Micturition Notes

Micturition Introduction

Micturition is a process by 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. The functional anatomy and nerve supply of the urinary bladder are essential for the process of micturition.

Functional Anatomy Of Urinary Bladder

• The urinary bladder consists of the body, neck, and internal urethral sphincter.
• The smooth muscle forming the body of the bladder is called the detrusor muscle. It is formed by three ill-defined layers of muscle fibers, viz.
• the inner longitudinal layer, the middle circular layer, and the outer longitudinal layer.
• At the posterior surface of the bladder wall, there is a triangular area called a trigone.
• At the upper angles of this trigone, two ureters enter the bladder.
• The lower part of the bladder is narrow and forms the neck. The distal end of the bladder is guarded by the internal urethral sphincter.
• This sphincter is made up of detrusor muscle. It opens towards the urethra. At the distal end of the urethra, there is an external urethral sphincter.
• It is made up of skeletal muscle fibers. Therefore, it is responsible for the voluntary control of micturition.

Nerve Supply To Urinary Bladder And Sphincters

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

Sympathetic Nerve Supply

• Preganglionic fibers of the sympathetic nerve arise from the first two lumbar segments (L1, and L2) of the spinal cord.
• After leaving the spinal cord, the fibers pass through the lateral sympathetic chain without any synapse in the sympathetic ganglia and finally terminate in the hypogastric ganglion.
• The postganglionic fibers arising from this ganglion form the hypogastric nerve, which supplies the detrusor muscle and internal sphincter.
• The function of the Sympathetic Nerve
• The stimulation of the sympathetic nerve causes relaxation of the detrusor muscle and constriction of the internal sphincter.
• It results in the filling of the urinary bladder and so, the sympathetic nerve is called the nerve of filling.

Parasympathetic Nerve Supply

• The preganglionic fibers of the parasympathetic nerve form the pelvic nerve or nervous origins.
• Pelvic nerve fibers arise from the second, third, and fourth sacral segments (S2, S2, and S3) of the spinal cord.
• These fibers run through the hypogastric ganglion and synapse with postganglionic neurons situated in close relation to the urinary bladder and internal sphincter.

Read And Learn More: Medical Physiology Notes

The function of the Parasympathetic Nerve

• The stimulation of the pelvic (parasympathetic) nerve causes contraction of the detrusor muscle and relaxation of the internal sphincter leading to the emptying of the urinary bladder.
• So, the parasympathetic nerve is called the nerve of emptying or the nerve of micturition.
• The pelvic nerve has also sensory fibers which carry impulses from stretch receptors present on the wall of the urinary bladder and urethra to the central nervous system.

Somatic Nerve Supply

• The external sphincter is innervated by the somatic nerve called the pudendal nerve.
• It arises from the second, third, and fourth sacral segments of the spinal cord.

The Function Of The Pudendal Nerve

It maintains the tonic contraction of the skeletal muscle fibers of the external sphincter and keeps the external sphincter constricted always. During micturition, this nerve is inhibited.

It causes relaxation of the external sphincter leading to voiding of urine. Thus, the pudendal nerve is responsible for the voluntary control of micturition.

Filling Of Urinary Bladder

Process Of Filling

Urine is continuously formed in the nephrons and it is transported drop by drop through the ureters into the urinary bladder. When urine collects in the pelvis of the ureter, the contraction sets up in the pelvis. The contraction is transmitted through the rest of the ureter in the form of peristaltic waves up to the trigone of the urinary bladder.

• Peristaltic wave usually travels at a velocity of 3 cm/ second. It develops at a frequency of 1-5 per minute. The pedestal wove now the urine into the bladder.
• After the kidney, the direction of the ureter is initially downward and outward. Then, it turns horizontally before entering the bladder.
• At the entrance of the ureters into the urinary bladder, a valvular arrangement is present.
• When the peristaltic wave pushes the urine toward the bladder, this valve opens towards the bladder.
• The position of the ureter and the valvular arrangement at the end of the ureter prevent the backflow of urine from the bladder into the ureter when the detrusor muscle contracts.
• Thus, urine is collected in the bladder drop by drop.
• A reasonable volume of urine can be stored in the urinary bladder without any discomfort and without much increase in pressure inside the bladder (intravesical pressure).
• It is due to the adaptation of the detrusor muscle. The relationship between the volume of urine and pressure in the urinary bladder is studied by cystometrogram.

