Physiology of the Urinary Tract - Micturition DSA

Question 1

What are the 3 anatomical divisions of the urinary tract? If there are multiple components of a division, name them too.

Answer 1

Upper urinary tract
 - calyces
 - pelvis
 - ureters
Bladder
Urethra

Question 2

What is the general function of the bladder?

Answer 2

Stores urine under low pressure

Expels urine under high pressure

Question 3

What is the general function of the urethra?

Answer 3

A conduit for carrying urine from the body

Question 4

1. A sphincter says what?

2. How many sphincters does the urethra have?

Answer 4

1. What?

2. 2 - controls voiding

Question 5

Describe the muscle composition and location of each urinary sphincter. Are they voluntary or involuntary?

Answer 5

1. Internal
   - involuntary smooth muscle
   - wall of bladder as it narrows towards the urethra
2. External
   - voluntary skeletal muscle
   - wraps around exterior of urethra where it joins bladder

Question 6

How do sensory afferents from the bladder wall reach the spinal column?

Answer 6

Via the pelvic splanchnic nerve and hypogastric plexus

Question 7

How do sensory afferents from the urethra reach the spinal column?

Answer 7

Via the pudendal nerve

Question 8

Where does the sensation of bladder fullness come from?

Answer 8

Stretch receptors in the bladder wall - in periurethral striated mm to be exact

Question 9

How does sensory afferent information from bladder fullness reach the brain?

Answer 9

Travels in dorsal columns of the spinal cord to the pontine and suprapontine micturition centers

Question 10

Where does sympathetic innervation for the bladder arise from?

Answer 10

From intermediolateral gray horn of spinal cord

- levels T10-L2

Question 11

How does sympathetic information travel from the spinal cord to the bladder and urethra?

Answer 11

Travels via hypogastric nerves

Question 12

What muscles of the urinary tract are sympathetic fibers inhibitory for? Excitatory?

Answer 12

Inhibitory:
detrusor muscle

Excitatory:
internal sphincter

Question 13

Does bladder pain and sensation of bladder fullness travel through the same neural pathways?

Answer 13

No

Question 14

Where does parasympathetic innervation for the bladder arise from?

Answer 14

Sacral detrusor nucleus
S2-S4

Note: nucleus in intermediolateral gray horn of the spinal cord, but at different levels than sympathetics

Question 15

What is the path of the efferent parasympathetic nerves out of the spinal cord?

Answer 15

Leave cord through ventral root
Pass through pelvic splanchnic nerve
Synapse on ganglia on/near bladder

Question 16

What muscles of the urinary tract are parasympathetic fibers inhibitory for? Excitatory?

Answer 16

Inhibitory:
internal sphincter

Excitatory:
detrusor muscle

Question 17

Where do the somatic motor neurons for the bladder originate from? Where are they located?

Answer 17

Sacral pudendal nucleus

- located in ventral horn of S2 spinal segment

Question 18

How do the somatic motor neurons reach their intended muscles?

Answer 18

Reach external sphincter via perineal branch of pudendal nerve

Also via inferior hypogastric plexus and pelvic splanchnic nerve

Question 19

Explain the reflex arc for the sacral micturition center.

Answer 19

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

Question 20

How does central control of micturition work?

Answer 20

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

Question 21

What is the location of the pontine micturition center?

Answer 21

locus coeruleus of the pons

Question 22

Why is central control important for bladder innervation?

Answer 22

Allows for conscious control of bladder

Question 23

How does autonomic innervation alter urine delivery to the bladder?

Answer 23

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

Question 24

When bladder stretch receptors are activated, where do they send sensory information to?

Answer 24

Sacral micturition center, which activates parasympathetic NS

Centers in brainstem and cerebral cortex, which suppress parasympathetic impulses

Question 25

What increases as bladder fills? What nerve impulses increase?

Answer 25

Wall tension
- LaPlace’s Law: T=Pr/2

Increased wall tension leads to increased frequency of afferent impulses from stretch receptors

Question 26

What steps make up late bladder filling?

Answer 26

1. Increasing wall tension
2. Increased firing of afferent impulses from stretch receptors
3. This overrides inhibition of sacral micturition centers by higher centers in brain
   - disinhibtion of central control
4. Parasympathetic impulses from sacral cord stimulate bladder contraction and inhibit contraction of internal urethral sphincter.
5. Urine enters posterior urethra.

Question 27

What happens after urine enters urethra?

Answer 27

1. Sensory impulses from the urethra to the sacral cord inhibit the somatic nerves responsible for constricting the external sphincter
2. The bladder continues to fill and contractions intensify. Sensory impulses from the bladder and urethra become more powerful.
3. When it’s the appropriate time, the external sphincter relaxes and urine is voided

Question 28

What happens with passive emptying of the bladder?

Answer 28

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)

Question 29

Summarize the bladder filling and voiding process.

Answer 29

1. Bladder begins filling
2. Stretch activates sensory inputs
3. Parasympathetics: bladder contracts, internal sphincter relaxes
4. Brainstem suppresses PNS
5. Somatic motor constricts external sphincter
6. Bladder continues filling
7. Urine enters urethra, activates stretch receptors
8. Sensory impulses intensify
9. External sphincter relaxes
10. Voiding

Question 30

What are the 4 basic types of micturition abnormalities?

Answer 30

Atonic bladder
Denervated bladder
Automatic bladder
Uninhibited neurogenic bladder

Question 31

What is the pathology behind atonic bladder?

Answer 31

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.

Question 32

What is the pathology behind denervated bladder?

Answer 32

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

Question 33

What is the pathology behind automatic bladder?

Answer 33

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.

Question 34

What is the pathology behind uninhibited neurogenic bladder?

Answer 34

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

Question 35

What is a urinary tract infection? What happens?

Answer 35

Bladder and urethral irritation from the infection
Uninhibited contraction of detrusor muscle with facilitation of the micturition reflex
Leads to urinary frequency and leakage