Introduction

Micturition is the mechanism by which the urinary bladder is emptied when it is full. Emptying of the bladder is a reflex action called a micturition reflex, which is regulated by the supraspinal centers and is aided by contraction of the perineal and abdominal muscles.

Micturition may also be referred to as urination or voiding. To understand the physiology of micturition, it is important to know about the organs and structures involved in the urination process. The kidneys are the main site of urine production, urine is continuously produced by nephrons and discharges into the urinary bladder drop by drop through the ureters. The ureters propel the urine from the kidneys to the bladder through a peristaltic contraction of the smooth muscle layer. As the urine is collected in the pelvis of the ureter, the contraction sets up in the pelvis. This contraction is transferred via the rest of the ureter in the form of a peristaltic wave to the trigone of the urinary bladder. Peristaltic waves typically travel at a velocity of 3 cm/second. It evolves at a frequency of 1 to 5 per minute. The peristaltic wave propels the urine to the bladder.

The bladder is a reservoir for urine, the urinary bladder is a muscular sac below the peritoneum and behind the pubic bones. Consists of the body and the neck, which is the lowest part of the bladder, and continues as a posterior urethra. The wall of the bladder is formed by a smooth muscle. The bladder retains the urine until micturition is desired it contract and expels the urine. The urine passes through the urethra. The urethra takes the urine from the bladder to the outside. Composition and volume of urine are not affected by the ureters, urinary bladder, or urethra, but those organs are responsible for the periodic elimination of urine.

What is micturition?

Micturition is the mechanism by which the urinary bladder is emptied when it becomes filled. In older children and adults, micturition can be controlled voluntarily to some extent. The functional anatomy and nerve supply of the urinary bladder are important for the process of micturition.

Micturition starts with the onset of urinary secretion in the fifth month of intrauterine life. It remains a purely reflex act until approximately 2–2.5 years of age, at which point it starts to be under voluntary control.

This process includes two main steps: first, the bladder fills progressively until the tension in its walls rises above the threshold level; this stimulates a second step, which is a nervous reflex called a micturition reflex, which empties the bladder or if it fails, at least induces a conscious desire to urinate. Since the micturition reflex is an autonomic spinal cord reflex, centers in the cerebral cortex or brain stem can also, inhibit or facilitate the micturition reflex.

Micturition reflex

Micturition reflex is a reflex by which micturition occurs. This reflex is obtained by stimulation of the stretch receptors on the walls of the urinary bladder and urethra. In particular, the receptors in the posterior urethra as this region starts to fill with urine at higher bladder pressures.

The reflex is activated long before the maximum capacity is reached even though the bladder can accommodate up to 800 mL of urine, or even more, but the intravesical pressure increases when about 300 to 400 mL of urine is accumulated in the bladder. This stretches the wall of the bladder resulting in stimulation of the stretch receptor and production of sensory (afferent) impulses that pass through the pelvic nerves to the sacral segments of the spinal cord and then reflexively return to the bladder through the motor (efferent) impulses that return along the parasympathetic nerve fibers through the same nerves to the bladder and the internal sphincter.

Motor impulses cause contraction of the detrusor muscle, while the internal urethral sphincter relaxes at the same time. When the 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 the sensory impulses of the bladder and urethra. These impulses, in turn, cause the reflex contraction of the bladder to increase further.

The cycle continues repeatedly until the force of contraction of the bladder reaches its maximum and the urine is completely voided. During micturition, the flow of urine is facilitated by an increase in abdominal pressure due to the voluntary contraction of the abdominal muscles.

When the reflex of the micturition becomes strong enough, it induces another reflex that passes through the pudendal nerves to the external sphincter to inhibit it. micturition will occur If this inhibition is more potent in the brain than in the voluntary constrictor signals to the external sphincter. If not, urination will not occur until the micturition reflex becomes more powerful and when the bladder fills still further.

The urine will flow into the urethra, and the bladder is emptied If the external urethral sphincter is voluntarily relaxed. by voluntary contraction of the external urethral sphincter Urination can be prevented. Bladder overdistension is extremely painful, and when this phase is reached there is a tendency for allowing a small amount of urine to escape, by involuntary relaxation of the external sphincter, provided there is no mechanical obstruction. The Involuntary loss of urine is referred to as (incontinence).

Higher Centers for Micturition

The micturition reflex is an autonomic spinal cord reflex present in the sacral and lumbar segments, but these spinal centers are regulated by higher centers. The Higher centers, that control micturition is of two types, inhibitory centers, and facilitatory centers.

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 primary cause of micturition, but higher centers typically perform the final control of micturition as follows:

Usually, voluntary urination is initiated in the following way:

First, a person voluntarily contracts his or her abdominal muscles, raising the pressure in the bladder and allowing extra urine to enter the bladder neck and posterior urethra under pressure, thereby stretching their walls. This stimulates the stretch receptors, which excites the micturition reflex and at the same time inhibits the external urethral sphincter.

Usually, all urine would be emptied, with rarely more than 5 to 10 milliliters left in the bladder. After urination, the female urethra is emptied by gravity, while the urine remaining in the male urethra is expelled by multiple contractions of the bulbospongiosus muscle.

1. Unless micturition is desired, the higher centers keep the micturition reflex partially inhibited.
2. The higher centers can prevent micturition, even if the micturition reflex occurs, by tonic contraction of the external bladder sphincter until a convenient time presents itself.
3. 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 urinary sphincter so that urination can occur.

Summary

Urine production is the function of the kidneys, and the removal of urine is the function of the ureters, the urinary bladder, and the urethra. Body cells produce waste products such as urea, creatinine, and ammonia that must be eliminated from the blood before they accumulate to toxic levels. Each ureter extends from the hilus of the kidney to the lower posterior of the urinary bladder.

The ureters pass obliquely through the wall of the bladder, and while there are no ureteral sphincters as such, the oblique passage tends to keep the ureters closed even during peristaltic waves, preventing the reflux of urine from the bladder. Peristalsis waves of the ureter smooth muscle layer propel urine toward the bladder.

Micturition is basically a spinal reflex that is facilitated and inhibited by higher brain centers and, is subject to voluntary facilitation and inhibition just like defecation. The micturition reflex is triggered in the wall of the urinary bladder by stimulation of the stretch receptors. When 300 to 400 ml of urine is collected in the bladder, afferent autonomic nerve fibers in the bladder wall sensitive to stretch are stimulated. In infants, this initiates a spinal reflex, and micturition occurs.

Urine passed in response to parasympathetic stimulation of the bladder, causing contraction of the detrusor muscle and relaxation of the internal urethral sphincter. When the nervous system is fully developed, the reflex of the micturition is stimulated, but the sensory impulses also pass upwards to the brain and there is a consciousness of the need to pass urine. By experienced and deliberate effort, contraction of the external urethral sphincter and pelvic floor muscles can prevent urination until it is convenient to empty the bladder.

The micturition reflex is an autonomic spinal cord reflex, but it can be inhibited or facilitated by brain centers. These centers include:

Micturition Inhibitor Centers Centers in the midbrain and cerebral cortex inhibit the urination by suppressing spinal micturition centers.

Micturition Facilitator centers Pons centers facilitate urination by spinal centers. Some cerebral cortex centers also facilitate urination.