Physio

• Micturition


  The process of bladder emptying, also known as urination or voiding
• Micturition is a reflex process, and in grown up children and adults, it can be controlled voluntarily to some extent
• Ureter


  • Smooth-muscle walled tubes that transmit urine from the kidneys to the urinary bladder
  • Peristaltic contractions propel the urine forward
  • Physiological valve that prevents backflow (urinary reflux)
• Urinary bladder


  • A hollow, distensible muscular organ that stores urine before being expelled by micturition
  • Trigone - a triangular area between 2 ureter openings and internal sphincters
  • 2 sphincters - internal urethral sphincter (smooth muscle, involuntary) and external urethral sphincter (skeletal muscle, voluntary)
• Innervations of the bladder

  • Parasympathetic
  • Somatic
  • Sympathetic
• Parasympathetic innervation


  Preganglionic fibers form the pelvic nerve, arising from S2-S4 spinal segments, terminating on the detrusor muscle and internal sphincter
• Functions of parasympathetic nerves

  Stimulation causes contraction of detrusor muscle and trigone, and relaxation of internal sphincter, leading to bladder emptying
• Somatic innervation


  External sphincter is innervated by the pudendal nerve, arising from S2-S4 spinal segments
• Function of somatic nerve


  Maintains tonic contraction of external sphincter, inhibited during micturition to allow relaxation and voiding
• Sympathetic innervation


  Preganglionic fibers arise from T10-L2 spinal segments, terminate in hypogastric ganglion, postganglionic fibers form hypogastric nerve
• Functions of sympathetic nerves

  Stimulation causes relaxation of detrusor muscle and constriction of internal sphincter, resulting in bladder filling
• Micturition reflex


  1. Initiated by stretch receptors in bladder wall

  2. Afferent signals to spinal cord

  3. Excites parasympathetic neurons to bladder, causing detrusor contraction and internal sphincter relaxation

  4. Inhibits somatic motor neurons to external sphincter, causing relaxation

  5. Urine expelled through urethra
• Voluntary control of micturition

  • Learned reflex that keeps micturition reflex inhibited until desired
  • Involves additional sensory fibers signaling bladder fullness
  • Brain stem and cortical centers receive this information and override the basic reflex
• Voluntary urination


  1. Deliberately relax external sphincter and pelvic diaphragm

  2. Bladder drops downward, opening internal sphincter

  3. Stretch receptors activated, causing detrusor contraction via micturition reflex

  4. Assisted by contracting abdominal and respiratory muscles to increase intra-abdominal pressure
• Higher centers for micturition

  • Brain stem (pons) has strong facilitative and inhibitory centers
  • Cerebral cortex has mainly inhibitory but can become excitatory centers
  • Keep micturition reflex partially inhibited except when desired
  • Can prevent micturition by tonic contraction of external sphincter
  • Can facilitate micturition reflex and inhibit external sphincter when desired
• Pontine micturition center (PMC)

  • Also known as Barrington's nucleus
  • Located in brainstem (medial dorsal pons)
  • Activation stimulates urination via parasympathetic activation and external sphincter relaxation
• Bladder distension


  • Transmits afferent signal to pons, resulting in perception of bladder fullness
  • Higher brain normally suppresses PMC, inhibiting micturition reflex
  • When appropriate, higher brain withdraws PMC suppression, allowing micturition reflex
• Atonic bladder


  • Loss of tone in detrusor muscle
  • Caused by destruction of sensory (pelvic) nerve fibers, so no stretch signals to spinal cord
  • Results in urine overflow in drops as it enters the bladder (overflow incontinence)
• Automatic bladder


  • Hyperactive micturition reflex with loss of voluntary control
  • Caused by spinal cord damage above sacral region
  • Even small amounts of urine elicit the micturition reflex, resulting in involuntary bladder emptying
• Nocturnal micturition


  • Involuntary voiding of urine during night, also called enuresis or bedwetting
  • Normal in infants and children under 3 due to incomplete myelination of motor nerves
  • Abnormal if persisting after 3 years, can be due to neurological disorders or psychological factors