Erection - Anatomy & Physiology

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  • When sexual receptivity of the female is established and sufficient arousal is accomplished in the male, erection and protrusion of the penis ensue.
  • Successful penile erection requires a complex series of neural and vasomotor reactions.
  • Erection of the penis is necessary for copulation and deposition of semen in the female reproductive tract.
  • Erection is characterized by a marked increase in rigidity of the penis.
  • Increased rigidity is the result of increase in arterial inflow of blood compared to venous outflow.
  • Erection requires that blood be trapped within the cavernous sinus of the penis.
  • Increased blood flow to the penis is brought about by vasodilation of the arterioles supplying it.

Erection of the Fibroelastic Penis

The Fibroelastic Penis in a Flaccid State

Copyright Nottingham University 2008.
  • Bull,Ram and Boar
  • Involves increased blood flow with subsequent increase in pressure and simultaneous relaxation of the retractor penis muscles.
  • Erection and protrusion involve straightening of the penis to eliminate the sigmoid flexure.
  • Does not increase significantly in diameter during erection.

Erection of the Musculovascular Penis

  • Stallion
  • Increases in diameter during erection.
  • Retractor penis muscle still relaxes during erection, but no sigmoid flexure.
  • Engorgement with blood plays a significant role in the highly vascular penis.


Erection of the penis requires:

  • Elevated arterial inflow
  • Dilation of corporal sinusoids
  • Restricted venous outflow
  • Elevated intrapenile pressure
  • Relaxation of retractor penis muscles

  • Engorgement of cavernous sinuses causes a blockage of venous circulation from the penis.
  • Contractions of ischiocavernous muscles cause compression of the penile veins.
    • Intermittent contractions of the muscles around the two crura create a pump-like action at the base of the penis.
    • Result in the build up of blood within the corpus cavernosum.
  • Very high pressure results.

Sensory Input and Local Vascular Response

Nervous Component

  • Arousal driven
  • Must be appropriate sensory stimuli (tactile, visual, olfactory, auditory) in order for the central nervous system to be stimulated for efferent events to cause erection.
    • Extrinsic stimuli are called erotogenic stimuli.

  • Stimuli cause afferent sensory nerves to fire.
  • Terminals synapse with neurons in the 'behaviour centre' in the Hypothalamus.
  • Hypothalamic neurones synapse with parasympathetic and sympathetic efferent neurons that control penile vascular smooth muscle (vascular tone).
  • Firing of Nonadrenergic, noncholinergic (NANC) parasympathetic neurones causes erection.
    • Release nitric oxide (NO) from their terminals.
    • NO is the principle neurotransmitter that drives the erectile process.
    • NO activates the enzyme guanylate cyclase.
    • Guanylate cyclase converts guanylate triphosphate (GTP) into cyclic guanyosine monophosphate (cGMP).
  • Causes corporal smooth muscle relaxation (vasodilation) and results in erection.

  • Under nonerotogenic conditions, cGMP is acted upon by PDE5 (phosphodiesterase 5).
  • PDE5 promotes conversion of cGMP to GMP.
  • Breakdown causes increased vascular tone.
    • Tonic contraction of arterial and corporal smooth muscles.
    • Outflow of blood from corpora cavernosa.
  • Loss of erection and return to usual flaccid state.

Local Vascular Component

  • When corporal smooth muscles relax because of cGMP, resistance to blood flow by penile arterioles and corporal sinusoids decreases.
  • Blood flow to the penis triples or quadruples.
  • When erection occurs, sinusoid pressure is so great that emissary veins are collapsed.
    • Venous outflow blocked
  • Penile erection maintained for as long as vasodilation of corporal smooth muscle takes place.

Vascular and Biochemical Control of Erection


  • Shaft of the Penis consists of two dorso-lateral corpora cavernosa and the corpus spongiosum.
  • Arterial blood is supplied by the internal pudendal artery that supplies dorsal and deep cavernosal arteries.
  • Corporal sinusoids are supplied by helicine arteries.
  • Deep dorsal vein and superficial dorsal vein drain the erectile tissues.

  • For more information on anatomy of the penis follow this link.

Flaccid Penis

  • Sinusoids are flattened because adrenergic nerves secrete noradrenaline (norepinephrine) that causes vasoconstriction.
  • Blood flow to cavernous tissue is low.
  • No erotogenic stimuli present
  • Nonadrenergic noncholinergic (NANC) parasympathetic neurons do not fire.
  • No release of Nitric Oxide (NO)
  • Vasoconstriction takes precedence over vasodilation.

Erect Penis

  • Erotogenic stimuli present
  • NANC neurones fire and release NO from their terminals.
  • NO activates the enzyme guanylate cyclase.
  • Guanylate cyclase converts guanylate triphosphate (GTP) into cyclic guanyosine monophosphate (cGMP).
  • Causes smooth muscle of corporal sinusoids to relax (vasodilation).
  • Cavernous sinusoids engorge with blood.
  • Intracorporal pressure increases dramatically.
  • Pressure compresses the venules through which blood exits the penis.
  • Blood trapped within the penis causing an erection.