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MICTURITION

· It 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.

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FUNCTIONAL ANATOMY OF URINARY BLADDER

· Urinary bladder consists of the body, neck and internal urethral sphincter. The smooth muscle forming the body of bladder is called detrusor muscle. 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.

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NERVE SUPPLY TO URINARY BLADDER AND SPHINCTERS · Urinary bladder and internal sphincter are supplied by sympathetic and parasympathetic divisions of autonomic nervous system, whereas, the external sphincter is supplied by the somatic nerve fibres

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FILLING OF URINARY BLADDER

· PROCESS OF FILLING · Urine is continuously formed in the nephrons and it is transported drop by drop through the ureters into the urinary bladder. When urine collects in the pelvis of ureter, the contraction sets up in pelvis. The contraction is transmitted through rest of the ureter in the form of peristaltic wave up to trigone of the urinary bladder. 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.

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· . At the entrance of ureters into urinary bladder, a valvular arrangement is present. When peristaltic wave pushes the urine towards the bladder, this valve opens towards the bladder. The position of ureter and the valvular arrangement at the end of ureter prevent the backflow 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 in bladder. It is due to the adaptation of detrusor muscle.

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MICTURITION REFLEX

· Micturition occurs by a reflex called micturition reflex. This reflex starts because of the stimulation of stretch receptors, situated on the wall of urinary bladder and urethra. The rise in pressure stretches the bladder resulting in stimulation of stretch receptors and generation of sensory impulses. The sensory impulses from the receptors reach the sacral segments of spinal cord via the sensory fibres of pelvic nerve. The motor impulses produced in spinal cord, travel through motor fibres of pelvic nerve towards bladder and internal sphincter. The motor impulses cause contraction of detrusor muscle and relaxation of internal sphincter so that, urine enters the urethra from the bladder.

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· . Once urine enters the urethra, the stretch receptors in the urethra are stimulated and send afferent impulses to spinal cord via pelvic fibres. These impulses inhibit pudendal nerve. So the external sphincter relaxes and micturition occurs.during micturition, the flow of urine is facilitated by the increase in the abdominal pressure due to the voluntary contraction of abdominal muscles.

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Higher Centres for Micturition

· Spinal centres for mictrition are present in sacral and lumbar segments. But these spinal centres are regulated by higher centres. The higher centres, which control micturition are of two types, inhibitory centres and facilitatory centres.

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APPLIED PHYSIOLOGY

· Atonic bladder ­ it is due to the destruction of sensory fibres from urinary bladder. It occurs in syphilis, tabes dorsalis or injury to sacral segments of spinal cord. Due to the absence of sensory impulses, the detrusor muscles loses the tone and become flaccid. The micturition contraction does not occur. So bladder is completely filled with urine. Later, overflow occurs in drops as and when urine enters the bladder. It is called overflow dribbling

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Automatic bladder ­ During spinal shock, after complete transsection of spinal cord above sacral segments, the urinary bladder loses the tone and fails to give response to the micturition reflex. So the bladder is completely filled and later urine overflows by dribbling. But, during recovery stage after shock period, the capacity of bladder is reduced. The voluntary control of micturition is lost. Whenever the bladder is filled with some amount of urine, there is automatic evacuation of bladder.

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The uninhibited neurogenic bladder ­ Due to lesion in some parts of brainstem, there is continuous excitation of spinal micturition centres by the higher centres resulting in uncontrollable micturition. The frequency of micturition is very much increased. Even a small quantity of urine collected in bladder will elicit the micturition reflex.

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· Nocturnal micturition ­ It is otherwise known enuresis or bedwetting. It occurs due to the absence of voluntary control of micturition. It is a common and normal process in infants and children below 3 years. It is because of the underdevelopment of voluntary control of micturition, which is due to incomplete myelination of motor nerve fibres of the bladder. When myelination is complete, voluntary control of micturition develops and enuresis stops in these children.

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If nocturnal micturition occurs after 3 years of age, it is considered abnormal. It occurs due to neurological disorders like lumbosacral vertebral defects. It can also occur due to psychological factors. Loss of voluntary control of micturition occurs even during the impairment of motor area of cerebral cortex.

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MICTURITION

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