Urinary Bladder and micturition

Parts: We can divide urinary bladder into:1) Body: which is the major

part in which the urine collects

2) Neck (posterior urethra): funnel-shaped that connects to urethra

Physiological anatomy of urinary bladder

1) Submucosa:The wall of UB is lined by a transitional epithelium that is continuous with that in the ureters. When the bladder is empty, the mucosa has numerous folds called rugae.As the bladder fills with urine these rugae flatten out and distend with little change in intravesical pressureThis results in high compliance of the bladder, so the volume of the bladder can ↑ from 10 ml to 400 ml with a pressure change of only 10 cm H2O

2) Submucosa:• It supports the mucous membrane. • It is composed of connective tissue with elastic

fibers. 3) Muscle layer (Detrusor muscle):• It is composed of smooth muscle. • The smooth muscle fibers are interwoven in all

directions and collectively these are called the detrusor muscle.

• It consist of a mixture of spiral and longitudinal muscle fibers

• It can increase the pressure in the bladder to 40 – 60 mmHg.

Physiological anatomy of urinary bladder

Physiological anatomy of urinary bladder

Bladder Neck : • It is a funnel shaped extension of the body toward the

urogenital triangle, to join the anterior urethra (external urethra).

• The lower 2-3 cm of the bladder neck is called the posterior or internal urethra.

• The ms fibers in the bladder neck are arranged in 3 layers: inner longitudinal, middle circular, and outer longitudinal.

• The inherent tone of the bladder neck ms, specially the middle layer, prevents emptying of the bladder until the appropriate time for micturition.

Physiological anatomy of urinary bladder

Innervations of UrinaryBladder

Autonomic Innervations of the bladder

Parasympathetic Supply Sympathetic Supply

Nerve Pelvic nerve Hypogastric NerveEfferents:Origin:Supply:

Functions

-LHCs of the S 2,3, and 4.-Body and neck of the bladder.

a) Contraction of bladder wall.b) Relaxation of the bladder neck → stimulation of the detrusor ms of the body causes longitudinal layers to open the bladder neck.

- L1,2, and 3.- Bladder neck

a) Contraction of bladder neck, specially the middle layer→ facilitate the storage of urine.b) Relaxation of the bladder wall by inhibiting the parasympathetic ganglia.

Afferents: a) Carry input from stretch receptors in the bladder neck..b) Detect bladder fullness.c) Carry pain and temperature sensation.

a) Transmit pain sensation b) Detect bladder fullness

Autonomic Innervations of the bladder

(Pelvic nerve)

(Hypogastric nerve)

The Pudendal nerves (AHCs of S 2,3,and 4) Its efferent fibers arise as the parasympathetic

nerves from the 2nd, 3rd and 4th sacral segments of the spinal cord but from the AHCs.

They supply and control the activity of the external urethral sphincter

Somatic Innervations of the bladder

Autonomic and Somatic Innervations of the bladder

In healthy individuals, the bladder has two discrete phases of

activity:

1. The storage phase, when urine is stored in the bladder; and

2. The voiding phase, when urine is released through the

urethra.

Functions of the Urinary Bladder

• During storage, bladder pressure stays low, because of the bladder's highly compliant nature.

• This property is helped by the presence of the transitional epithelium.

• This function is studied by a curve or plot between bladder (intravesical) pressure against the volume of fluid in the bladder (called a cystometrogram)

Storage Phase

Def.• It is a curve which studies the relationship between

intravesical pressure (IVP) and volume.

Cystometrogram

Method:•It is done by inserting a catheter into the bladder to fill and empty the bladder by water, and through special transducer, the intravesical pressure is recorded.•First, the bladder is emptied from urine and the pressure is recorded, then the bladder is filled with 50 ml water and the pressure is recorded for every ↑ in the volume.

0 100 200 300 400 500 Intravesical Volume (ml)

Intr

aves

ical

Pre

ssu

re (

cm H

2O2)

0

20

40

60

80

Ia

Ib

II

Cystometrogram

Ia Ib II

Initial slight riseLong flat

segmentSharp rapid rise

Produced by the

1st increment of

volume by about

50 ml.

Produced by

further increase

in volume up to

400 ml.

