Micturition Introduction .
[1]. Micturition is a process by which urine is voided from the urinary bladder.
[2]. It is a reflex process. However, in grown up children and adults, it can be controlled voluntarily to some extent.
[3]. The functional anatomy and nerve supply of urinary bladder are essential for the process of micturition.
Functional Anatomy of Urinary Bladder & Urethra .
Urinary Bladder .
[1]. Urinary bladder is a triangular hollow organ located in lower abdomen. It consists of a body and neck.
[2]. Wall of the bladder is formed by smooth muscle. It consists of three ill-defined layers of muscle fibers called detrusor muscle, viz. the inner longitudinal layer, middle circular layer and outer longitudinal layer.
[3]. Inner surface of urinary bladder is lined by mucus membrane.
[4]. In empty bladder, the mucosa falls into many folds called rugae.
[5]. At the posterior surface of the bladder wall, there is a triangular area called trigone.
[6]. At the upper angles of this trigone, two ureters enter the bladder.
[7]. Lower part of the bladder is narrow and forms the neck. It opens into urethra via internal urethral sphincter.
Urethra .
[1]. Male urethra has both urinary function and reproductive function. It carries urine and semen.
[2]. Female urethra has only urinary function and it carries only urine. So, male urethra is structurally different from female urethra.
Male Urethra .
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| Male urinary bladder and urethra . |
[1]. Male urethra is about 20 cm long. After origin from bladder it traverses the prostate gland, which lies below the bladder and then runs through the penis .
[2]. Throughout its length, the urethra has mucus glands called glands of Littre.
[3]. Male urethra is divided into three parts:
1. Prostatic urethra
2. Membranous urethra
3. Spongy urethra.
1. Prostatic urethra .
[1]. Prostatic urethra is 3 cm long and it runs through prostate gland. The prostatic fluid is emptied into this part of urethra through prostatic sinuses.
[2]. Sperms from vas deferens and the fluid from seminal vesicles are also emptied into prostatic urethra via ejaculatory ducts .
[3]. Part of the urethra after taking origin from neck of bladder before entering the prostate gland is known as pre-prostatic urethra.
[4]. Its length is about 0.5 to 1.5 cm. This part of urethra is considered as part of prostatic urethra.
2. Membranous urethra .
[1]. Membranous urethra is about 1 to 2 cm long.
[2]. It runs from base of the prostate gland through urogenital diaphragm up to the bulb of urethra.
3. Spongy urethra .
[1]. Spongy urethra is also known as cavernous urethra and its length is about 15 cm.
[2]. Spongy urethra is surrounded by corpus spongiosum of penis.
[3]. It is divided into a proximal bulbar urethra and a distal penile urethra.
[4]. Penile urethra is narrow with a length of about 6 cm.
[5]. It ends with external urethral meatus or orifice, which is located at the end of penis.
[6]. The bilateral bulbourethral glands open into spongy urethra. Bulbourethral glands are also called Cowper glands.
Female Urethra .
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| Female urinary bladder and urethra . |
[1]. Female urethra is narrower and shorter than male urethra. It is about 3.5 to 4 cm long.
[2]. After origin from bladder it traverses through urogenital diaphragm and runs along anterior wall of vagina.
[3]. Then it terminates at external orifice of urethra, which is located between clitoris and vaginal opening.
Urethral Sphincters .
There are two urethral sphincters in urinary tract:
1. Internal urethral sphincter .
2. External urethral sphincter.
1. Internal Urethral sphincter .
[1]. This sphincter is situated between neck of the bladder and upper end of urethra.
[2]. It is made up of smooth muscle fibers and formed by thickening of detrusor muscle.
[3]. It is innervated by autonomic nerve fibers. This sphincter closes the urethra when bladder is emptied.
2. External Urethral sphincter .
[1]. External sphincter is located in the urogenital diaphragm.
[2]. This sphincter is made up of circular skeletal muscle fibers, which are innervated by somatic nerve fibers.
