Micturition
Introduction :-
Micturition- Urination, also known as urine production, is the physiological process by which the bladder empties its contents into the urethra and out of the body. This seemingly simple process is based on a complex interplay between the nervous, muscular and urinary systems. The urination process, regulated by the nervous system, is the accumulation of produced urine in the bladder and its regular elimination. It is an automatic process, whereas in adults and older children it can be regulated consciously to some extent. The functional structure and nerve supply of the bladder are crucial to the micturition process.
Anatomy of the Urinary System :-
The bladder’s functions include holding pee in a distensible reservoir and releasing its contents when needed. There are two primary components of the urine bladder: the body and the trigone. The body is a hollow sac made of detrusor muscle, with a layer of transitional epithelium lining the inside and resting on the lamina propria.
The innermost layer’s transitional epithelium has efferent arterioles, or Vas deferens, with corrugations that straighten out over time as the bladder fills. The bladder’s body expands during filling, and the muscles contract as a result of the micturition reflex to push the contents out. The urethra, two ureter apertures, and the trigone are joined to form a tiny triangular area. The smooth muscles that surround the urethral opening make up the bladder’s internal sphincter. Tonically, the muscles tighten until the bladder’s internal pressure rises to a point where the bladder can be opened. The urogenital diaphragm’s striated muscle forms the bladder’s external sphincter, which is located farther away. Except for when it relaxes under reflex or voluntary control during micturition, this muscle is also tonically tensed constantly.
Anatomy of the Urethra :-
The male urethra serves both reproductive and urinary purposes. It transports both semen and urine. The female urethra is only used for urination and is used to transfer pee. As a result, the urethras in men and women have different structures.
Male urethra: The length of the male urethra is 20 cm. It begins in the bladder, travels via the prostate gland (located beneath the bladder), and ends in the penis. The urethra is lined with mucus glands known as glands of Littre.
There are three separate parts to the male urethra:
1. The urethra prostatic
2. The urethra membrane
3. The Spongy urethra
Female urethra- The urethra of a female is shorter and thinner than that of a male. Its length ranges from 3.5 to 4 cm. Following its start from the bladder, it passes through the urogenital diaphragm and proceeds down the vagina’s front wall. Beyond that, it concludes at the urethral external orifice, which is situated between the their clitoris and the outlet of the vagina.
Bladder Filling Mechanism :-
Normally, the bladder fills with urine from the two ureters. Peristaltic waves flow from the renal pelvis through the ureters to the bladder. With each wave, a stream of urine enters the bladder. The waves move at a speed of 20-25 mm per second with a frequency of 1-5 waves per minute. With each wave, urine enters the bladder and the intravesical pressure increases. The pressure then quickly falls again (to almost the initial level) due to relaxation of the detrusor muscle. In this way, the bladder volume increases due to the gradual relaxation of the detrusor muscle , increasing the amount of urine stored in the bladder, but without a significant increase in pressure (Laplace’s law). This process continues until the urine volume (i.e., bladder volume) is about 400 ml, after which the bladder walls no longer relax and the pressure increases without a significant decrease. In this way, the bladder gradually fills up. When about 350-400 ml of urine has been collected, a normal urge to urinate occurs. By voluntary efforts, the start of urination can be delayed until a maximum of about 700-800 ml of urine has accumulated in the bladder, when urination becomes urgent and painful. From this stage onwards, further inhibition is not possible and urination begins automatically. Under normal circumstances, both the internal and external sphincter muscles remain tensed and contracted.
Process of Micturition :-
1. The process of micturition involves coordinated contraction of the smooth muscles of the bladder wall (detrusor), the muscles of the abdominal wall and pelvic floor, fixation of the chest the diaphragm and wall, as well as the internal and exterior sphincters relaxing.
2. The process of micturition can be examined by inserting a catheter into the bladder, connecting it to a suitable manometer, and examining the process under various conditions (cystometrography). If we compare the normal micturition process with the behavior of a denervated bladder, we can see the following facts: In a denervated bladder, the ability to regulate tone is lost, and urination begins much earlier than in a normal bladder.
Urine begins, but the process is not yet complete; the bladder is only partially emptied, and there is always some residual urine remaining. In a normal bladder, the ability to regulate tone is perfect, but urination begins rather later (i.e., a more forceful filling is possible), and the micturition process is a complete process without any residual urine. This striking difference in behavior between a normal and a denervated bladder indicates that under normal conditions the bladder does not simply behave like a simple muscular organ, but its normal activity requires nerves. Put another way, the health of the bladder’s nerves determines one’s capacity for regular urination. These considerations indicate that urination is dependent on a neural reflex.
