Editing Urination (section)

===Anatomy of the bladder and outlet===
{{multiple image|total_width=440
|image1=Gray1140.png|caption1=The interior of the bladder
|image2=Gray1142.png|caption2=Location of [[external urethral orifice (male)|external urethral orifice in adult human male]]
|image3=
Female and Male Urethra.jpg|caption3=Location of the bladder and urethra in adult human male and female ([[Anatomical plane|sagittal section]])
}}
{{Main article|Urinary bladder|Urethra}}

The main organs involved in urination are the [[urinary bladder]] and the [[urethra]]. The [[smooth muscle]] of the bladder, known as the [[detrusor]], is innervated by [[sympathetic nervous system]] fibers from the [[lumbar]] [[spinal cord]] and [[parasympathetic]] fibers from the [[Sacrum|sacral]] spinal cord.<ref name="wennemer2008">{{cite web
|author=Heidi K. Wennemer |url=http://www1.va.gov/SpinalCordBoston/page.cfm?pg=21 |title=Urinary Incontinence &ndash; Part 2 |publisher=[[United States Department of Veterans Affairs]] |date=7 July 2008 |access-date=24 March 2013 |archive-date= 25 September 2008 |archive-url=https://web.archive.org/web/20080925191459/http://www1.va.gov/SpinalCordBoston/page.cfm?pg=21 }}</ref> Fibers in the [[pelvic splanchnic nerve|pelvic nerves]] constitute the main afferent limb of the voiding reflex; the parasympathetic fibers to the bladder that constitute the excitatory efferent limb also travel in these nerves. Part of the urethra is surrounded by the [[external sphincter muscle of male urethra|male]] or [[external sphincter muscle of female urethra|female external urethral sphincter]], which is innervated by the somatic [[pudendal nerve]] originating in the cord, in an area termed [[Onuf's nucleus]].<ref>{{cite journal |vauthors=Rajaofetra N, Passagia JG, Marlier L, Poulat P, Pellas F, Sandillon F, Verschuere B, Gouy D, Geffard M, Privat A | title = Serotoninergic, noradrenergic, and peptidergic innervation of Onuf's nucleus of normal and transected spinal cords of baboons (Papio papio) | journal = [[J. Comp. Neurol.]] | volume = 318 | issue = 1 | pages = 1&ndash;17 | year = 1992 | pmid = 1374763 | doi = 10.1002/cne.903180102 | s2cid = 23190313 }}{{subscription required}}</ref>

Smooth muscle bundles pass on either side of the urethra, and these fibers are sometimes called the [[internal urethral sphincter]], although they do not encircle the urethra. Further along the urethra is a sphincter of skeletal muscle, the sphincter of the membranous urethra (external urethral sphincter). The bladder's epithelium is termed [[transitional epithelium]] which contains a superficial layer of dome-like cells and multiple layers of stratified cuboidal cells underneath when evacuated. When the bladder is fully distended the superficial cells become squamous (flat) and the stratification of the cuboidal cells is reduced in order to provide lateral stretching.