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Micturition

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... reversal of function in which contractile forces predominate in the bladder body ... send a message to the nervous system, small contractile waves occur in the ... – PowerPoint PPT presentation

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Title: Micturition


1
  • 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.

6
  • 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.

7
  • 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.

8
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10
  • 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.

11
  • 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.

12
  • 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

13
  • 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.

14
  • 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

15
  • 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

16
  • 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.

17
  • 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.

18
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
19
  • 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
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