The act of Micturition

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

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

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

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.

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

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.

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 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’.

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’.

L1

L2

L3

Sympathetic nerve supply

Sympathetic

chain

Hypogastric

ganglion

Hypogastric

nerveUrethra

External sphincter

Parasympathetic nerve supply S2

S3

S4

S2

S3

S4

Pelvic nerve

Pudendal nerve

Innervation of the bladder

Somatic nerve supply

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.

InnervationParasympathetic Pre-glanglionic S2, S3, S4 unite to form Pelvic nerves Post-ganglionic onto detrusor muscle & internal sphincter

Sympathetic Pre-ganglionic L1, L2, L3

Post-ganglionic onto trigone, neck, & internal sphincter Little to do with bladder contraction o--------- o------------------------------------------ Ach NE

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 S1 and S2; innervate external

sphincterMediate voluntary control of micturition

Anatomy of Micturition

• 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

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.

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.

Bladder Filling and Micturition

Bladder Filling:1. Empty bladder: 0 pressure2. 30 - 50 mls of urine 5 - 10 cm

H2O3. 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 H2O to more than 100 cm H2O

c. Caused by micturition reflex

Cystometrogram

100 200 300 400

Intravesical volume (mL)

Intravesical pressure

(cm of

Water)

80

60

40

20

0

0

IaIb

Micturition Con’t…

2. Micturition reflex (does not need the brain)a. Filling stimulates sensory stretch receptorsb. Afferent impulses in Pelvic nervec. Signal reflexively sent back to bladder via efferent parasympathetic fibers in the Pelvic nerved. 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 H2O)

g. Voluntary relaxation of external sphincter allows for urination

h. Flow thru urethra stimulates parasympathic system, sustaining bladder contraction

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 receptorDetrusor muscle

PELVIC NERVE(Sensory Fibers)

PELVIC NERVE(Parasympathetic

Motor Fibers)

Micturitioncontractions

Basal cystometrogram

Volume (milliliters)

Intr

ave

sical

pre

ssu

re(c

en

tim

ete

rs o

f w

ate

r)

la

lb

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 ………..

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.

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.

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

• 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.

Spinal cord

Nervous control of micturition

Brain stem & Cerebral cortex

Voluntary Control of Micturition

1. Micturition reflex can be inhibited by:a. Ponsb. 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 sphincterb. Voluntary relaxation of external sphincter

Problems:Atonic Bladder - destruction of sensory fibers

Traumatic spinal cord injuryOverflow incontinence.

Automatic Bladder - spinal cord injury above sacral region

Micturition reflex is intact but uncontrolled

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

Abnormalities of micturition

1.Atonic bladderThis 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 .