Micturation (1)

1. MICTURATION

2. Physiological Anatomy of Urinary
Bladder
• Smooth muscle hollow vesicle. The fibers are
arranged in spiral, longitudinal and circular
bundles.
Urinary Bladder is composed of
The body – comprised of Detrusor muscle –
responsible for emptying of bladder during
micturition.
The Trigone – small triangular area near the
mouth of bladder through which both ureter and
urethra pass.

4.  The internal sphincter – trigonal muscle fiber get
interlaced around the opening of urethra forming
internal sphincter.
• The natural tone of internal sphincter prevents
emptying of bladder until pressure in the bladder rises
above threshold level.
 The external sphincter – beyond the bladder neck,
encircled by a ring of voluntary ( skeletal type ) muscle
known as external sphincter.
• Sphincter remains tonically contracted which prevents
constant dribbling of urine. But it can be reflexly or
voluntarily relaxed.

5. • The physiological capacity of bladder varies with age, at
birth 20–50 ml, at 1 year–200 ml, and young adult male –
600 ml.
• Anatomical capacity of bladder is 1 L. it is the capacity just
before bladder rupture occurs.
• The urine stored in bladder remains unchanged in chemical
composition since luminal surface of transitional epithelium
forms a complete barrier to passage of water and solute.
• 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

6. Vesicoureteral Reflux
• 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 occlusion of the ureter. As a result some of the
urine in the bladder is propelled backward into the ureter.
Ureterorenal reflex
• The Ureters are well supplied with pain nerve fibers. When
a ureter is blocked e.g. 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.

10. Innervation of Urinary Bladder
Motor innervation
 Parasympathetic innervation. Pelvic nerve
• Efferent fiber derived from 2nd, 3rd, and 4th, sacral segment ( mainly
s₂ and s₃ ).
• Fibers carry motor impulses to the bladder causing contraction of
Detrusor muscle and emptying of bladder.
• These fibers are inhibitory to internal sphincter.
• If these fibers are destroyed normal micturition is not possible.

11.  Sympathetic innervation
• The nerve arise in the T₁₁ to L₂ segments. These fibers are
said to be inhibitory to detrusor muscle and motor to
sphincter vesicae.
• Sympathetic activity are not involved in micturition
• Increased sympathetic discharge to the bladder occurs
during ejaculation and help to prevent the reflux of sperm
from prostatic urethra into bladder.
 Somatic motor innervation
• Pudendal nerve ( S₂, S₃ and S₄ ) supplies the external
sphincter which is voluntary.

12. Sensory innervation
 Sensation of bladder distension
• Afferent from detrusor stretch receptors travel to the spinal
cord via pelvic splanchnic nerve.
• In the spinal cord, fibers of awareness of bladder distention
run in the posterior column to reach the spinal, pontine and
suprapontine micturition center.
 Sensation of bladder pain
• Pain fibers are stimulated by excessive distension or spasm
of bladder wall or by stone, inflammation or malignant
disease irritating the bladder.

13. • In the spinal cord, fibers carrying pain sensation
run in the lateral spinothalamic tract.
Urethral sensation
• Sensation of imminent voiding associated with
maximal bladder filling, reach the spinal cord via
Pudendal nerve.
• In the spinal cord, fibers carrying urethral
sensation travel in dorsal column.

17. Micturition
• Micturition is the process by which urinary bladder empties
when filled.
• It begins in the 5th month of intrauterine life with the onset
of urinary secretion. It remains purely a reflex act until
approx 2-2.5 year of age at which time it begins to come
under voluntary control.
Events in process of micturition
• Filling of urinary bladder
• Emptying of urinary bladder

18. Filling of Bladder
• 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

19. Volume and pressure changes during
filling
• The normal bladder is completely empty at end of
micturition at same time intravesicular pressure is equal
to intra abdominal pressure.
• As the bladder is filled up, adjust its tone and a fairly
large volume of urine can be accommodated with
minimal alternations in intravesicular pressure.
• This is possible because of phenomenon of adaptation.
It occurs because of inherent property of plasticity of
detrusor muscle and because of law of Laplace.

20. Law of Laplace
• According to this low, pressure in the spherical
organ is inversely proportional to its radius, the tone
remaining constant. i.e. if radius is more, pressure is
less and if radius is less, pressure is more provided
tone remain constant.
• If the tone remains constant, increases in radius
decreases the pressure and vice versa.
Law of Laplace – P α T/R
• In the bladder, as volume of fluid rises, the radius
increases due to relaxation of detrusor therefore
pressure rise is almost nil.

21. Cystometry
• The process of studying the relationship between
intravesical volume and pressure.
• Cystometrogram – graphical record of this relationship.
Method of recording Cystometrogram
• A double lumen catheter is introduced into the bladder. The
fluid is introduced through one lumen and pressure changes
are recorded by connecting the other lumen to the pressure
transducer of polygraph.

22. • The bladder is filled with 50 ml increments of water and at each volume,
intravesical pressure is recorded.
Normal Cystometrogram shows 3 phase of filling.
• Phase la – pressure rises from 0 to 10 cm of H₂O, when about 50 ml of
fluid is collected in the bladder.
• Phase lb – Plateau phase which lasts till the bladder volume is 400 ml.
during this phase pressure in the bladder does not change much and remains
approx 10 cm H₂O because of adaptation of bladder by relaxation.
• Phase ll. – phase start beyond 400 ml volume when pressure begins to rise
markedly, triggering the micturition reflex.
• Normally, the voiding contraction raises intravesicular pressure by about 20
to 40 cm of H₂O. if voiding is avoided, the pressure rises from 10 cm of
H₂O onward.
• Beyond 600 ml, the urge to void urine becomes almost unbearable.