Cystometrogram Definition

• Cystometry is the technique used to demonstrate the relationship between intravesical pressure and the volume of urine in the bladder.
• Cystometrogram is the graphical registration (recording) of pressure changes in the urinary bladder in relation to the rise in the volume of urine collected in it.

Method of Recording Cystometrogram

• A double-lumen catheter is introduced into the urinary bladder. One of the lumens is used to infuse fluid into the bladder and the other one is used to record the pressure changes by connecting it to a suitable recording instrument.
• First, the bladder is emptied completely. Then, a small and known quantity of fluid is introduced into the bladder at regular intervals.
• The 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.

Description of Cystometrogram Cystometrogram shows three segments.

Segment 1

• Initially, when the urinary bladder is empty, the intravesical pressure is 0.
• When about 100 mL of fluid is collected, the pressure rises sharply to about 10 cm H20.

Segment 2

• This segment shows the plateau, i.e. the intravesical pressure remains more or less at 10 cm H20 (level of segment 1) without any change even after introducing 300400 mL of fluid. It is because of the adaptation of the urinary bladder by relaxation.
• It is in accordance with the law of Laplace.

Law of Laplace:

• According to this law, the pressure in a spherical organ is inversely proportional to its radius, and the tone remains constant.
• That is, if the radius is more, the pressure is less and if the radius is less the pressure is more, provided the tone remains constant.
• P=T/R
• R Where P = Pressure T = Tension R = Radius
• Urinary bladder obeys Laplace law. In the bladder, the increases as the urine is filled.
• At the same time, ns also increases due to the relaxation of the detrusor; a because of this, the pressure rise is almost zero.
• When about 100 mL of urine is collected, the pressure rises to about 10 cm H20 and now, the desire for micturition occurs.
• The desire for urination is associated with a vague feeling in the perineum.
• An additional volume of about 200-300 mL of urine can be collected in the bladder without much increase in pressure.
• However, when the total volume rises beyond 400 mL, the pressure rises sharply, and the urge for micturition starts. Still, voluntary control of micturition is possible.
• And, beyond 600-700 mL of urine, voluntary control starts failing.

Segment 3

• As the pressure increases with a collection of 300-400 mL of fluid, the contraction of the detrusor muscle becomes intense, increasing the consciousness and the urge for micturition. Still, voluntary control is possible.
• The voluntary control is possible up to the volume of 600-700 mL at which the pressure rises to about 35-40 cm H20.
• When the intravesical pressure rises above 40 cm of water, the contraction of the detrusor muscle becomes still more intense.
• And, voluntary control of micturition is not possible. Now, pain sensation develops.

Micturition Reflex

• It is the reflex by which micturition occurs. This reflex is elicited by the stimulation of stretch receptors situated on the wall of the urinary bladder and urethra.
• When about 300-400 mL of urine is collected in the bladder, the pressure inside the bladder increases.
• The rise in pressure stretches the wall of the bladder resulting in the stimulation of stretch receptors and the generation of sensory impulses.
• The sensory (afferent) impulses from the receptors reach the sacral segments of the spinal cord via the sensory fibers of the pelvic (parasympathetic) nerve.
• The motor (efferent) impulses produced in the spinal cord, travel through motor fibers of the pelvic nerve towards the bladder and internal sphincter.
• The motor impulses cause contraction of the detrusor muscle and relaxation of the internal sphincter so that, urine enters the urethra from the bladder.

Once urine enters the urethra, the stretch receptors in the urethra are stimulated and send afferent impulses to the spinal cord via pelvic nerve fibers. These impulses inhibit the pudendal nerve. So, the external sphincter relaxes and micturition occurs.

