Erection - Anatomy & Physiology

Introduction

• 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.

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Erection of the Fibroelastic Penis

• 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.

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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.

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Mechanism

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.

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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.

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Vascular and Biochemical Control of Erection

Anatomy

• 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.