MICTURITION

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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 urinary bladder and urethra are essential for the process of micturition. 


URETHRAL SPHINCTERS
There are two urethral sphincters in urinary tract:
1. Internal urethral sphincter
2. External urethral sphinct
INTERNAL URETHRA SPHINCTER .
This sphincter is situated between neck of the bladder and upper end of urethra.
It is made up of smooth muscle fibers and formed by thickening of detrusor muscle.
It is innervated by autonomic nerve fibers. This sphincter closes the urethra when bladder is emptied.
EXTERNAL SPHINCTER 
External sphincter is located in the urogenital diaphragm.
This sphincter is made up of circular skeletal muscle fibers, which are innervated by somatic nerve fibers.


NERVE SUPPLY TO URINARY BLADDER AND SPHINCTERS
Urinary bladder and the internal sphincter are supplied by sympathetic and parasympathetic divisions of autonomic nervous system
 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 spinal cord.
After leaving spinal cord, the fibers pass through lateral sympathetic chain without any synapse in the sympathetic ganglia and finally terminate in 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 detrusor muscle and constriction of the internal sphincter.
It results in filling of urinary bladder.
PARASYMPATHETIC NERVE SUPPLY
Preganglionic fibers of parasympathetic nerve form the pelvic nerve or nervus erigentes.

Pelvic nerve fibers arise from second, third and fourth sacral segments ( S2-S4) of spinal cord.

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 
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.  

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
External sphincter is innervated by the somatic nerve called pudendal nerve.
It arises from second, third and fourth sacral segments of the spinal cord.
SYMPATHETIC NERVE SUPPLY
Preganglionic fibers of sympathetic nerve arise from first two lumbar segments (L1 and L2) of spinal cord.

After leaving spinal cord, the fibers pass through lateral sympathetic chain without any synapse in the sympathetic ganglia and finally terminate in 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 detrusor muscle and constriction of the internal sphincter.
It results in filling of urinary bladder.
PARASYMPATHETIC NERVE SUPPLY
Preganglionic fibers of parasympathetic nerve form the pelvic nerve or nervus erigentes.

Pelvic nerve fibers arise from second, third and fourth sacral segments ( S2-S4) of spinal cord.

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
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.
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
External sphincter is innervated by the somatic nerve called pudendal nerve.
It arises from second, third and fourth sacral segments of the spinal cord.
FUNCTION of PUDENDAL NERVE
Pudendal nerve 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 external sphincter leading to voiding of urine.
Thus, the pudendal nerve is responsible for voluntary control of micturition.
FILLING OF URINARY BLADDER
Urine is continuously formed by nephrons and it flows into urinary bladder drop by drop through ureters.

When urine collects in the pelvis of ureter, the contraction sets up in pelvis. This contraction is transmitted through rest of the ureter in the form of peristaltic wave up to trigone of the urinary bladder.
Peristaltic wave usually travels at a velocity of 3 cm/second.
It develops at a frequency of 1 to 5 per minute. 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.

 At the entrance of ureters into urinary bladder, a valvular arrangement is present
When peristaltic wave pushes the urine towards bladder, this valve opens towards the bladder.

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.

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). It is due to the adaptation of detrusor muscle.
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 spinal cord via the sensory fibers of pelvic (parasympathetic) nerve.

Motor (efferent) impulses produced in spinal cord, travel through motor fibers of pelvic nerve towards bladder and internal sphincter.

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.

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 further increase in reflex contraction of 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 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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