3. In a denervated bladder, which functions simply as a muscular pouch, moderate filling is enough to raise the pressure to the appropriate level and trigger muscle contraction, thereby initiating urination. Once a certain amount of urine has been expelled, the intravesical pressure falls below a critical level and further contractions cease until the required amount of urine has been accumulated again, thus leaving a residual urine. In a normal bladder, however, when urination begins, the intravesical pressure does not decrease, although fluid is expelled. Because during the process, many sensory impulses emanate from the bladder and urethra, reflexively stimulating further contractions of the bladder until the process is completed.
4. The voluntary micturition process is subject to a great deal of control. It has been experimentally shown that by voluntary effort urination can be prevented for longer periods or initiated sooner. Will can affect both the sympathetic and parasympathetic nerve supplies of the bladder, altering their activity. During normal urination, it is assumed that a certain inhibition of the bladder is voluntarily released, releasing the lower centers. This is the beginning of micturition. If the bladder is not adequately filled, contraction of the bladder can be induced by stimulating the pelvic splanchnic nerves through voluntary efforts.
5. Furthermore, increased intra-abdominal pressure caused by voluntary contraction of the abdominal muscles
and diaphragm puts pressure on the bladder, activating the micturition reflex. In infants, due to lack of myelination of the pyramidal tract, the act of urination is not subject to voluntary control, but other centers function appropriately so that the act is completed, albeit involuntary.
Reflex of Micturition :-
As the bladder radius gradually increases due to the accumulation of urine from the ureters, tension in the bladder wall increases. This tension in the bladder wall stimulates the proprioceptive end organs in the bladder wall, which send afferent impulses to the spinal micturition reflex center (S2-S4), the micturition reflex center in the brainstem (pons) and voluntary micturition. Center paracentral lobule of the cerebral cortex. Normally, the spinal reflex center for micturition is inhibited by impulses from the brainstem centers even when the bladder is full. When the opportunity to empty the bladder arises, the inhibition of the higher centers on the spinal reflex center is released and the micturition reflex begins. parasympathetic efferent impulses that travel to the bladder wall’s detrusor muscle from the spinal reflex center. Contraction of the detrusor muscle causes the posterior urethra to dilate and shorten, resulting in the passage of a urine bolus into the posterior urethra. Urine in the posterior urethra stimulates spinal cord centers that reflexively relax the external urethral sphincter and allow urine to flow through the urethra. The micturition reflex is a “self-renewing process,” meaning that the initial contraction of the bladder further stimulates receptors that further increase the reflex contraction of the bladder. This starts a cycle that repeats over and over until the bladder reaches a strongly contracted state. After a while, the reflex weakens and the self-renewing cycle ends.
Micturition Centers :-
Central control of urination occurs at four levels: (1) the cortex, (2) the hypothalamus, (3) the brainstem, and (4) the spinal cord. Urination is controlled by all of these centers jointly.
Cortical Centers: The centers controlling the bladder are probably located in the motor areas of the cortex and in the upper part of the postcentral gyrus. Motor fibers arising from the cortex are located near the pyramidal tracts of the spinal cord and are intermingled with spinocerebellar fibers. Electrical stimulation of the premotor cortex increases intravesical pressure, causing urination. Bilateral damage to cortical areas (motor areas 4 and 6) leads to loss of voluntary control over the act of micturition, i.e., spontaneous retention of urine is lost.
Hypothalamic centers: Increased detrusor tone has been observed after electrical stimulation of the anterior hypothalamic nucleus, and decreased tone after stimulation of the posterior hypothalamic nucleus.
Brainstem centers: The inferior section of the superior colliculus causes the bladder to become highly excitable, initiating urination after only a few milliliters of urine have been collected. The inferior section of the inferior colliculus reduces bladder function to a spinal state. The dominant centers for Barrington’s first and second reflexes are in the hindbrain.
Spinal centers: These centers remain in the second, third, and fourth sacral segments. If these spinal centers are damaged, the movements become involuntary and incomplete. These spinal centers can be reflexively excited by various kinds of sensory stimuli. The spinal centers are under the inhibitory control of the cerebral cortex and the midbrain. Irritation of the pelvic organs, for example by intestinal parasites in children, may cause involuntary urination reflexively. This is one of the most common causes of nocturia in children. If the spinal cord above the sacral region is damaged, spontaneous urination does not occur. Muscular tone of the bladder wall (detrusor) and sphincter is lost for a period of time. There is no reflex relaxation of the sphincter or contraction of the detrusor. After a while, however, the detrusor muscle regains some tone, and small contractions occur, but not enough to overcome the previously restored sphincter tone. After 1-2 weeks, the resistance of the urethra decreases. The bladder accumulates with urine and gradually becomes overstretched, causing an increase in intravesical pressure. If this increased pressure exceeds the tone of the sphincter, a small amount of urine will dribble out and more urine will remain in the bladder. This condition is called urinary retention with overflow. From the above, it is clear that the phenomenon of urination involves a series of complex reflexes controlled by many centers. It is also interesting to note that both the somatic and autonomic nervous systems are involved in this process. It is one of the few bodily functions that both the autonomic and somatic nerve systems may regulate simultaneously.