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.

^{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.

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:

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.

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.

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.

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.