Micturition is defined as a process during which urine is voided from the urinary bladder.

It is a reflex process. However, in grown up children and adults, it can be controlled voluntarily to some extent.

Urinary Bladder

Urinary bladder consists of the body, neck and internal urethral sphincter.

Smooth muscle forming the body of bladder is called detrusor muscle.

At the posterior surface of bladder wall, there is a triangular area called trigone.

At upper angles of this trigone, two ureters enter the bladder.

Lower part of bladder is narrow and forms the neck.

Distal end of the bladder is guarded by internal urethral sphincter.

This sphincter is made up of detrusor muscle. It opens towards urethra.

At the distal end of urethra, there is an external urethral sphincter.

It is made up of skeletal muscle and is responsible for voluntary control of micturition.

Male urethra has both urinary function and reproductive function. It transports urine and semen.

Female urethra has only urinary function and it transports only urine. So, male urethra is structurally different from female urethra.

Male Urethra

Male urethra is about 20 cm long.

After taking origin from bladder, it traverses the prostate gland and then runs through penis.

Throughout its length, the urethra has numerous glands called glands of Littre.

Male urethra is divided into three parts:

1. Prostatic urethra.
2. Membranous urethra.
3. Spongy urethra.

Female Urethra

Female urethra is narrower and shorter than male urethra.

It is about 3.5 cm to 4 cm long.

After arising from bladder, it traverses through urogenital diaphragm and runs along anterior wall of vagina.

Then it terminates at external orifice of urethra which is located between clitoris and vaginal opening.

Urinary tract has two urethral sphincters:

1. Internal urethral sphincter.
2. External urethral sphincter.

1. Internal Urethral Sphincter

This sphincter is situated between neck of the bladder and upper end of urethra.

It is made up of smooth muscle fibers and formed by thickening of detrusor muscle.

2. External Urethral Sphincter

External sphincter is located in urogenital diaphragm.

This sphincter is made up of circular skeletal muscle fibers.

Urinary bladder and the internal sphincter are supplied by sympathetic and parasympathetic divisions of autonomic nervous system whereas, external sphincter is supplied by somatic nerve fibers.

Sympathetic Nerve Supply

Preganglionic fibers of sympathetic nerve arise from first two lumbar segments (L1 and L2) of spinal cord.

After leaving spinal cord, the fibers pass through lateral sympathetic chain without any synapse in sympathetic ganglia and finally terminate in hypogastric ganglion.

Postganglionic fibers arising from this ganglion form the hypogastric nerve, which supplies detrusor muscle and internal sphincter.

Function of Sympathetic Nerve

Sympathetic nerve causes relaxation of detrusor muscle and constriction of the internal sphincter.

It results in filling of urinary bladder and so, the sympathetic nerve is called nerve of filling.

Parasympathetic Nerve Supply

Preganglionic fibers of parasympathetic nerve form the pelvic nerve or nervus erigens.

Pelvic nerve fibers arise from second, third and fourth sacral segments (S2, S3 and S4) of spinal cord.

These fibers run through hypogastric ganglion and synapse with postganglionic neurons situated in close relation to urinary bladder and internal sphincter.

Function of Parasympathetic Nerve

Pelvic (parasympathetic) nerve causes contraction of detrusor muscle and relaxation of the internal sphincter, leading to emptying of urinary bladder.

So, parasympathetic nerve is called the nerve of emptying or nerve of micturition.

Pelvic nerve has also the sensory fibers which carry impulses from stretch receptors present on the wall of urinary bladder and urethra to the central nervous system.

Somatic Nerve Supply

External sphincter is innervated by somatic nerve called the pudendal nerve.

It arises from second, third and fourth sacral segments of the spinal cord.

Function of Pudendal Nerve

Pudendal nerve maintains tonic contraction of skeletal muscle fibers of external sphincter and keeps external sphincter contracted always.

During micturition, this nerve is inhibited. It causes relaxation of external sphincter leading to voiding of urine.

Process of Filling

Urine is continuously formed in nephrons and it is transported drop by drop through ureters into the urinary bladder.

Peristaltic waves transport urine from pelvis of kidney through ureter.

A reasonable volume of urine can be stored in urinary bladder without any discomfort and without much increase in pressure inside the bladder (intravesical pressure).

It is due to the adaptation of detrusor muscle.

Relationship between the volume of urine and pressure in urinary bladder is studied by cystometrogram.

Cystometry is a technique used to study the relationship between intravesical pressure and volume of urinary bladder.

Cystometrogram is the graphical registration (recording) of pressure changes in urinary bladder in relation to volume of urine collected in it.

Significance of Cystometrogram

Cystometrogram helps to assess the functioning of urinary bladder during filling.

It is useful to measure the pressure inside the bladder.

It also helps to determine size of the bladder and quantity of urine the bladder can hold without any ill effects.

Method of Recording Cystometrogram

A double lumen catheter is introduced into the urinary bladder through urethra.

One lumen is used to infuse fluid into the bladder and the other one is used to record pressure changes by connecting it to a suitable recording instrument.

First, bladder is emptied completely.

