Transcript Document

MICTURITION
Dr Mangala Gunatilake
Dept. of Physiology
Smooth muscle of
the bladder
(detrusor muscle)
is arranged in
spiral, longitudinal
and circular
bundles. The
muscle bundles
pass on either side
of the urethra are
called the internal
urethral spincter.
Body
Trigone (on the
posterior wall)
Neck
External urethral spincter
Filling of the bladder
The walls
of ureters
contain
smooth
muscle
arranged in
spiral,
longitudinal
and circular
bundles.
When urine collects in
the renal pelvis, the
pressure in the pelvis
increases. This increase
in the pressure initiates a
peristaltic contraction
beginning in the pelvis
and spreading downward
along the ureter to force
urine toward the bladder.
Peristaltic waves occur
1-5 times/minute
The ureters pass obliquely through the detrusor muscle and it passes
little further underneath the bladder mucosa. This oblique passage tends
to keep the ureters closed except during peristaltic waves, preventing
reflux of urine from the bladder.
In some people, the distance that the ureter courses through the bladder
mucosa is less than normal, so that contraction of the bladder during
micturition does not always lead to complete occulusion of the ureter.
As a result some of the urine in the bladder is propelled backward into
the ureter. This is called ‘Vesicoureteral reflux’.
Ureterorenal reflex
The ureters are well supplied with pain nerve fibers. When a ureter is
blocked eg. by a ureteral stone, there will be intense reflex constriction
which is associated with very severe pain. These pain impulses cause a
sympathetic reflex back to the kidney to constrict the renal arterioles,
thereby decreasing urinary output from that kidney. This effect is known
as ‘Ureterorenal reflex’.
Innervation of the bladder
Sympathetic nerve
supply
Parasympathetic nerve supply
S2
L1
S3
L2
Pelvic nerve
L3
S4
Sympathetic
chain
Somatic
nerve supply
Hypogastric
ganglion
S2
S3
S4
Hypogastric
nerve
Urethra
External sphincter
Pudendal nerve
Sympathetic nerve supply and Internal urethral sphincter apparently
play no role in micturition. They prevent reflux of semen into the
bladder during ejaculation.
Parasympathetic nerve supply
Sensory fibers in the pelvic nerve carry impulses from stretch receptors
present on the wall of the urinary bladder to the spinal centre of
micturition. Stimulation of parasympathetic efferent fibers causes
contraction of detrusor muscle leading to emptying of urinary bladder.
Somatic nerve supply
This maintains the tonic contractions of the skeletal muscle fibers of the
external sphincter, so that this sphincter is contracted always. During
micturition this nerve is inhibited, causing relaxation of the external
sphincter and voiding of urine.
What is micturition?
Spinal cord reflex activity.
* facilitated or inhibited by higher centers
* voluntary facilitation or inhibition
The relationship between the volume of urine and pressure in the
urinary bladder (intravesical pressure) can be studied by inserting a
double lumen catheter and emptying the bladder. Then the pressure is
recorded by connecting one lumen of the catheter to a suitable recording
instrument while introducing water or air through the other lumen. The
graphical recording of the pressure changes in the urinary bladder in
relation to rise in the volume of urine collected in it is called
cystometrogram. Cystometry is the technique used to demonstrate this
relationship.
Cystometrogram
80
Intravesical
60
pressure
(cm of
40
Water)
20
0
0
Ib
Ia
100
200
300
Intravesical volume (mL)
400
When the urinary bladder is empty, the intravesical pressure is zero.
When about 50 mL of fluid is collected, the pressure rises sharply
(Ia)to about 10 cm H2O (Ia in the cystometrogram). The pressure in
the bladder remains more or less constant with further addition of
about 350 mL of urine (Ib) in an adult. This is in accordance with law
of Laplace. In the bladder tension increases as the urine is filled. At the
sametime, the radius also increases due to relaxation of the detrusor
muscle. Because of this, the pressure rise is almost nil.
When bladder wall stretches during filling it will initiate a reflex
contraction which has lower threshold. That does not trigger
micturition reflex. When bladder is filled about 300 – 400 mL of urine,
there will be sharp rise in the intravesical pressure as the micturition
reflex is triggered.
When, urine of about 400 mL is collected, the contraction of detrusor
muscle becomes intense, increasing the consciousness and the urge for
micturition. At this point also voluntary control is possible. Beyond 600
– 700 mL of urine voluntary control starts failing.
Intravesical pressure
(centimeters of water)
Micturition
contractions
lb
la
Volume (milliliters)
Filling of the bladder ………..
Filling of the bladder – partially filled
Reflex contractions
Acute increase in pressure
Contractions relax spontaneously
Pressure falls back to baseline
Bladder continues to fill
Reflex contractions – more frequently and powerful
Spinal centres of micturition which are present in scral and lumbar
segments are regulated by higher centres in the brain stem
(Facilitatory area in the pontine region and inhibitory area in the mid
brain). The threshold for the voiding reflex is adjusted by the activity
of facilitatory and inhibitory centres.
When the micturition is facilitated, perineal muscles and external
urethral sphincter are relaxed, the detrusor muscle contracts and urine
passes out through the urethra. During micturition, the flow of urine
is facilitated by increase in the abdominal pressure due to voluntary
contractions of abdominal muscles.
After urination, the female urethra empties by gravity. Urine
remaining in the urethra of the male is expelled by several
contractions of the bulbocavernosus muscle.
Nerve endings
sensitive to stretch
Stimulates
contraction of
detrusor muscle
Spinal cord
Simple reflex control of micturition seen in
infants. The ability of voluntary control
(inhibition) develops at the age of 2 – 3 years.
Brain stem &
Cerebral cortex
Nervous control of micturition
Spinal cord
Abnormalities of micturition
1. Atonic bladder
This is due to destruction of sensory nerve fibers from urinary
from the bladder. When the dorsal sacral roots are interrupted by
diseases of the dorsal roots such as tabes dorsalis or when there is
crush injury to sacral segments of spinal cord, person looses
bladder control (abolition of reflex contractions of the bladder).
Bladder muscle looses the tone (hypotonic) and becomes flaccid).
Bladder fills to the capacity and overflows few drops at a time
through the urethra (overflow incontinence or overflow dribbling).
2. Automatic bladder (Spastic neurogenic
bladder)
During spinal shock after complete transection of spinal cord
above sacral centres of micturition, the urinary bladder looses its
tone and becomes flaccid and unresponsive. So, the bladder is
completely filled, and later urine overflows by dribbling. After the
spinal shock has passed, the voiding reflex returns although there is
no voluntary and higher centre control.
Whenever, the bladder is filled with some amount of urine, there is
automatic evacuation of the bladder.
3. Uninhibited neurogenic bladder
Due to a lesion in some parts of brain stem (interrupting most
of the inhibitory signals), there is continuous excitation of
spinal micturition centres by the higher centres. There is
uncontrollable micturition. Even a small quantity of urine
collected in bladder will elicit the micturition reflex increasing
the frequency of micturition.
Nocturnal micturition (Bed wetting)
This is normal in infants and children below 3 years. It occurs due to
incomplete myelination of motor nerve fibers of the bladder
resulting loss of voluntary control of micturition .