• Once a micturition reflex begins, it is self-regenerative, i.e. the initial contraction of the bladder further activates the receptors to cause still further increase in sensory impulses from the bladder and urethra.
• These impulses, in turn, cause a further increase in reflex contraction of the bladder. The cycle continues repeatedly until the force of contraction of the bladder reaches the maximum and the urine is voided out completely.
• During micturition, the flow of urine is facilitated by the increase in abdominal pressure due to the voluntary contraction of abdominal muscles.

Higher Centers for Micturition

Spinal centers for micturition are present in sacral and lumbar segments. But, these spinal centers are regulated by higher centers. The higher centers, which control micturition, are of two types, inhibitory centers, and facilitatory centers.

Inhibitory centers for micturition: The centers in the midbrain and cerebral cortex inhibit micturition by suppressing spinal micturition centers.

Facilitatory centers for micturition: The centers in pons facilitate micturition via spinal centers. Some centers in the cerebral cortex also facilitate micturition.

Voiding of urine

Applied Physiology

The abnormalities of micturition are of four types:

1. Atonic bladder
2. Automatic bladder
3. The uninhibited neurogenic bladder
4. Nocturnal micturition.

1. Atonic Bladder-Effect Of Destruction Of Sensory Nerve Fibers

• The atonic bladder is the urinary bladder with a loss of tone in the detrusor muscle.
• It is caused by the destruction of sensory (pelvic) nerve fibers of the urinary bladder.
• Due to the destruction of sensory nerve fibers, the bladder is filled up without any stretch signals to the spinal cord.
• Due to the absence of stretch signals, the detrusor muscle loses its tone and becomes flaccid.
• The micturition contraction does not occur. So, the bladder is completely filled with urine.
• Later, overflow occurs in drops as and when urine enters the bladder. It is called overflow incontinence or overflow dribbling.
• This type of bladder is also called flaccid neurogenic bladder or hypoactive neurogenic bladder.

Conditions of Destruction of Sensory Nerve Fibers

• Spinal injury: During the first stage (stage of spinal shock) after injury to sacral segments of the spinal cord, the bladder becomes atonic
• Syphilis: Syphilis results in the degenerative nervous disorder called tabes dorsal which is characterized by the degeneration of dorsal (sensory) nerve roots. Degeneration of sensory nerve roots of sacral region develops atonic bladder. The atonic bladder in the Tabes dorsal is called the tabetic bladder.

2. Automatic Bladder

• Automatic bladder refers to the urinary bladder that is characterized by a hyperactive micturition reflex. In this condition, the voluntary control of micturition is lost.
• So, even with a small amount of urine collected in the urinary bladder, the micturition reflex occurs resulting in the emptying of the urine.
• This occurs during the second stage (stage of recovery) after the complete transaction of the spinal cord above the sacral segments.
• During the first stage (stage of spinal shock) after complete transection of the spinal cord above sacral segments, the urinary bladder loses the tone and becomes atonic resulting in overflow incontinence.
• During the second stage after the shock period, the micturition reflex returns. However, voluntary control is lacking because of the absence of inhibition or facilitation of micturition by higher centers.
• There is hypertrophy of detrusor muscles so that the capacity of the bladder reduces. Some patients develop hyperactive micturition reflexes.

3. The Uninhibited Neurogenic Bladder

• It is the urinary bladder with frequent and uncontrollable micturition caused by lesions in the brainstem.
• It is also called spastic neurogenic bladder or hyperactive neurogenic bladder.
• The lesion in the midbrain causes continuous excitation of spinal micturition centers resulting in frequent and uncontrollable micturition.
• Even a small quantity of urine collected in the bladder will elicit the micturition reflex. This type of bladder is known as an uninhibited neurogenic bladder.

4. Nocturnal Micturition: Nocturnal micturition is the involuntary voiding of urine during the night. It is otherwise known as enuresis or bed wetting, 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 the underdevelopment of voluntary control of micturition which is due.
• to incomplete myelination of motor nerve fibers, of the bladder. When myelination is complete, voluntary control of micturition develops and enuresis stops in these children.
• If nocturnal micturition occurs after 3 years of age it is considered abnormal.
• It occurs due to neurological disorders like lumbosacral vertebral defects.
• It can also occur due to psychological factors. Loss of voluntary control of micturition occurs even during the impairment of the motor area of the cerebral cortex.