Micturition reflex | Nocturnal micturition

 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-regene-
rative, 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.

ABNORMALITIES OF MICTURITION

ATONIC BLADDER – EFFECT OF
DESTRUCTION OF SENSORY NERVE FIBERS

Atonic bladder is the urinary bladder with loss of tone in detrusor muscle. It is also called flaccid neurogenic bladder or hypoactive neurogenic bladder. It is caused by destruction of sensory (pelvic) nerve fibers of urinary bladder.

Due to the destruction of sensory nerve fibers, the bladder is filled without any stretch signals to spinal cord. Due to the absence of stretch signals, detrusor muscle loses the tone and becomes flaccid. So the bladder is completely filled with urine without any micturition.

Now, urine overflows in drops as and when it
enters the bladder. It is called overflow incontinence or overflow dribbling.

Conditions of Destruction of Sensory Nerve Fibers

 1. Spinal injury: During the first stage (stage of spinal shock) after injury to sacral segments of spinal cord the bladder becomes atonic

 2. Syphilis: Syphilis results in the degenerative nervous disorder called tabes dorsalis, which is characterized by the degeneration of dorsal (sensory) nerve roots . Degeneration of sensory nerve roots of sacral region develops atonic bladder. The atonic bladder in tabes dorsalis is called tabetic bladder.

AUTOMATIC BLADDER 

Automatic bladder is the urinary bladder characterized by hyperactive micturition reflex with loss of voluntary control. So, even a small amount of urine collected in the bladder elicits the micturition reflex resulting in emptying of bladder.

This occurs during the second stage (stage of
recovery) after complete transection of spinal cord above the sacral segments. During the first stage (stage of spinal shock) after complete transection of spinal cord above sacral segments, the urinary bladder loses the tone and becomes atonic resulting in overflow incontinence.

During the second stage after shock period, the
micturition reflex returns. However, the voluntary control is lacking because of absence of inhibition or facilitation of micturition by higher centers. There is hypertrophy of
detrusor muscles so that the capacity of bladder reduces. Some patients develop hyperactive micturition reflex.

UNINHIBITED NEUROGENIC BLADDER

Uninhibited neurogenic bladder is the urinary bladder with frequent and uncontrollable micturition caused by lesion in midbrain. It is also called spastic neurogenic bladder or hyperactive neurogenic bladder.

The lesion in midbrain causes continuous excitation of spinal micturition centers resulting in frequent and uncontrollable micturition. Even a small quantity of urine collected in bladder will elicit the micturition reflex.

NOCTURNAL MICTURITION

Nocturnal micturition is the involuntary voiding of urine during night. It is otherwise known as enuresis orbedwetting. 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
incomplete myelination of motor nerve fibers of the bladder. When myelination is complete, voluntary control of micturition develops and bedwetting stops.

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