Nerve Supply to Urinary Bladder & Sphincters .
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| Nerve supply to urinary bladder and urethra . |
Urinary bladder and the internal sphincter are supplied by sympathetic and parasympathetic divisions of autonomic nervous system where as, the external sphincter is supplied by the somatic nerve fibers .
Sympathetic Nerve Supply .
[1]. Preganglionic fibers of sympathetic nerve arise from first two lumbar segments (L1 and L2) of spinal cord.
[2]. After leaving spinal cord, the fibers pass through lateral sympathetic chain without any synapse in the sympathetic ganglia and finally terminate in hypogastric ganglion.
[3]. The postganglionic fibers arising from this ganglion form the hypogastric nerve, which supplies the detrusor muscle and internal sphincter.
Function of Sympathetic Nerve .
[1]. The stimulation of sympathetic (hypogastric) nerve causes relaxation of detrusor muscle and constriction of the internal sphincter.
[2]. It results in filling of urinary bladder and so, the sympathetic nerve is called nerve of filling.
Parasympathetic Nerve Supply .
[1]. Preganglionic fibers of parasympathetic nerve form the pelvic nerve or nervus erigens.
[2]. Pelvic nerve fibers arise from second, third and fourth sacral segments (S1, S2 and S3) of spinal cord.
[3]. 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 .
[1]. Stimulation of parasympathetic (pelvic) 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.
[2]. Pelvic nerve has also the 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 .
[1]. External sphincter is innervated by the somatic nerve called pudendal nerve.
[2]. It arises from second, third and fourth sacral segments of the spinal cord.
Function of Pudendal Nerve .
[1]. Pudendal nerve maintains the tonic contraction of the skeletal muscle fibers of the external sphincter and keeps the external sphincter constricted always.
[2]. During micturition, this nerve is inhibited. It causes relaxation of external sphincter leading to voiding of urine. Thus, the pudendal nerve is responsible for voluntary control of micturition.
Filling of Urinary Bladder .
Process of Filling .
[1]. Urine is continuously formed by nephrons and it flows into urinary bladder drop by drop through ureters.
[2]. When urine collects in the pelvis of ureter, the contraction sets up in pelvis.
[3]. This contraction is transmitted through rest of the ureter in the form of peristaltic wave up to trigone of the urinary bladder.
[4]. Peristaltic wave usually travels at a velocity of 3 cm/second. It develops at a frequency of 1 to 5 per minute.
[5]. The peristaltic wave moves the urine into the bladder. After leaving the kidney, the direction of the ureter is initially downward and outward. Then, it turns horizontally before entering the bladder.
[6]. At the entrance of ureters into urinary bladder, a valvular arrangement is present.
[7]. When peristaltic wave pushes the urine towards bladder, this valve opens towards the bladder.
[8]. The position of ureter and the valvular arrangement at the end of ureter prevent the back flow of urine from bladder into the ureter when the detrusor muscle contracts. Thus, urine is collected in bladder drop by drop.
[9]. 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).
[10]. It is due to the adaptation of detrusor muscle. This can be explained by cystometrogram.
Cystometrogram .
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| Cystometrogram . |
[1]. Cystometry is the technique used to study the relationship between intravesical pressure and volume of urine in the bladder.
[2]. Cystometrogram is the graphical registration (recording) of pressure changes in urinary bladder in relation to volume of urine collected in it.
Method of Recording Cystometrogram .
[1]. A double lumen catheter is introduced into the urinary bladder. One of the lumen 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.
[2]. First, the bladder is emptied completely. Then, a known quantity of fluid is introduced into the bladder at regular intervals.
[3]. The intravesical pressure developed by the fluid is recorded continuously.
[4]. A graph is obtained by plotting all the values of volume and the pressure. This graph is the cystometrogram .
Description of Cystometrogram .
Cystometrogram shows three segments.
Segment I .
[1]. Initially, when the urinary bladder is empty, the intravesical pressure is 0.
