Micturition

Question 1

Name the efferent nerves and the corresponding receptors involved in micturition?

Answer 1

Pelvic nerve (parasympathetic involuntary) to M3 receptors on the detrusor muscle. 
Hypogastric nerve (sympathetic) to B3 receptors of the detrusor muscle, and A1 receptors of the internal sphincter.
Pudendal nerve (somatic) to the nicotinic receptors on the external sphincter.

Question 2

What neurotransmitter does each nerve release?

Answer 2

Pelvic efferent - ACh to M3 receptor.
Hypogastric - Noradrenaline to B3 and A1 receptors.
Pudendal - ACh to nicotinic receptor.

Question 3

Which spinal region do the 3 efferent nerves arise from?

Answer 3

Sacral - Pelvic and pudendal.

Thoraco lumbar - Hypogastric.

Question 4

Describe the function of the 3 efferent nerves?

Answer 4

Pelvic: Contraction of the detrusor muscle.
Hypogastric: Relaxes the detrusor muscle, and contracts the internal sphincter.
Pudendal: Contracts the external sphincter.

Question 5

Name the afferent nerve involved in micturition. What is its role?

Answer 5

Pelvic sensory. Goes from the detrusor muscle to the sacral region. Increases firing rate in response to stretch.

Question 6

Describe the physiology of an empty bladder situation?

Answer 6

Little stretch, so slow impulses are sent by the pelvic sensory to the sacral region.
This stimulates the hypogastric nerve from the thoraco-lumbar region to stimulate the sympathetic response. It releases NorAdrenaline to the A1 receptors of the internal sphincter causing contraction and to the B3 receptors to cause relaxation of the bladder detrusor.
The pontine micurition centre sends signals to the pelvic efferent in the sacral region to inhibit bladder contraction, and also to the pudendal nerve to act on nicotinic receptors to cause external sphincter contraction.

Question 7

Describe the physiology of a full bladder situation?

Answer 7

Urine causes stretch of the bladder, which causes the sensory pelvic nerve to increase firing to the sacral region.
The sacral region stimulates the pontine micturition centre.
The pons signals to the thoracolumbar region to inhibit the hypogastric nerve to stop relaxation of the detrusor, and no contract (so relax) in the internal sphincter.
The pons also stimulates the pelvic efferent from the sacral regions to contract the detrusor.
The pons also inhibits the pudendal nerve in the sacral region so there is no contraction of the external sphincter (so it relaxes).
Urine can be voided.

Question 8

Outline the voiding reflex?

Answer 8

Pelvic sensory afferent nerve continues to signal to the sacral region of the spinal cord, which causes pelvic efferent fibres to keep contracting the detrusor, causing continual urination and all urine to be voided.