Micturition

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• Micturition

2

• Both the filing and emptying phases of urinary
  bladder function are dependent on the coordinated
  control of a storage chamber, the bladder body,
  and its outlet, the urethra.
• The essence of this control is the ability of the
  bladder to increase in volume at relatively low,
  but sufficient, intra-vesical pressure to prevent
  the over distension of the bladder while the
  bladder outlet (urethra) remains in a contracted
  or tonic state to maintain bladder continence.

3

• Several reports indicate that the smooth muscle
  in the bladder body is in a constant state of
  controlled contraction during the filling stage,
  while the bladder outlet is closed.
• Bladder emptying is accompanied by a reversal of
  function in which contractile forces predominate
  in the bladder body smooth muscle with a
  concomitant reduction in outlet resistance of the
  bladder neck and urethra associated with the
  relaxation of the smooth muscle.
• While there is still considerable controversy
  regarding the mechanisms that control
  micturition, there is little question that the
  mechanical events occur in a coordinated manner
  to facilitate storage and rapid emptying of the
  bladder.

4

• The bladder wall has three main layers of muscle
• - mucosa
• - submucosa
• - detrusor muscle.
• The mucosa is the innermost layer and is composed
  of transitional cell epithelium.
• The submucosa lies immediately beneath the mucosa
  and its basement membrane. It is composed of
  blood vessels which supply the mucosa with
  nutrients and the lymph nodes which aid in the
  removal of waste products.
• The detrusor is a thick layer of smooth muscle
  which expands to store urine and contracts to
  expel urine.
• The urethra is a small tube which leads from the
  floor or neck of the urinary bladder to the
  outside of the body.

5

• Storage and emptying of the bladder are regulated
  by the internal and external urethral sphincters.
• Sphincters are made up of a ring-like band of
  muscle fibres close off a natural opening in the
  body.
• Sphincters are normally in a closed position and
  need stimulation to open.
• Continence depends on two factors normal lower
  urinary tract support and normal sphincteric
  function.
• Lying below the internal sphincter is the
  external sphincter which is made up of smooth
  muscle mixed with striated, or striped, muscle of
  the pelvic floor or pelvic diaphragm.
• Unlike the smooth muscles that an individual can
  not consciously control, the striated muscles of
  the external sphincter allow for voluntary
  interruption of abdominal pressure to prevent
  urine leakage, such as occurs in coughing or
  sneezing.

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• These three sets of muscles must work in close
  unison to control the various stages of urinary
  bladder filling and emptying.
• During the filling stage, only minimal activity
  is needed to produce closure of the external
  urethral sphincter.
• At a certain point during bladder filling, the
  internal pressure within the bladder becomes
  strong enough to activate stretch receptors in
  the bladder wall (and posterior urethra)
• When these stretch receptors send a message to
  the nervous system, small contractile waves occur
  in the detrusor muscle and the internal urethral
  sphincter automatically relaxes and becomes
  funnel shaped.
• This information is transmitted up the spinal
  cord via the spinothalamic tracts to the central
  nervous system.

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• The external sphincter must now be consciously
  tightened, and the urge to urinate becomes very
  apparent. To urinate, a person must relax the
  external sphincter.
• The advantage of this system is that, during the
  early stages of bladder filling, a person remains
  unaware of the slowly accumulating urine and is
  not required to keep the external sphincter
  tightly closed.
• This only becomes necessary when enough urine
  collects to relax the internal sphincter.

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• Approximately 300 cc of urine within the bladder
  is necessary before the intravesicle pressure
  rises enough for the brain to recognize a sense
  of bladder fullness.
• With low bladder volumes, the sympathetic nervous
  system is stimulated and parasympathetic system
  is inhibited resulting in internal sphincter
  contraction and detrusor relaxation.
• When the bladder is full and micturition is
  desired, the inhibitory signals from the brain
  are replaced by impulses which stimulate the
  parasympathetic system resulting in detrusor
  contraction, and inhibit the sympathetic system
  resulting in internal sphincter relaxation.
• The intravesicle pressure then rises to a point
  at which it exceeds the resistance within the
  urethra, and urine flows out of the bladder.

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• Once the bladder has emptied, the brain again
  sends impulses resulting in parasympathetic
  inhibition and sympathetic stimulation resulting
  in detrusor relaxation and internal sphincter
  contraction. The urinary bladder is again ready
  to be filled with urine.

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• Micturition Reflex
• Stretch of bladder wall
• Increase activity in sensory neurones
• Increase parasympathetic activity
• Increase detrusor muscle contraction
• Inhibition of pudendal nerve
• Results in skeletal muscle relaxing
• Urine voided

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• The brain sends inhibitory signals when detrusor
  relaxation is desired
• Excitatory signals when detrusor contraction is
  desired.
• This information from the brain is carried down
  the spinal cord to the urinary bladder via the
  dorsal columns and corticospinal tracts.
• The bladder has somatic, parasympathetic, and
  sympathetic innervation.

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• Neural Innervation
• The micturition reflex is completely autonomic
  spinal cord reflex but it can be inhibited or
  facilitated by centres in the brain
• These centres include
• Brain stem, in particular pons
• Centres located in cerebral cortex

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• The principle nerve supply of the bladder is by
  way of the pelvic nerves
• These connect with the spinal cord through the
  sacral plexus, mainly connecting with cord
  segments S-2 and S-3
• Coursing through the pelvic nerves are both
  sensory and motor nerve fibres
• The sensory fibres detect the degree of stretch
  in the bladder wall
• Stretch signals from the posterior urethra
  especially strong and mainly responsible for
  initiating reflexes that cause bladder emptying

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• Motor nerves transmitted in pelvic nerves are
  parasympathetic fibres
• Terminate in wall of bladder innervating detrusor
  muscle
• The pudendal nerve is the somatic component of
  bladder innervation (skeletal motor fibres) and
  innervates the external sphincter.
• When stimulated, it produces contraction of the
  external urethral sphincter.
• The external sphincter plays an infrequent role
  in maintaining continence, since it is only able
  to remain tightly contracted for a short period
  of time.
• This sphincter normally contracts with transient
  increases in intra-abdominal pressure such as
  occurs with coughing sneezing, and laughing.

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• The parasympathetic nerve fibres arise from the
  2nd through the 4th segments of the sacral spinal
  cord, and innervate the detrusor muscle.
• Stimulation occurs when the individual desires
  micturition.
• When stimulated, the detrusor contracts resulting
  in elevated intravesicular pressure.
• The internal urinary sphincter is innervated by
  the sympathetic nerves fibres exiting mainly from
  the L-2 segment of the spinal cord.

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Pelvic nerves
Excitation of stretch receptors when 300ml of
urine
Relayed to parasympathetic NS
Pelvic nerves
Contraction of bladder
Bladder outlet pulled open, increase in pressure
micturition
Pudendal impulses nerve inhibited
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• The micturition reflex is a single complete cycle
  of
• 1. Progressive and rapid increase of pressure
• 2. A period of sustained pressure
• 3. Return of the pressure to the basal tone of
  the bladder
• Once a micturition reflex has occurred but has
  not succeeded in emptying the bladder, the
  nervous elements of the reflex remain in an
  inhibited state for 1 minute up to an hour before
  the reflex occurs again
• As bladder becomes more and more filled,
  micturition reflexes occur more and more often
  and more and more powerfully