[2]. When about 100 mL of fluid is collected, the pressure rises sharply to about 10 cm H2 O.
Segment II .
[1]. Segment II shows the plateau, i.e. no change in intra vesical pressure.
[2]. It remains at 10 cm H2 O even after introducing 300 to 400 mL of fluid.
[3]. It is because of adaptation of urinary bladder by relaxation. It is in accordance with law of Laplace.
Law of Laplace .
[1]. According to this law, the pressure in a spherical organ is inversely proportional to its radius, the tone remaining constant.
[2]. That is, if radius is more, the pressure is less and if radius is less the pressure is more, provided the tone remains constant.
T = R х P
Where, P = Pressure T = Tension R = Radius
[3]. Accordingly in the bladder, the tension increases as the urine is filled. At the same time, the radius also increases due to relaxation of detrusor muscle. Because of this, the pressure does not change and plateau appears in the graph.
[4]. With 100 mL of urine and 10 cm H2 O of intravesical pressure, the desire for micturition occurs. [5]. Desire for micturition is associated with a vague feeling in the perineum. But it can be controlled voluntarily.
[6]. An additional volume of about 200 to 300 mL of urine can be collected in bladder without much increase in pressure. However, when total volume rises beyond 400 mL, the pressure starts rising sharply.
Segment III .
[1]. As the pressure increases with collection of 300 to 400 mL of fluid, the contraction of detrusor muscle becomes intense, increasing the consciousness and the urge for micturition.
[2]. Still, voluntary control is possible up to volume of 600 to 700 mL at which the pressure rises to about 35 to 40 cm H2 O.
[3]. When the intravesical pressure rises above 40 cm water, the contraction of detrusor muscle becomes still more intense. And, voluntary control of micturition is not possible.
[4]. Now, pain sensation develops and micturition is a must at this stage.
Micturition Reflex .
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| Micturition Reflex . |
[1]. Micturition reflex is the reflex by which micturition occurs. This reflex is elicited by the stimulation of stretch receptors situated on the wall of urinary bladder and urethra.
[2]. When about 300 to 400 mL of urine is collected in the bladder, intravesical pressure increases.
[3]. This stretches the wall of bladder resulting in stimulation of stretch receptors and generation of sensory impulses.
Pathway for Micturition Reflex .
[1]. Sensory (afferent) impulses from the receptors reach the sacral segments of spinal cord via the sensory fibers of pelvic (parasympathetic) nerve.
[2]. Motor (efferent) impulses produced in spinal cord, travel through motor fibers of pelvic nerve towards bladder and internal sphincter.
[3]. Motor impulses cause contraction of detrusor muscle and relaxation of internal sphincter so that, urine enters the urethra from the bladder . Once urine enters urethra, the stretch receptors in the urethra are stimulated and send afferent impulses to spinal cord via pelvic nerve fibers.
[4]. Now the impulses generated from spinal centers inhibit pudendal nerve. So, the external sphincter relaxes and micturition occurs.
[5]. Once a micturition reflex begins, it is self-regenerative, i.e. the initial contraction of bladder further activates the receptors to cause still further increase in sensory impulses from the bladder and urethra. [6]. These impulses, in turn cause further increase in reflex contraction of bladder.
[7]. The cycle continues repeatedly until the force of contraction of bladder reaches the maximum and the urine is voided out completely.
[8]. During micturition, the flow of urine is facilitated by the increase in the abdominal pressure due to the voluntary contraction of abdominal muscles.
Higher Centers for Micturition .
[1]. Spinal centers for micturition are present in sacral and lumbar segments. But, these spinal centers are regulated by higher centers.
[2]. The higher centers, which control micturition are of two types, inhibitory centers and facilitatory centers.
Inhibitory centers for micturition .
Centers in midbrain and cerebral cortex inhibit the micturition by suppressing spinal micturition centers.
Facilitatory centers for micturition .
Centers in pons facilitate micturition via spinal centers. Some centers in cerebral cortex also facilitate micturition .
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