Then, a known quantity of fluid is introduced into the bladder at regular intervals.

Intravesical pressure developed by the fluid is recorded continuously.

A graph is obtained by plotting all the values of volume and pressure. This graph is the cystometrogram.

Segments of Cystometrogram

Cystometrogram shows three segments:

Segment I

Initially, when urinary bladder is empty, intravesical pressure is 0.

When about 100 mL of fluid is collected, the pressure rises sharply to about 10 cm H₂O.

Segment II

This segment shows the plateau, i.e., intravesical pressure remains more or less at 10 cm H₂O without any change even after introducing 300 to 400 mL of fluid.

It is because of adaptation of urinary bladder by relaxation.

Law of Laplace

According to this law, in a spherical organ subjected to external pressure, the tone remaining constant, pressure is less if radius is more and pressure is more if radius is less, provided tone remains constant.

Urinary bladder obeys Laplace law.

In urinary bladder, the tension increases as urine is filled. At the same time, radius also increases due to adaptation of detrusor muscle. Because of this, the pressure rise is almost zero.

Segment III

As pressure increases with collection of 300 to 400 mL of fluid, the contraction of detrusor muscle becomes intense, increasing the consciousness and urge for micturition.

Still, voluntary control is possible up to a volume of 600 to 700 mL, at which the pressure rises to about 35 to 40 cm H₂O.

When intravesical pressure rises above 40 cm H₂O, the contraction of detrusor becomes more intense. And, voluntary control of micturition is not possible.

Now, pain sensation develops and micturition should take place.

Micturition reflex is a reflex by which micturition occurs.

This reflex is elicited by stimulation of stretch receptors present on the wall of urinary bladder and urethra.

Sensory (Afferent) Impulses

When about 300 to 400 mL of urine is collected in bladder, the pressure inside bladder increases.

This pressure stretches the wall of bladder resulting in stimulation of stretch receptors and generation of sensory impulses.

Sensory impulses from receptors are carried to the sacral segments of spinal cord via sensory fibers of pelvic nerve.

Motor (Efferent) Impulses

Motor impulses produced in spinal cord travel through motor fibers of pelvic nerve towards bladder.

Motor impulses cause contraction of detrusor muscle and relaxation of internal sphincter so that urine enters urethra from bladder.

Once urine enters urethra, stretch receptors in urethra are stimulated and send afferent impulses to spinal cord via pelvic nerve fibers.

The afferent impulses inhibit pudendal nerve. So, the external sphincter relaxes and micturition occurs.

Self-Regenerative Nature of Micturition Reflex

Once a micturition reflex begins, it is self-regenerative, i.e., initial contraction of bladder fluid activates stretch receptors to cause still further increase in sensory impulses from the bladder and urethra.

Sensory impulses, in turn, cause further increase in reflex contraction of bladder.

This cycle continues repeatedly until force of contraction of bladder reaches maximum and urine is voided out.

Role of Voluntary Control

During micturition, flow of urine is facilitated by increase in abdominal pressure due to the voluntary contraction of abdominal muscles.

Sequence of Micturition Reflex

Spinal centers for micturition are present in sacral and lumbar segments.

Spinal centers are regulated by higher centers.

Types of Higher Centres

1. Inhibitory centers: situated in midbrain and cerebral cortex.
2. Facilitatory centers: situated in pons and cerebral cortex.

1. Atonic Bladder: Effect of Destruction of Sensory Nerve Fibers

Atonic bladder is the urinary bladder with loss of tone in detrusor muscle.

It is caused by destruction of sensory (pelvic) nerve fibers to urinary bladder.

Due to destruction of sensory nerve fibers, urinary bladder is filled up without any stretch signals to spinal cord.

Detrusor muscle loses the tone and becomes flaccid.

So, micturition does not occur and bladder is fully filled with urine.

Later, overflow occurs in drops and when urine enters the bladder. It is called overflow incontinence or overflow dribbling.

It occurs in spinal injury and syphilis.

2. Automatic Bladder

Automatic bladder is the urinary bladder characterized by hyperactive micturition reflex with loss of voluntary control.

So, with small amount of urine collected in the bladder, micturition reflex occurs resulting in emptying of bladder.

This occurs in transaction of spinal cord above the sacral segments.

3. Uninhibited Neurogenic Bladder

It is the urinary bladder with frequent and uncontrollable micturition.

It is caused by lesion in midbrain.

Lesion in midbrain causes continuous excitation of spinal micturition centers resulting in frequent and uncontrollable micturition.

Even a small quantity of urine collected in bladder will elicit the micturition reflex.

4. Nocturnal Micturition

Nocturnal micturition is the involuntary voiding of urine during night. It is otherwise known as enuresis or bedwetting.

It occurs due to the absence of voluntary control of micturition.

It is a common and normal process in infants and children below 3 years.

It is because of incomplete myelination of motor nerve fibers of bladder.

When myelination is complete, voluntary control of micturition develops and bedwetting stops.

Nocturnal micturition occurs after 3 years of age because of neurological disorders such as lumbosacral vertebral defects and impairment of motor area of cerebral cortex.