Micturition :
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 study of functional anatomy and nerve supply of urinary bladder are required to study the process of micturition.
Physiology of urinary bladder and urethra:
Urinary bladder:
The urinary bladder is a triangular hollow organ located in the lower abdomen. It consists of a body and neck. 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. the inner surface of the urinary bladder is lined by mucous membrane. In the empty bladder, the mucosa falls into many folds called rugae.at the posterior surface of the bladder wall, there is a triangular area called 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. It opens into urethra via internal urethral sphincter.
Urethra:
Male urethra has both urinary function and reproductive function. It carries urine and semen. Female urethra has only urinary function and it carries only urine. So, male urethra is structurally different from the female urethra.
Male urethra
The 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.
male urinary bladder Female urethra
The female urethra is narrower and shorter than male urethra. It is about 3.5 to 4 cm long. After origin from the bladder, it traverses through the urogenital diaphragm and runs along the anterior wall of the vagina. Then it terminates at external orifice of the urethra, which is located between the clitoris and vaginal opening.
Nerve supply to urinary bladder and sphincters
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 sympathetic nerve arise from first two lumbar segments (l1 and l2) of the spinal cord. After leaving spinal cord, the fibers pass through 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.
Function of sympathetic nerve
The stimulation of sympathetic (hypogastric) nerve causes relaxation of the detrusor muscle and constriction of the internal sphincter. It results in filling of the urinary bladder and so, the sympathetic nerve is called nerve of filling.
Parasympathetic nerve supply
Preganglionic fibers of parasympathetic nerve for the pelvic nerve or nervus erigens. Pelvic nerve fibers arise from second, third and fourth sacral segments (s1, s2, and s3) of the spinal cord. These fibers run through hypogastric ganglion and synapse with postganglionic Neurons situated in close relation to the urinary bladder and Internal sphincter.
Micturition reflex
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. When about 300 to 400 ml of urine is collected in the bladder, intravesical pressure increases. This stretches the wall of bladder resulting in stimulation of stretch receptors and generation of sensory impulses.
Pathway for micturition reflex
Sensory (afferent) impulses from the receptors reach the sacral segments of the spinal cord via the sensory fibers of pelvic (parasympathetic) nerve. Motor (efferent)
Impulses produced in the spinal cord, travel through motor fibers of pelvic nerve towards bladder and internal sphincter. Motor impulses cause contraction of the detrusor muscle and relaxation of 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. Now the impulses
Generated from spinal centers inhibit 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 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 bladder reaches the maximum and the urine is voided out completely. 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
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 facilitators 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. micturition process
Introduction to Micturition
Functional anatomy of urinary bladder
What Is Micturition
micturition reflex