Produced by

further increase

in volume above

400 ml.

Cystometrogram

• Segment Ib is a manifestation of the law of Laplace, which states that the pressure in a spherical viscus is equal to twice the wall tension divided by the radius.

• In the case of the bladder, the tension increases as the organ fills, but so does the radius.

• Therefore, the pressure increase is slight until the organ is relatively full.

Cystometrogram

P=2T/r

Micturition

Micturition

Def:

It is the periodic evacuation of urinary bladder through urethra

Mechanism: • Micturition is fundamentally a spinobulbospinal

reflex facilitated and inhibited by higher brain centers and, like defecation, subject to voluntary facilitation and inhibition.

• Center: sacral segments 2, 3 & 4.• Receptors: stretch (receptor) in the wall of

bladder.• Afferent & efferent: pelvic nerve.• Response:

1. Contraction of detrusor muscle (body).

2. Relaxation of internal sphincter of urethra.

3. Relaxation of external urethral sphincter via the pudendal nerve which is somatic nerve originating from AHC of sacral segment 2, 3, & 4.

Micturition Reflexes

↑ IVP

Stretch receptors

Afferents Pelvic Nerve

Center S2,3,4,

EfferentPelvic Nerve

Contraction of wall

Relaxation of int. sphincter

Relaxation of ext. sphincter

MICTURITION REFLEX

Voluntary Control of Micturition

• 1) Cerebral cortex: Motor cortex exerts a voluntary control of micturition either stimulation or inhibition.

• 2) Hypothalamus: There is facilitatory area in the hypothalamus.

• 3) Midbrain: Inhibition.• 4) Pons: facilitation

Higher Centers Control Micturition

1. Keeping the micturition reflex partially inhibited all the time except when there is a desire for micturition.

2. Prevent the micturition even when the reflex is initiated until appropriate time allows.

Higher Centers Control Micturition

Voluntary Initiation of MicturitionVoluntary initiation of micturition • Relaxation of perineal ms causing traction on the bladder• Contraction of anterior abdominal wall and diaphragm to

increase intra-abdominal pressure compressing bladder.• Relaxation of external urethral sphincter.• Flow of urine in urethra intensification of the reflex.• If the condition unfavourable, the higher center will

delay micturition until convenient time by:• 1) Inhibition of sacral segment of micturition.• 2) Stimulation of external urethral sphincter.• 3) Contractions of perineal muscle decrease intravesical

pressure.

• If the bladder fluid content exceeds 700 ml, urine starts to dribble in spite of the voluntary control

Uninhibited Neurogenic Bladder

• Caused by Lack of Inhibitory Signals from the Brain. • This condition derives from partial damage in the

spinal cord or the brain stem that interrupts most of the inhibitory signals.

• Which results in frequent and relatively uncontrolled micturition.

• Therefore, facilitative impulses passing continually down the cord keep the sacral centers so excitable that even a small quantity of urine elicits an uncontrollable micturition reflex, thereby promoting frequent urination.

Automatic/Reflex Bladder • Caused by Spinal Cord Damage Above the Sacral Region. If the

spinal cord is damaged above the sacral region but the sacral cord segments are still intact.

• typical micturition reflexes can still occur. • However, they are no longer controlled by the brain. • During the first few days to several weeks after the damage to the

cord has occurred, the micturition reflexes are suppressed because of the state of “spinal shock” caused by the sudden loss of facilitative impulses from the brain stem and cerebrum.

• However, if the bladder is emptied periodically by catheterization to prevent bladder injury caused by overstretching of the bladder, the excitabilityof the micturition reflex gradually increases until typical micturition reflexes return; then, periodic (but unannounced) bladder emptying occurs.

Atonic Bladder • Caused by Destruction of Sensory Nerve Fibers.• Micturition reflex contraction cannot occur if the sensory nerve fibers from

the bladder to the spinal cordare destroyed, thereby preventing transmission of stretch signals from the bladder.

• When this happens, a person loses bladder control, despite intact efferent fibers from the cord to the bladder and despite intact neurogenic connections within the brain.

• Instead of emptying periodically, the bladder fills to capacity and overflows a few drops at a time through the urethra. This is called overflow incontinence.

• A common 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.