Micturition

Micturition

Bladder gradually distended

• Leads to coordinated expulsion of urine

• Modified by conscious control

  • Micturition centre signals to UMNs → stimulates sacral parasympathetic neurons → detrusor muscle contraction (bladder voiding)

  • UMNs inhibit somatic pudendal nerve → relaxation of urethralis muscle (external urethral sphincter)

Sympathetic motor

• preganglionic = lumbar splanchnic nerves

• synapse: caudal mesenteric ganglion

• Postganglionic fibres = hypogastric nerves

  • enter pelvic plexus on wall of rectum

    • Direct inhibition of pelvic nerves → promotes relaxation of detrusor muscle via β2-adrenergic (also β3) [storage phase - low pressure filling]

    • Prevents leakage → internal sphincter constriction via α1-adrenoceptors

    • Constant tone via pudendal nerve = main barrier (closed urethralis)

Parasympathetic

• Preganglionic = sacral spinal → pelvic plexus as nervi pelvini → pelvic nerve

  • S1-S3 (small)

  • S2-S4 (large)

• Synapse: in bladder wall

Postganglionic = short → detrusor muscle

• Emptying recap

  • detrusor muscle contraction

  • inhibition of hypogastric nerve → inhibits adrenergic tone → internal urethral sphincter

  • pudendal nerve inhibition → urethralis relaxation

Central integration - micturition centre

• Pons

• Stimulated by bladder stretch receptors

• Coordinates urethralis relaxation + detrusor contraction

• Voluntary control involves cerebral cortex → learned behaviour overrides autonomic stimulation to an extent

Sensory innervation

• Stretch receptors in bladder

  • bladder stretch signals → pelvic nerves → sacral spinal cord → brain

• Urethra: flow, distension, pain

  • Uroliths → tube distension

  • urethra stretch signal → pudendal nerve → sacral spinal cord → brain

Somatic innervation

Pudendal nerve

• Small animals S1-3

• Large animals S3-5

• Innervates urethralis (striated)

• External anal sphincter

• Other striated muscles → pelvic + perineal regions

• Voluntary control and continence

Lower urinary tract

• Ureters

  • Propels urine via peristalsis

  • Intramural ureter inserts obliquely into bladder

    • Angle created → valvular function of ureteral ostium

      • Compressive force of full bladder adds to valvular function → variable flow from ureter

• Bladder

  • Bladder trigone → 2 ureteral ostia + urethral entrance

• Urethra

  • Urethral crest → raised mucosa at proximal urethra

All urothelium (transitional)

• Variable number of layers + angles

• Designed to allow bladder or urethra distension

• Unique to urinary tract

Muscles

• Detrusor

  • Meshwork of smooth muscle fibres

  • Spherical

  • Contracts to expel urine

• Internal urethral sphincter

  • Smooth muscle around bladder neck

  • Contracts to prevent leakage

• Urethralis

  • Skeletal muscle

  • Surrounds pelvic urethra

  • Squeezes urethra when contracted

Urethra (female)

• Short

• Opens at external urethral orifice → junction of vagina + vestibule

  • Vestibule different embryological origin from vagina

  • Vestibule: vulvar labia (vulva) → + external urethral orifice

  • Vagina proper: external urethral orifice (cranial limit of vestibule) to the cervix

Urethra (male)

• Male hormones → elongation of penis that elongates urethra

  • Clitoris + penis same embryonic tissue

• Prostate exists

  • Surrounds part of urethra (esp dog)

  • Enlarges with age from androgen stimulation → benign prostatic hypertrophy

    • → Compression of the urethra (specifically the prostatic urethra)
      difficulty urinating (stranguria)

    • incomplete bladder emptying → more urination attempts

  • Dogs → may enlarge dorsally and obstruct rectum → tenesmus

    • Persistent need to defacate → constant straining