9.6 Micturition, Autonomic Bladder Innervation; Renal Pathologies

Micturition (Urination)

  • Urine flow to bladder

    • Urine reaches the bladder through peristaltic contractions of the ureters

    • Stretch receptors in the bladder wall are activated when the bladder fills, initiating the micturition reflex.

  • Urge to urinate

    • Typically, the urge to urinate appears when the bladder contains about 200ml of urine.

    • At approximately 500ml, contractions of the detrusor muscle force open the internal urethral sphincter.

  • Involuntary urination

    • In infants, the neural pathway for voluntary control of the external urethral sphincter is not established, leading to involuntary urination

    • In the elderly, the external urethral sphincter may be weak, causing involuntary urination if not powerful enough to resist the pressure

Micturition reflex (Spinal cord control)

  • The micturition reflex is mediated by the spinal cord, with inputs from the brain.

    • Detrusor muscle (surrounding bladder): Contracts involuntarily

    • Internal urethral sphincter: Relaxes involuntary

    • External urethral sphincter: Relaxes voluntarily, allowing urine to flow through the urethra and out of the body

Autonomic bladder innervation

  • Bladder control: primarily controlled by the autonomic nervous system, with voluntary control facilitated and inhibited by higher brain centres.

  • Innervation details:

    • Detrusor muscle:

      • Sympathetic nervous system (SNS): relaxes the detrusor muscle via β3 receptors

      • Parasympathetic nervous system (PNS): Contracts the detrusor muscle via M3 receptors

    • Internal urethral sphincter:

      • Sympathetic nervous system: Contracts the internal urethral sphincter via a1a1 receptors

    • External urethral sphincter:

      • Controlled by somatic motor fibres from the pudendal nerve.

Bladder innervation

  • Motor fibres (efferents)

    • Sympathetic and parasympathetic fibres carried in hypogastric and pelvic nerves

    • Somatic fibres innervating sphincter carried in pudendal nerve

  • Sensory fibres (afferents)

    • Aδ-fibres respond to bladder distension and contraction

    • C-fibres respond to noxious stimuli

Autonomic control of bladder filling

  • Detrusor muscle: Relaxed, due to sympathetic stimulation

  • Internal urethral sphincter: Contracted, due to sympathetic stimulation

  • External urethral sphincter: Contracted, under somatic control

Autonomic control bladder voiding

  • Detrusor muscle: Contracts, due to parasympathetic stimulation

  • Internal urethral sphincter: Relaxed, due to reduced sympathetic stimulation

  • External urethral sphincter: Relaxed, under voluntary control

Micturition centres in the brain

  • Cortical centre: Located in the paracentral lobule, inhibits the pontine center

  • Brainstem center (Pons): Facilitates micturition and coordinates the reflex

  • Spinal cord (Scral spinal cord): Parasympathetic reflex fr bladder evacuation

Neurogenic bladder

  • Dysfunction of the bladder due to nervous system diseases:

    • Central nervous system: Stroke, spinal cord injury (SCI), ALS, parkinson’s disease, multiple scelrosis

    • Peripheral nervous system: Disc herniation

  • Types of neurogenic bladder

    • Detrusor hyperreflexia: Involuntary, uninhabited contraction of the detrusor muscle (spastic bladder)

    • Detrusor sphincter dyssynergia: Bladder outlet obstruction due to contraction of the detrsor muscle with simultaneous activation of the urethral sphincter

    • Areflexia: Failure of the detrusor muscle to contract, leading to a flaccid bladder

Effects of medications on bladder tone

  • Beta-3 receptor agonists:

    • Reduce contraction of the detrusor muscle

  • Muscarinic receptor antagonists:

    • Reduce contraction of the detrusor muscle

  • Alpha-1 adrenergic antagonists:

    • Relax the internal urethral sphincter

Urinary tract infection (UTI)

  • Bacterial infection in any part of the urinary system

    • Lower UTI - bladder, urethra (considered uncomplicated)

    • Upper UTI - ureters, kidneys (considered serious)

  • Approximately 1 in 20 and 1 in 2 women will expience in their lifetime

    • It is more common in women due to the shorter urethra

  • Symptoms: Painful urination, increased frequency, urgency

  • Treatment: Antibiotics and hydration

Kidney stones

  • Formation: Crystallisation of minerals and salts in urine. Approximately 1 in 10 men and 1 in 35 women will experience in their lifetime

  • Pain: Occurs when stones obstruct the ureters

  • Causes: Excess calcium, dehydration

  • Treatment: Increased water intake, pain relief, ultrasound, or surgery if necessary

Renal failure

  • When the kidneys cannot filter wastes from the blood and can no longer maintain homeostasis

  • Virtually all systems in the body are affected

    • Blood pressure increases

    • Anaemia develops due to decrease in red blood cell production, which may result from kidney dysfunction and reduced erythropoietin levels.

    • Central nervous system problems such as sleeplessness, seizures, delirium and coma

  • Acute renal failure: Sudden reduction in kidney function due to ischemia, obstruction, or nephrotoxic drugs. Recovery is possible but not guaranteed.

  • Chronic renal failure: Progressive decline in kidney function over time, often due to uncontrolled diabetes or hypertension. generally irreversible, but progression can be slowed.

Dialysis

  • Purpose: Artificial filtration of the blood when kidneys fail

  • Process: Blood is filtered through a membrane, Removing waste products and excess fluid

  • Frequency: Often done multiple times per week, lasting several hours per session.