int. 99 (1): 177–182. PMID 17034495. doi:10.1111/j.1464-410X.2006.06530.x.
  23. ↑ Tanagho, Emil A. (et al.), Smith's General Urology. London, 2000. (ISBN 0-8385-8607-4)
  24. ↑ NHS Direct – Health encyclopaedia -Erectile dysfunction
  25. ↑ "Semen Collection from Dogs". Arbl.cvmbs.colostate.edu. 2002-09-14. Retrieved 2012-01-29.
  26. ↑ Shoshani, p. 80.
  27. ↑ William O. Reece (4 March 2009). Functional Anatomy and Physiology of Domestic Animals. John Wiley & Sons. ISBN 978-0-8138-1451-3. Retrieved 22 July 2013.
  28. ↑ James R. Gillespie; Frank Bennie Flanders (28 January 2009). Modern Livestock & Poultry Production. Cengage Learning. ISBN 978-1-4283-1808-3. Retrieved 22 July 2013.
  29. ↑ Köhncke, M.; Leonhardt, K. (1986). "Cryptoprocta ferox" (PDF). Mammalian Species (254): 1–5. doi:10.2307/3503919. Retrieved 19 May 2010.
  30. 1 2 "The Stallion: Breeding Soundness Examination & Reproductive Anatomy". University of Wisconsin-Madison. Archived from the original on 2007-07-16. Retrieved 7 July 2007.
  31. ↑ James Warren Evans (15 February 1990). The Horse. W. H. Freeman. ISBN 978-0-7167-1811-6. Retrieved 22 July 2013.
  32. ↑ Sarkar, A. (2003). Sexual Behaviour In Animals. Discovery Publishing House. ISBN 978-81-7141-746-9.
  33. ↑ Juan C. Samper, Ph.D.; Jonathan F. Pycock, Ph.D.; Angus O. McKinnon (2007). Current Therapy in Equine Reproduction. Elsevier Health Sciences. p. 176. ISBN 978-0-7216-0252-3. Retrieved 22 July 2013.
  34. ↑ Juan C. Samper (2009). Equine Breeding Management and Artificial Insemination. Elsevier Health Sciences. ISBN 978-1-4160-5234-0. Retrieved 22 July 2013.
  35. ↑ Frank B. Gill (6 October 2006). Ornithology. Macmillan. pp. 414–. ISBN 978-0-7167-4983-7. Retrieved 5 December 2012.
  36. ↑ McCracken, Kevin G. (2000). "The 20-cm Spiny Penis of the Argentine Lake Duck (Oxyura vittata)" (PDF). The Auk. 117 (3): 820–825. doi:10.2307/4089612.
  37. ↑ McCracken, Kevin G.; Wilson, Robert E.; McCracken, Pamela J.; Johnson, Kevin P. (2001). "Sexual selection: Are ducks impressed by drakes' display?" (PDF). Nature. 413: 128. PMID 11557968. doi:10.1038/35093160.
  38. ↑ Gabrielle Morrissey (27 January 2005). Urge: Hot Secrets For Great Sex. HarperCollins Publishers. p. 6. ISBN 978-0-7304-4527-2. Retrieved 22 July 2013.
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