Physiology of the Urinary Tract - Micturition DSA Flashcards
What are the 3 anatomical divisions of the urinary tract? If there are multiple components of a division, name them too.
Upper urinary tract - calyces - pelvis - ureters Bladder Urethra
What is the general function of the bladder?
Stores urine under low pressure
Expels urine under high pressure
What is the general function of the urethra?
A conduit for carrying urine from the body
- A sphincter says what?
2. How many sphincters does the urethra have?
- What?
2. 2 - controls voiding
Describe the muscle composition and location of each urinary sphincter. Are they voluntary or involuntary?
- Internal
- involuntary smooth muscle
- wall of bladder as it narrows towards the urethra - External
- voluntary skeletal muscle
- wraps around exterior of urethra where it joins bladder
How do sensory afferents from the bladder wall reach the spinal column?
Via the pelvic splanchnic nerve and hypogastric plexus
How do sensory afferents from the urethra reach the spinal column?
Via the pudendal nerve
Where does the sensation of bladder fullness come from?
Stretch receptors in the bladder wall - in periurethral striated mm to be exact
How does sensory afferent information from bladder fullness reach the brain?
Travels in dorsal columns of the spinal cord to the pontine and suprapontine micturition centers
Where does sympathetic innervation for the bladder arise from?
From intermediolateral gray horn of spinal cord
- levels T10-L2
How does sympathetic information travel from the spinal cord to the bladder and urethra?
Travels via hypogastric nerves
What muscles of the urinary tract are sympathetic fibers inhibitory for? Excitatory?
Inhibitory:
detrusor muscle
Excitatory:
internal sphincter
Does bladder pain and sensation of bladder fullness travel through the same neural pathways?
No
Where does parasympathetic innervation for the bladder arise from?
Sacral detrusor nucleus
S2-S4
Note: nucleus in intermediolateral gray horn of the spinal cord, but at different levels than sympathetics
What is the path of the efferent parasympathetic nerves out of the spinal cord?
Leave cord through ventral root
Pass through pelvic splanchnic nerve
Synapse on ganglia on/near bladder
What muscles of the urinary tract are parasympathetic fibers inhibitory for? Excitatory?
Inhibitory:
internal sphincter
Excitatory:
detrusor muscle
Where do the somatic motor neurons for the bladder originate from? Where are they located?
Sacral pudendal nucleus
- located in ventral horn of S2 spinal segment
How do the somatic motor neurons reach their intended muscles?
Reach external sphincter via perineal branch of pudendal nerve
Also via inferior hypogastric plexus and pelvic splanchnic nerve
Explain the reflex arc for the sacral micturition center.
Afferent fibers from the detrusor mm and urethra travel into spinal column via dorsal horn, to sacral micturition center.
Afferents then excite sacral detrusor nucleus, causing detrusor contraction
Inhibit sacral pudendal nucleus, causing relaxation of sphincters
How does central control of micturition work?
Neurons originate in gray matter of brain, synapse onto pontine micturition center
Pontine micturition center sends neuron fibers to sacral micturition center and thoracolumbar sympathetic outputs
Pontine micturition center coordinates activity of bladder and sphincters - makes everything work together
What is the location of the pontine micturition center?
locus coeruleus of the pons
Why is central control important for bladder innervation?
Allows for conscious control of bladder
How does autonomic innervation alter urine delivery to the bladder?
Via peristalsis of urine through ureters
Sympathetic stimulation:
decreases frequency of peristaltic contractions in ureter
- decreases urine delivery to bladder
Parasympathetic stimulation:
increases frequency of peristaltic contractions
- increases urine delivery to bladder
When bladder stretch receptors are activated, where do they send sensory information to?
Sacral micturition center, which activates parasympathetic NS
Centers in brainstem and cerebral cortex, which suppress parasympathetic impulses
What increases as bladder fills? What nerve impulses increase?
Wall tension
- LaPlace’s Law: T=Pr/2
Increased wall tension leads to increased frequency of afferent impulses from stretch receptors
What steps make up late bladder filling?
- Increasing wall tension
- Increased firing of afferent impulses from stretch receptors
- This overrides inhibition of sacral micturition centers by higher centers in brain
- disinhibtion of central control - Parasympathetic impulses from sacral cord stimulate bladder contraction and inhibit contraction of internal urethral sphincter.
- Urine enters posterior urethra.
What happens after urine enters urethra?
- Sensory impulses from the urethra to the sacral cord inhibit the somatic nerves responsible for constricting the external sphincter
- The bladder continues to fill and contractions intensify. Sensory impulses from the bladder and urethra become more powerful.
- When it’s the appropriate time, the external sphincter relaxes and urine is voided
What happens with passive emptying of the bladder?
Tension of the bladder wall declines as urine is voided
A decreasing radius maintains intravesical pressure, despite decreasing wall tension
This enables the bladder to empty to its residual volume (<25% of capacity)
Summarize the bladder filling and voiding process.
- Bladder begins filling
- Stretch activates sensory inputs
- Parasympathetics: bladder contracts, internal sphincter relaxes
- Brainstem suppresses PNS
- Somatic motor constricts external sphincter
- Bladder continues filling
- Urine enters urethra, activates stretch receptors
- Sensory impulses intensify
- External sphincter relaxes
- Voiding
What are the 4 basic types of micturition abnormalities?
Atonic bladder
Denervated bladder
Automatic bladder
Uninhibited neurogenic bladder
What is the pathology behind atonic bladder?
AKA flaccid neuropathic bladder
Cause: destruction of sensory inputs from bladder to sacral cord (diabetes, crush injury, syphilis, MS)
Stretch information is no longer transmitted; bladder contractions are no longer initiated.
Bladder becomes flaccid and filled to capacity, called “overflow incontinence;” eventually distended, thin-walled.
What is the pathology behind denervated bladder?
AKA hypertrophic areflexic bladder
Cause: destruction of both afferent and efferent fibers between bladder and cord
Initially: detrusor contractions cease; bladder becomes flaccid, distended
Later: detrusor regains spontaneous activity; bladder shrinks, muscle wall hypertrophies
What is the pathology behind automatic bladder?
AKA spastic neuropathic bladder
Cause: injury or severing of spinal cord above sacral region: cuts off communication with brain
Acute phase: ‘spinal shock’ temporarily suppresses micturition reflex → flaccid neuropathic bladder
Micturition reflex can gradually recover & become exaggerated, resulting in spasticity - control by brain is lost.
What is the pathology behind uninhibited neurogenic bladder?
AKA autonomic dysreflexia bladder
Cause: destruction of tracts carrying inhibitory impulses (- · - · -) from brain
Facilitatory inputs (- - -) remain intact; micturition ctr. is continually stimulated
Micturition is activated by small amounts of urine
Detrusor hypertrophies, bladder capacity reduced
What is a urinary tract infection? What happens?
Bladder and urethral irritation from the infection
Uninhibited contraction of detrusor muscle with facilitation of the micturition reflex
Leads to urinary frequency and leakage
