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* 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.
 
* 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'''.
 
** Extrinsic stimuli are called '''erotogenic stimuli'''.
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* Stimuli cause afferent sensory nerves to fire.
 
* Stimuli cause afferent sensory nerves to fire.
 
* Terminals synapse with neurons in the ''''behaviour centre'''' in the [[Endocrine_System_-_Hypothalamus_-_Anatomy_%26_Physiology#Hypothalamus|Hypothalamus]].
 
* Terminals synapse with neurons in the ''''behaviour centre'''' in the [[Endocrine_System_-_Hypothalamus_-_Anatomy_%26_Physiology#Hypothalamus|Hypothalamus]].
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** Release nitric oxide (NO) from their terminals.
 
** Release nitric oxide (NO) from their terminals.
 
** NO is the principle neurotransmitter that drives the erectile process.  
 
** NO is the principle neurotransmitter that drives the erectile process.  
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** NO activates the enzyme guanylate cyclase.
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** Guanylate cyclase converts guanylate triphosphate (GTP) into cyclic guanyosine monophosphate (cGMP).
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* Causes corporal smooth muscle relaxation (vasodilation) and results in erection.
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* Under nonerotogenic conditions, cGMP is acted upon by PDE5 (phosphodiesterase 5).
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* PDE5 promotes conversion of cGMP to GMP.
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* Breakdown causes increased vascular tone.
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** Tonic contraction of arterial and corporal smooth muscles.
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** Outflow of blood from '''corpora cavernosa'''.
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* Loss of erection and return to usual flaccid state.
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'''''Local Vascular Component'''''
 
'''''Local Vascular Component'''''
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