Editing Normal Erectile Physiology (section)

==== Nitric oxide ====
* '''<span style="color:#ff0000">Principal neurotransmitter mediating erection</span>'''
* '''<span style="color:#ff0000">Synthesized by nitric oxide synthase (NOS)</span>'''
** '''<span style="color:#ff0000">Nitric oxide synthase</span>'''
*** '''<span style="color:#ff0000">3 isoforms; all 3 have been identified in the corpus cavernosum:</span>'''
***# '''<span style="color:#ff0000">Neuronal nitric oxide synthase (</span><span style="color:#0000ff">n</span><span style="color:#ff0000">NOS) in the nitrergic nerves: responsible for the i</span><span style="color:#0000ff">NNN</span><span style="color:#ff0000">itiation</span>'''
***# '''<span style="color:#ff0000">Endothelial nitric oxide synthase (</span><span style="color:#0000ff">e</span><span style="color:#ff0000">NOS): contributes to the maint</span><span style="color:#0000ff">EEE<span style="color:#ff0000">nance of smooth muscle relaxation and erection</span>'''
***# '''<span style="color:#0000ff">i</span><span style="color:#ff0000">NOS</span>'''
* '''<span style="color:#ff0000">Released from (2):</span>'''
*# '''<span style="color:#ff0000">Non-adrenergic/non-cholinergic (NANC) neurotransmission from postganglionic parasympathetic stimulation [from the cavernous nerves]</span>'''
*# '''<span style="color:#ff0000">Endothelium</span>'''
* '''<span style="color:#ff0000">Stimulates production of cGMP from GMP</span>''' by activating guanylate cyclase in smooth muscle
**'''<span style="color:#ff0000">Increased cGMP results in smooth muscle relaxation</span>''' by activating cGMP-dependent protein kinases, which phosphorylate certain proteins and ion channels, resulting in:
**# Opening of the potassium channels and hyperpolarization
**# Sequestration of intracellular calcium by the endoplasmic reticulum
**# Inhibition of voltage-dependent calcium channels, blocking calcium influx
* '''<span style="color:#ff0000">The smooth muscle regains its tone when cGMP is degraded by phosphodiesterase</span>'''

===== '''<span style="color:#ff0000">Phosphodiesterase (PDE)</span>''' =====
* '''<span style="color:#ff0000">Enzyme that degrades cGMP to GMP</span>'''
*11 subtypes
** '''All subtypes, except PDE6 which is specifically expressed in photoreceptor cells, have been identified in the corpus cavernosum'''
** '''<span style="color:#ff0000">PDE5 is the principal PDE for the termination of cavernous cGMP signaling</span>'''