Physiology-of-micturition-reflex

First MBBS Lecture

1. The act of Micturition

2. Micturition
•Urinary bladder is temporary reservoir for urine
•When bladder become full & surroundings are
favorable for voiding urine ,subject passes urine
•The process of voiding urine is known as
micturition

3. Anatomy
•The urinary bladder is a smooth muscle chamber
composed of two main parts:
–(1) the body, the major part, collects urine
–(2) the neck, a funnel-shaped extension of the body,
passing inferiorly and anteriorly into the urogenital
triangle and connecting with the urethra

4. Body

Trigone (on the
posterior wall)
Neck
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. External urethral spincter

5. The bladder neck (posterior urethra) is 2 to 3 centimeters
long, and its wall is composed of detrusor muscle
interlaced with a large amount of elastic tissue. The
muscle in this area is called the internal sphincter.

6. •Beyond the posterior urethra, the urethra
passes through the urogenital diaphragm,
which contains a layer of muscle called the
external sphincter of the bladder.

7. Filling of the 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
The walls of
ureters
contain
smooth
muscle
arranged in
spiral,
longitudinal
and circular
bundles.

8. 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.

9. 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. This is called
„Vesicoureteral reflux‟.

10. 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‟.

11. L
1
L
2
L
3
Sympathetic nerve
supply
Sympathetic
chain

Hypogastric
ganglion

Hypogastric
nerve Urethra
External sphincter
Parasympathetic nerve
supply
S
2
S
3
S
4
S
2
S
3
S
4


Pelvic nerve
Pudendal nerve
Innervation of the bladder
Somatic
nerve
supply

12. 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.

13. Innervation
Parasympathetic
Pre-glanglionic  S
2, S
3, S
4 unite to form Pelvic nerves
Post-ganglionic  onto detrusor muscle & internal sphincter
Sympathetic
Pre-ganglionic  L
1, L
2, L
3
Post-ganglionic  onto trigone, neck, & internal sphincter
Little to do with bladder contraction
o--------- o------------------------------------------
Ach NE

14. Innervation con’t…
Afferents (sensory nerves)
Pelvic nerve: impulses due to bladder fullness; micturition reflex;
pain impulses
Hypogastric nerve: pain impulses
Pudendal nerve: sensory impulses from urethra
Somatic Efferent (Pudendal nerve)
Impulses originate in S
1 and S
2; innervate external sphincter
Mediate voluntary control of micturition

15. Anatomy of Micturition

16. •Internal sphincter
- detrusor muscle in the bladder neck whose
tone normally keeps the bladder neck and
posterior urethra empty of urine and
therefore prevents
emptying of the bladder until the pressure in
the main part of the bladder exceeds a
critical level

•External sphincter
- layer of voluntary skeletal muscle which
surrounds the urethra as it passes through
the urogenital diaphragm
- under voluntary control and can conciously
prevent urination even when involuntary
controls are attempting to empty the
bladder

17. Micturition Reflex
•As bladder fills sensory stretch receptors
send signals via pelvic nerves to sacral
segments of spinal cord.

•Parasympathetic stimulation of the bladder
smooth muscle via the same pelvic nerves
occurs.

•It is “self-regenerative”, subsides, then re-
generates again until the external
sphincter is relaxed and urination can
occur.

18. What is micturition?

Spinal cord reflex activity.

* facilitated or inhibited by higher centers

* voluntary facilitation or inhibition

19. 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.

20. Bladder Filling and Micturition
Bladder Filling:
1. Empty bladder: 0 pressure
2. 30 - 50 mls of urine  5 - 10 cm H
2O
3. 50 - 300 ml little pressure change
4. With filling, increased
activity of external sphincter
(maintains continence, or control of
excretory functions)
5. > 300 - 400 ml  discomfort;
leads to urgency
Start of Micturition:
1. As bladder fills, micturition (bladder)
contractions begin to appear
a. Last from a few seconds to more than
a minute
b. Pressure peaks (micturition waves)
may rise a few cm H
2O to more than 100
cm H
2O
c. Caused by micturition reflex

21. Cystometrogram
100 200 300 400
Intravesical volume (mL)
Intravesic
al
pressure
(cm of
Water)
80
60
40
20
0
0
Ia
Ib

22. Micturition Con’t…
2. Micturition reflex (does not need the
brain)
a. Filling stimulates sensory stretch
receptors
b. Afferent impulses in Pelvic nerve
c. Signal reflexively sent back to
bladder via efferent parasympathetic
fibers in the Pelvic nerve
d. Detrusor muscle contracts, then
relaxes
2. Micturition reflex - continued
e. As bladder fills, micturition reflex occurs
more frequently, with greater
contraction of bladder wall (positive
feedback loop)
f.Micturition powerful enough then
another signal is sent through
Pudendal nerve to inhibit external
sphincter (internal relaxes passively
when pressure is 20 - 40 cm H
2O)
g.Voluntary relaxation of external
sphincter allows for urination
h.Flow thru urethra stimulates
parasympathic system, sustaining
bladder contraction

23. Micturition Reflex
•stretch reflex initiated by filling of the bladder with
urine which results in bladder wall contraction
•mediated by sensory stretch receptors in the
bladder wall,specially by receptors in the posterior
urethra

BLADDER
SPINAL CORD
(sacral segments)
Sensory stretch receptor Detrusor muscle
PELVIC NERVE
(Sensory Fibers)
PELVIC NERVE
(Parasympathetic
Motor Fibers)

24. Micturition
contractions
Volume (milliliters)
Intravesical pressure

(centimeters of water)

la
lb

25. 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
Filling of the bladder ………..

26. 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.

27. Stimulates
contraction of
detrusor muscle
Nerve endings
sensitive to
stretch
Spinal cord
Simple reflex control of micturition seen in infants.
The ability of voluntary control (inhibition) develops
at the age of 2 – 3 years.

28. Facilitation or Inhibition of
Micturition by the Brain
•The micturition 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

29. •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 micturition reflex
partially inhibited, except when micturition is desired.
–2. The higher centers can prevent micturition, even if
the micturition reflex occurs, by continual tonic
contraction of the external urethral sphincter until a
convenient time presents itself.
–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 urethral sphincter so that urination can occur.

30. Spinal cord
Nervous control of micturition
Brain stem &
Cerebral
cortex

32. Voluntary Control of Micturition
1. Micturition reflex can be inhibited by:
a. Pons
b. Cerebral cortex
2. Voluntary contraction of external bladder sphincter means emptying can be delayed
even if a micturition reflex occurs (can go and stop voluntarily)
3. Voluntary emptying:
a. Contraction of abdominal muscles causes ↑ pressure in bladder
 micturition reflex and inhibition of external sphincter
b. Voluntary relaxation of external sphincter
Problems:
Atonic Bladder
- destruction of sensory fibers
Traumatic spinal cord injury
Overflow incontinence.

Automatic Bladder
- spinal cord injury above sacral region
Micturition reflex is intact but
uncontrolled

33. Micturition Abnormalities
•Atonic Bladder - destruction of
sensory fibers
–Traumatic spinal cord injury
–Overflow incontinence.

•Automatic Bladder - spinal cord
injury above sacral region
–Micturition reflex is intact but
uncontrolled

34. 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).

35. 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.

36. 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.

37. 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 .