What is Micturition

Question 1

What is Micturition

Answer 1

Process by which the urinary bladder empties when it becomes filled.
Involves two main steps:

1. Bladder fills until tension in its walls rises above a threshold level
2. Nervous reflex called the micturition reflex that empties the bladder. Failuremto generate a nervous reflex will cause a conscious desire to urinate

Micturition reflex is an autonomic spinal cord reflex, it can also be inhibited or facilitated by centers in the cerebral cortex or brain stem

Question 2

Physiological Anatomy of the Bladder

Answer 2

There are two main sphincters which are present:

1. Internal sphincter and
2. External Sphincter

Question 3

Internal sphincter

Answer 3

Natural muscle tone normally keeps the bladder neck and posterior urethra empty of urine, prevents emptying of the bladder until the pressure in the main part of the bladder rises above a critical threshold.

Question 4

External Sphincter

Answer 4

Under voluntary control of the nervous system and can be used to consciously prevent urination even when involuntary controls are attempting to empty the
bladder

Question 5

Innervation of the Bladder

Answer 5

The SNS communicates with the bladder via the hypogastric nerve (T12 – L2) - relaxation of the detrusor muscle which functions to promote urine retention

The PNS communicates with the bladder via the pelvic
nerve (S2-S4) - contraction of the detrusor muscle which stimulates micturition

The somatic nervous supply allows voluntary control over micturition. It innervates the external urethral sphincter, via the pudendal nerve (S2-S4). It can cause it to constrict (retention phase) or relax (micturition phase)

Sensory nerves found in the bladder wall and signal the brain when the bladder becomes full, need to urinate

Question 6

How is urine transported from the Kidneys to the Bladder via the Ureters

Answer 6

1. It enters the Collecting ducts
2. Renal calyces stretches the calyces to increase their
   pacemaker activity
3. Initiates peristaltic contractions that spread to the renal pelvis
4. Peristaltic contractions continue downward along the length of the ureter
5. Urine enters the bladder

Question 7

Micturition Reflex/ Wave

Answer 7

Stretch receptors send sensory impulses to the sacral
region of the spinal cord, in response parasympathetic
neurons from the spinal cord to the bladder are activated and this causes the smooth muscle on the bladder wall to contract

Sensory impulses via the sacral region of the spinal cord also stimulates the CNS (Pons and cerebrum) which results in a conscious desire to urinate

If urination is inconvenient, brain sends impulses down
the spinal cord to inhibit the micturition reflex - impulses are carried via somatic motor neurons to keep the external sphincter contracted which prevents urination

When urination is desired, the brain decreases action
potentials in the somatic motor neurons , causing the
external sphincter to relax

Question 8

Does a micturition reflex necessarily cause micturition?

Answer 8

1. Internal sphincter’s intrinsic tone is overcome during a micturition reflex, causing it to relax
2. External sphincter under voluntary control –normally closed and must be voluntarily relaxed
3. Higher centers decide when to relax upon receiving afferent (sensory) signals

Question 9

Micturition reflex is “Self-regenerative”

Answer 9

The initial micturition reflex initiates further stimulation of the stretch receptors, strong enough to the initiate next reflex

Inducing a repetitive cycle until the bladder has reached a strong degree of contraction

The self-regenerative reflex begins to fatigue and the regenerative cycle of the micturition reflex ceases, permitting the bladder to relax
13

Question 10

What happens in summary in Micturition:

Micturition Reflex

Self-Regenerative

Answer 10

A micturition reflex occurs in the following way:

Stretch receptors send sensory/afferent impulses to the spinal cord, which stimulates the parasympathetic or efferent impulses to bladder causing the detrusor muscle in the bladder to contract

The self-regenerative aspect occurs in the following way:

Further stimulation of the stretch receptors causes initiation of the next reflex, repetitive
cycle until the self regenerative reflex begins to fatigue.

Question 11

Facilitation or Inhibition of Micturition by the Brain

Answer 11

The CNS (Pons) keeps the micturition reflex partially inhibited, except when micturition is desired

Prevent micturition, even if the micturition reflex occurs, by contraction of the external bladder
sphincter until a convenient time presents itself

When it is time to urinate, the CNS can facilitate the sacral micturition centers to help initiate a micturition reflex and at the same time inhibit the external urinary sphincter so that urination can occur

Question 12

Involuntary Urination

Answer 12

1. Person voluntarily contracts the abdominal muscles
2. Increases the pressure in the bladder and allows extra urine to enter the bladder neck and posterior
   urethra under pressure, thus stretching their walls
3. This action stimulates the stretch receptors, which excites the micturition reflex and simultaneously
   inhibits the external urethral sphincter
4. All the urine will be emptied, from the bladder (rarely, more than 5 to 10 milliliters left in the bladder

Question 13

Abnormalities of Micturition :

Types

Answer 13

Atonic Bladder and Incontinence:

Automatic Bladder

Uninhibited Neurogenic Bladder

Question 14

Atonic Bladder and Incontinence

Answer 14

Caused by damage to the sensory nerve fibers from the bladder to the spinal cord or crush injury to the sacral region of the spinal cord:

Results in:

1. Loss of bladder control
2. Bladder fills to capacity and overflows (overflow incontinence)

Question 15

Automatic Bladder

Answer 15

Caused by Spinal Cord Damage Above the Sacral Region

Results:

Micturition reflex can still occur but are no longer controlled by the brain

Question 16

Uninhibited Neurogenic Bladder

Answer 16

Caused by partial damage in the spinal cord or the brain stem that interrupts most of the inhibitory signals

1. The facilitative impulses (greater than inhibitory impulses) passing down the spinal cord keep the sacral centers so excitable that even a small quantity of urine elicits an uncontrollable micturition reflex, thereby promoting

Question 17

Vesicoureteral Reflex

Answer 17

The normal tone of the detrusor muscle in the bladder wall tends to compress the ureter, preventing backflow(Reflux) of urine from the bladder when the pressure builds up during either micturition or bladder compression,

The distance that the ureter courses through the bladder is very short thus the contraction does not completely closed of the opening which can result in urine flowing back into the ureter from the bladder(Abnormality)

The reflex can result in:

1. Enlargement of the ureters
2. In severe cases: Damage to the renal calyces and medulla which is as a result of increased pressure in those regions

Question 18

Ureterorenal Reflex

Answer 18

Ureters are well supplied by pain nerve fibers

When the ureter becomes blocked(Kidney stone), intense reflex constriction occurs, which is associated with severe pain.

The pain impulse causes a sympathetic reflex back to the kidneys to constrict the arterioles thereby decreasing urine output from the body-This effect is known as the Ureterorenal Reflex

It is important for preventing excessive flow of fluid into pelvis of the kidney with a blocked ureter.

Question 19

Cystometrogram

Answer 19

Measures the amount of urine in the bladder

Cystometric analysis conducted on a patient can yield a cystometrogram indicating any problems with filling and emptying of the bladder

Micturition contraction(Dashed spikes) are caused as the bladder fills due to stretch reflexes initiated by sensory stretch receptor in the bladder wall