PHYS - Micturition

Question 1

RENAL PELVIS

Answer 1

• Also called pyelum
• Site of many different types of kidney cancers
• Where 2-3 major calyces converge
• Funnels urine into ureter by pyelouteric peristalsis
  
  • Thought to be “myogenic”, initiating from the muscle and unaffected by blocked nerve signals
  • Now know that there are interstitial pacemaker cells at the pelvi-calyceal junction that propagate a signal distally to the renal pelvis
• Force of contraction is regulated by the rate of urine flow
• Hydronephrosis – distension and dilation of the renal pelvis and calyces caused by obstructed urine flow leading to increased pressure in kidneys
  
  • Can cause progressive atrophy of the kidney

Question 2

KIDNEY STONES

Answer 2

• Generally don’t cause symptoms until they start to pass:
  
  • Extreme, persistent pain in back and sides
  • Vomiting
  • Blood in urine
  • Fever and chills
• Sites where kidney stones can lodge:
  
  • Ureteric junction with renal pelvis
  • Where ureter passes over the iliac vessels
  • Vesicoureter junction where ureter enters bladder
• Types of kidney stones
  
  • Calcium-oxalate: most common
    
    • Eating too much calcium/vitamin D; or by medicine/genetics
  • Struvite: can be most harmful
    
    • Caused by kidney infections; affect women more than men
  • Uric acid
    
    • Caused by eating too much animal protein or genetics
  • Cysteine: most rare
    
    • Caused by cystinuria – a genetic kidney disease
• Increased likelihood of kidney stones if:
  
  • Male
  • White
  • Overweight
  • Don’t drink enough water
  • Prior kidney infection
  • Previous kidney stone
  • FHx of kidney stones
  • Too much animal protein, calcium, vitamin D (or medicines containing)
  • Geographic location (ions in water supply)

Question 3

ORIGIN OF EC COUPLING IN THE URETER

Answer 3

• Electrical impulse originates from pacemaker cells at the pyeloureteral complex
• Renal pelvis is spontaneously active and the impulse propagates toward and down ureter creating a peristaltic event that moves a bolus of urine into the bladder
• Latent pacemakers (usually quiescent) exist along length of ureter to help move peristaltic impulse
  
  • Aberrant activation of these pacemakers can cause antiperistaltic waves causing urine reflux
• Highest frequency of electrical activity is at the pyeloureteral complex

Question 4

BLADDER FILLING AND MICTURITION

Answer 4

• Bladder function = storage of urine
• Anatomy of bladder wall
  
  • Adventitia
  • Detrusor muscle
  • Mucosa
    
    • Submucosa or suburothelium
    • LP (CT)
    • Urothelium – contains lots of sensory nerve fibers
• Critical regions of bladder function
  
  • Urothelium - mucosal cells and sensory receptors
  • Suburothelium - afferent nerve pathway
  • Muscularis - efferent nerve pathway; motor zone
• Micturition: bladder filling → increased P → urothelium releases ATP → opens ion channels → afferent impulse → spinal cord/CNS (PAG) → efferent (parasympathetic mostly) impulse to detrusor → ACh and ATP release → muscle contraction → micturition

Question 5

NERVOUS CONTROL OF BLADDER

Answer 5

• Pelvic Nerve (parasympathetic, S2-S4, pelvic ganglion)
  
  • ACh → (+) M₃ → contracts detrusor muscle
  • ATP → (+) P2X₁ → contracts detrusor muscle
  • NO → (-) GC → relaxes urethral SM
• Hypogastric Nerve (sympathetic, T10-L2, inferior mesenteric ganglion)
  
  • NE → (-) β₃ → relaxes detrusor muscle
  • NE → (+) α₁ → contraction of sphincter SkM
• Pudendal Nerve (somatic, S2-S4, pelvic ganglion)
  
  • ACh → (-) N → contraction of sphincter SkM

Question 6

NERVOUS CONTROL OF FILLING AND MICTURITION

Answer 6

• Filling
  
  • Low stimulation from parasympathetic afferent fibers of pelvic nerve transmits information to CNS about bladder state (pressure, volume)
  • Sympathetic hypogastric nerve stimulation: contraction of bladder outlet and relaxation of detrusor
  • Somatic pudendal nerve stimulation: contraction of external urethral sphincter SkM
• Micturition
  
  • Increased stimulation from afferent pelvic nerve stimulates parasympathetic nerves and inhibits sympathetic and pudendal nerves
  • Parasympathetic efferent pelvic nerve stimulates contraction of detrusor and relaxation of bladder outlet
  • Inhibition of hypogastric and pudendal nerves allows contraction of detrusor muscle and relaxation of the sphincter.

Question 7

SENSORY INPUT TO THE BLADDER

Answer 7

• Receptors on urothelial lining of bladder release ATP to afferent nerve terminal
• Can be stimulated by:
  
  • pH
  • temperature (thermal)
  • mechanical
  • irritants
• Aδ afferent fibers
  
  • Mechanoreceptors
  • Participate in physiological micturition reflex
• C afferent fibers
  
  • Nociception (pain/harm sensation) during pathology
  • Mechanically insensitive
• Hyperalgesia is a condition in which the phenotypic function of a receptor changes so it becomes more sensitive to other signals and fires more often

Question 8

CONTRACTION OF THE DETRUSOR MUSCLE

Answer 8

• Smooth muscle contraction, stimulated by Ca2+ release
• 3 main mechanisms:
  
  • Entry of Ca2+
  • Release of Ca2+ from cellular stores
  • Sensitization of contractile apparatus by inhibition of MLCP

Question 9

BLADDER COMPLIANCE

Answer 9

• Very compliant muscular wall → as urine fills, muscle relaxes and bladder experiences little to no change in pressure
  
  • Myogenic mechanism
  • Change in intravesical pressure → neurogenic urination stimulus
• Cystometrogram
  
  • Records pressure changes during bladder filling and voiding
  • No voiding until a sensory threshold of pressure is released
  • With each bladder contraction, since on a cystometrogram as a pulse of pressure, a fraction of urine is ejected

Question 10

URINARY INCONTINENCE

Answer 10

• Loss of bladder control → unplanned release of urine
• Stress Urinary Incontinence (SUI)
  
  • Leaks during laughing, coughing, sneezing…events that increase abdominal pressure
  • Most common bladder problem in young women
• Urge Incontinence (HSIHP)
  
  • Spasm of detrusor muscle that create the sensation of needing to urinate, even with a mostly empty bladder → leaks of urine
  • Usually occurs as a result of muscle or nerve damage
• Overactive Bladder (OAB)
  
  • Another name for urge incontinence, some individuals can hold it until they reach a restroom