MICTURITION

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MICTURITION

• It is a process by which urine is voided from the
  urinary bladder. It is a reflex process. However,
  in grown up children and adults, it can be
  controlled voluntarily to some extent.

2
FUNCTIONAL ANATOMY OF URINARY BLADDER

• Urinary bladder consists of the body, neck and
  internal urethral sphincter. The smooth muscle
  forming the body of bladder is called detrusor
  muscle. At the posterior surface of the bladder
  wall there is a triangular area called trigone.
  At the upper angles of this trigone, two ureters
  enter the bladder.

3
NERVE SUPPLY TO URINARY BLADDER AND SPHINCTERS

• Urinary bladder and internal sphincter are
  supplied by sympathetic and parasympathetic
  divisions of autonomic nervous system, whereas,
  the external sphincter is supplied by the somatic
  nerve fibres

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FILLING OF URINARY BLADDER

• PROCESS OF FILLING
• Urine is continuously formed in the nephrons and
  it is transported drop by drop through the
  ureters into the urinary bladder. When urine
  collects in the pelvis of ureter, the contraction
  sets up in pelvis. The contraction is transmitted
  through rest of the ureter in the form of
  peristaltic wave up to trigone of the urinary
  bladder. The peristaltic wave moves the urine
  into the bladder. After leaving the kidney, the
  direction of the ureter is initially downward and
  outward. Then, it turns horizontally before
  entering the bladder.

5

• . At the entrance of ureters into urinary
  bladder, a valvular arrangement is present. When
  peristaltic wave pushes the urine towards the
  bladder, this valve opens towards the bladder.
  The position of ureter and the valvular
  arrangement at the end of ureter prevent the
  backflow of urine from bladder into the ureter
  when the detrusor muscle contracts. Thus urine is
  collected in bladder drop by drop. A reasonable
  volume of urine can be stored in urinary bladder
  without any discomfort and without much increase
  in pressure in bladder. It is due to the
  adaptation of detrusor muscle.

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MICTURITION REFLEX

• Micturition occurs by a reflex called micturition
  reflex. This reflex starts because of the
  stimulation of stretch receptors, situated on the
  wall of urinary bladder and urethra. The rise in
  pressure stretches the bladder resulting in
  stimulation of stretch receptors and generation
  of sensory impulses. The sensory impulses from
  the receptors reach the sacral segments of spinal
  cord via the sensory fibres of pelvic nerve. The
  motor impulses produced in spinal cord, travel
  through motor fibres of pelvic nerve towards
  bladder and internal sphincter. The motor
  impulses cause contraction of detrusor muscle and
  relaxation of internal sphincter so that, urine
  enters the urethra from the bladder.

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• . Once urine enters the urethra, the stretch
  receptors in the urethra are stimulated and send
  afferent impulses to spinal cord via pelvic
  fibres. These impulses inhibit pudendal nerve. So
  the external sphincter relaxes and micturition
  occurs.during micturition, the flow of urine is
  facilitated by the increase in the abdominal
  pressure due to the voluntary contraction of
  abdominal muscles.

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Higher Centres for Micturition

• Spinal centres for mictrition are present in
  sacral and lumbar segments. But these spinal
  centres are regulated by higher centres. The
  higher centres, which control micturition are of
  two types, inhibitory centres and facilitatory
  centres.

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APPLIED PHYSIOLOGY

• Atonic bladder it is due to the destruction of
  sensory fibres from urinary bladder. It occurs in
  syphilis, tabes dorsalis or injury to sacral
  segments of spinal cord. Due to the absence of
  sensory impulses, the detrusor muscles loses the
  tone and become flaccid. The micturition
  contraction does not occur. So bladder is
  completely filled with urine. Later, overflow
  occurs in drops as and when urine enters the
  bladder. It is called overflow dribbling

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• Automatic bladder During spinal shock, after
  complete transsection of spinal cord above sacral
  segments, the urinary bladder loses the tone and
  fails to give response to the micturition reflex.
  So the bladder is completely filled and later
  urine overflows by dribbling. But, during
  recovery stage after shock period, the capacity
  of bladder is reduced. The voluntary control of
  micturition is lost. Whenever the bladder is
  filled with some amount of urine, there is
  automatic evacuation of bladder.

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• The uninhibited neurogenic bladder Due to
  lesion in some parts of brainstem, there is
  continuous excitation of spinal micturition
  centres by the higher centres resulting in
  uncontrollable micturition. The frequency of
  micturition is very much increased. Even a small
  quantity of urine collected in bladder will
  elicit the micturition reflex.

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• Nocturnal micturition It is otherwise known
  enuresis or bedwetting. It occurs due to the
  absence of voluntary control of micturition. It
  is a common and normal process in infants and
  children below 3 years. It is because of the
  underdevelopment of voluntary control of
  micturition, which is due to incomplete
  myelination of motor nerve fibres of the bladder.
  When myelination is complete, voluntary control
  of micturition develops and enuresis stops in
  these children.

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• If nocturnal micturition occurs after 3
  years of age, it is considered abnormal. It
  occurs due to neurological disorders like
  lumbosacral vertebral defects. It can also occur
  due to psychological factors. Loss of voluntary
  control of micturition occurs even during the
  impairment of motor area of cerebral cortex.