24. Emptying of Bladder
• It is basically a reflex action called Micturition reflex, controlled by supraspinal centers and is assisted by
contraction of perineal and abdominal muscles.
Micturition reflex
Initiation – by stimulation of stretch receptor.
Stimulus – filling of bladder by 300 to 400 ml of urine constitutes adequate stimulus for Micturition reflex
Under normal circumstances, first urge to empty the bladder occurs at approx 150 ml of urinary volume
but it can be easily suppressed.
Afferents - from stretch receptors in the detrusor muscle and urethra travel along the pelvic splanchnic nerves and
enter spinal cord through dorsal roots to S₂, S₃ and S₄ segments to reach sacral micturition center.
• Sacral Micturition Center – formed by sacral detrusor nucleus and sacral Pudendal nucleus.
• Afferent of micturition reflex excite the neuron of detrusor nucleus and afferent inhibit Pudendal nucleus
thereby exciting external sphincter. Efferent – from the sacral detrusor nucleus are preganglionic
parasympathetic fibers which relay in ganglia near or within bladder and urethra. fibers are excitatory to detrusor
muscle and inhibitory to internal sphincter.
Response
• Once a micturition reflex begins, it is self-regenerative, i.e. initial contraction of the bladder activates the stretch
receptors to cause a greater increase in sensory impulses from the bladder and posterior urethra,
• Which causes a further increase in reflex contraction of the bladder; thus, the cycle is repeated again and again
until the bladder has reached a strong degree of contraction..
• After a few seconds, the self-regenerative reflex begins to fatigue and the regenerative cycle ceases, permitting
the bladder to relax.
• Once a micturition reflex has occurred but not succeeded in emptying the bladder, the nervous elements of this
reflex usually remain in an inhibited state for a few minutes to 1 hour or more before another micturition reflex
occurs.
• As the bladder becomes more and more filled, micturition reflexes occur more and more often and more and
more powerfullyOnce the micturition reflex becomes powerful enough, it causes another reflex,
• Which passes through the Pudendal nerves to the external sphincter to inhibit it.
• If this inhibition is more potent in the brain than the voluntary constrictor signals to the external sphincter,
urination will occur.
• If not, urination will not occur until the bladder fills still further and the micturition reflex becomes more
powerful

25. Facilitation or Inhibition of Micturition
by the Brain
This reflex is a completely autonomic spinal cord
reflex, but it can be inhibited or facilitated by
centers in the brain. These centers include
• (1) strong facilitative and inhibitory centers in
the brain stem, located mainly in the Pons, and
• (2) several centers located in the cerebral cortex
that are mainly inhibitory but can become
excitatory

26. The micturition reflex is the basic cause of
micturition, but the higher centers normally
exert final control of micturition as follows:
• 1. The higher centers keep the reflex partially
inhibited, except when micturition is desired.
• 2. The higher centers can prevent micturition,
even if the reflex occurs, by continual tonic
contraction of the external bladder sphincter
until a convenient time presents itself.

27. • 3. When it is time to urinate, the cortical
centers can facilitate the sacral micturition
centers to help initiate a micturition reflex and
• At the same time inhibit the external urinary
sphincter so that urination can occur.

28. Voluntary Urination
• A person voluntarily contracts his abdominal
muscles, which increases the pressure in the
bladder and
• Allows extra urine to enter the bladder neck and
posterior urethra under pressure, thus stretching
their walls.
• This stimulates the stretch receptors, which
excites the micturition reflex and simultaneously
inhibits the external urethral sphincter

29. Abnormalities of Micturition
Atonic Bladder
• Micturition reflex contraction cannot occur if the sensory
nerve fibers from the bladder to the spinal cord are
destroyed.
• Thereby preventing transmission of stretch signals from the
bladder.
• Person loses bladder control, despite intact efferent fibers
from the cord to the bladder and despite intact neurogenic
connections within the brain.

30. • Instead of emptying, the bladder fills to capacity and
overflows a few drops at a time through the urethra.
This is called overflow incontinence
• Cause of atonic bladder is crush injury to the sacral
region of the spinal cord.
• Certain diseases can also cause damage to the dorsal
root nerve fibers that enter the spinal cord.
• For example, syphilis can cause constrictive fibrosis
around the dorsal root nerve fibers, destroying them.
This condition is called tabes dorsalis, and the resulting
bladder condition is called tabetic bladder.

31. Automatic bladder
• Spinal cord is damaged above sacral region but the sacral
cord segments are still intact, micturition reflexes can still
occur.
• They are no longer controlled by the brain during the first
few days to several weeks after the damage to the cord has
occurred,
• Micturition reflexes are suppressed because of “spinal
shock” caused by the sudden loss of facilitative impulses
from the brain stem and cerebrum.
• So, even a small amount of urine collected in bladder elicits
micturition reflex resulting emptying bladder.

32. Uninhibited Neurogenic Bladder
• Caused by lack of inhibitory signal from brain. which
results in frequent and relatively uncontrolled micturition.
• This condition derives from partial damage in the spinal
cord or the brain stem that interrupts most of the inhibitory
signals.
• Therefore, facilitative impulses passing continually down
the cord keep the sacral centers so excitable.
• Even a small quantity of urine elicits an uncontrollable
micturition reflex, thereby promoting frequent urination.

33. Nocturnal Micturition (Bedwetting )
• It is the involuntary voiding of urine during night.
• It is due to absence of voluntary control of micturition and
is common, normal process in infants and children below 3
years.
• It is because of incomplete myelination of motor nerve
fibers of bladder. When myelination complete voluntary
control of micturition develops and bedwetting stop.
• If bedwetting occurs after 3 years of age, it is consider
abnormal, occurs due to neurological disorder like
lumbosacral vertebral defect.