Physiology of Micturition & Assessment of Renal Function

Question 1

What is the physiology of micturition?

Answer 1

1. The first urge to void starts when the bladder contains around 100mls or urine.
2. This urge stays the same until around 400mls when a sense of urgency for micturition is felt as the intravesical pressure increases.
3. During the filling phase, both the internal ‘sphincter’ and external sphincters of the bladder are contracted. The signal for the sphincters to stay contracted come from motor neuron signals from the CNS.
4. So an increase in parasympathetic activity leads to an increase in contraction of the detrusor muscle (muscle of bladder). This contraction increases the pressure felt in the bladder felt at S2-S4.
5. There is also sensory innervation sent from the bladder to the CNS. As the bladder fills there is an increased discharge in afferent nerves to the spinal cord that excite parasympathetic outflow and inhibit sympathetic outflow.
6. Somatic motorneurons holding the external sphincter closed are inhibited.
7. The combination of parasympathetic activation and inhibition of sympathetic nerves results in the external sphincter relaxing and the smooth muscle (detrusor) contracting to allow for voiding.
8. This combination of parasympathetic activation and sympathetic inhibition is known as the spinal reflex.
9. The spinal reflex is activated when the bladder contains around 300-350mls of urine.
10. Delaying of micturition is accomplished by descending pathways from the brain (cortex and brainstem) which inhibit the parasympathetic and stimulate the somatic motor nerves to the external sphincter telling them to remain closed. This overrides the signal from the bladder stretching.
11. Voluntary initiation involves stimulating the parasympathetic fibres and inhibiting the sympathetic fibres.

Question 2

What is the micturition reflex?

Answer 2

1. The micturition reflex is:
   
   1. Stimulation of the parasympathetic system to contract the detrusor muscle of the bladder – the stimulation of the parasympathetic system occurs by an increase in the firing rate of AP’s from the stretched detrusor muscle to the sacral region of the spinal cord.
   2. Inhibition of the sympathetic system to relax the external sphincter of the urethra – this occurs via signals from the brain to inhibit the somatic motor innervation of the external sphincter.

Question 3

How can voluntary control of micturition be achieved?

Answer 3

1. If it is undesirable to void the bladder, then the micturition reflex can be overridden
2. This is achieved by brain signals which travel to the sacral region of the spinal cord and inhibit the contraction of the detrusor muscle of the bladder
3. Brain signals also ensure that the signals to the external sphincter are maintained so that it remains closed.
4. Voluntary initiation is controlled in a similar way by brain centres in the cortex and brainstem:
   
   1. Parasympathetic is activated to contract the bladder muscle
   2. Sympathetic is deactivated to relax the external sphincter and allow voiding of the bladder.