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Micturition - Detailed Anatomy
Micturition: Detailed Anatomy
Overview
Key Innervation Patterns
Parasympathetic ACTIVATION
- Parasympathetic nervous system ACTIVATES urination via:
- Detrusor muscle activation
- Internal urethral sphincter inhibition.
Sympathetic INHIBITION
- Sympathetic nervous system INHIBITS urination via:
- Detrusor muscle inhibition
- Internal urethral sphincter activation
Somatomotor INHIBITION
- Somatomotor system INHIBITS urination via:
- External urethral sphincter activation
Detailed Anatomy & Physiology
Anatomical Structures
The major functional structures of the lower urinary system are:
- The urinary bladder.
- The urethra.
- The bladder wall muscle: the detrusor muscle.
- The internal urethral sphincter.
- The external urethral sphincter.
- The ureters, which empty into the bladder.
sympathetic nervous system
Origins
- The sympathetic preganglionic system originates in the intermediolateral cell column from T12 – L2.
- Along the descending aorta, lies the inferior mesenteric ganglion (the lowermost prevertebral ganglion).
Innervation
- There is supraspinal innervation of the intermediolateral cell column, from which excitatory sympathetic efferents project to the inferior mesenteric prevertebral ganglion.
parasympathetic nervous system
Origins
- The parasympathetic preganglionic system originates in the intermediolateral cell column of S2 – S4 and involves the parasympathetic vesical ganglia along the bladder wall.
Innervation
- There is supraspinal innervation of the intermediolateral cell column, from which excitatory parasympathetic efferents project to the parasympathetic vesical ganglia via the pelvic splanchnic nerve.
Somatomotor efferents
Origins
- Within the S2 – S4 spinal cord slice, in the anterior horn of the spinal cord, lies Onuf 's nucleus (aka nucleus of Onufrowicz).
Innervation
- There is supraspinal innervation of Onuf's nucleus, and excitatory somatomotor fibers project from Onuf's nucleus to the external urethral sphincter within the pudendal nerve, which inhibits urination.
Parasympathetic Actions
- Postganglionic parasympathetic fibers excite the bladder wall, which contracts to expel urine.
- Postganglionic parasympathetic fibers inhibit the internal urethral sphincter:
- Inhibition of the sphincter allows urine to empty the bladder.
Sympathetic Actions
- Postganglionic sympathetic fibers inhibit the detrusor muscle:
- Inhibition of the detrusor inhibits urination.
- Postganglionic sympathetic fibers inhibit the parasympathetic vesical ganglia:
- Inhibition of this ganglia inhibits urination.
- Postganglionic sympathetic fibers excite the internal urethral sphincter:
- Prevents bladder emptying.
Summary points of urinary innervation
Primary Urination Regulators
- The main regulators are the detrusor muscle, internal urethral sphincter, and external urethral sphincter.
- Sympathetic fibers inhibit bladder wall contraction and excite internal urethral sphincter constriction, which inhibits urination.
- Parasympathetic fibers excite bladder wall contraction and inhibit internal urethral sphincter constriction, which promotes urination.
- Somatomotor efferents provide tonic activation of the external urethral sphincter, which inhibits urination.
Supraspinal Control
Supraspinal control of urination
Overview
- The pontine micturition (bladder emptying) and continence (bladder filling) command centers and innervate the preganglionic parasympathetic and sympathetic spinal cord regions and Onuf's nucleus.
Anatomy
- The pontine micturition center lies in the medial (M) region of the dorsolateral pontine tegmentum and the pontine continence center lies ventro-lateral to it in the lateral (L) region.
Afferent Innervation
- These regions receive afferents from many brain regions, including the periaqueductal gray area, the hypothalamus, cerebellum, and certain limbic system regions.
- Barrington first described the pontine micturition center, so it is often referred to as the Barrington nucleus.
Peripheral Nerve Innervation
Peripheral Nerve Innervation
Now, let's conclude with the peripheral nerves that carry the aforementioned autonomic and somatic fibers.
From proximal to distal,
- Postganglionic thoracolumbar sympathetic fibers initially travel within the superior hypogastric plexus
- And then emerge as the hypogastric nerve. It is the transition zone between the superior and inferior hypogastric plexuses.
- Both the hypogastric and pelvic splanchnic nerves join within the inferior hypogastric plexus (aka the pelvic plexus), which provides both sympathetic and parasympathetic innervation to the bladder and urethra.
- They then pass into the vesical plexus.