Urinary13 - Micturition

Question 1

3 features of the pontine micturition centre (PMC)

What is it?
L Region
M Region

Answer 1

1. ) What is it? - collection of neuronal bodies involved in the supraspinal regulation of micturition
   - found in the rostral pons in the brainstem
2. ) L Region - facilitates storage of urine by closure of the external urethral sphincter (EUS)
   - stimulates somatic motor fibres (pudendal nerve) to cause contraction of the EUS
3. ) M Region - facilitates voiding of urine by contraction of the detrusor muscle in the bladder
   - stimulates the parasympathetic fibres to cause contraction of the detrusor muscle

Question 2

6 features of the storage of urine

Cerebral Cortex
L Centre
Sympathetic Innervation
Sensory Afferents
Somatic Innervation
Inhibitory Projections

Answer 2

1. ) Cerebral Cortex - stimulates the L centre
   - also inhibits the M centre
2. ) L Centre - stimulates the sympathetic efferents
   - can also stimulates somatic motor fibres
3. ) Sympathetic Innervation - originate between T10-L2
   - travel to the bladder via the hypogastric nerve (L1-L2)
   - relaxation of the detrusor muscle (beta-3 receptors)
   - contraction of the IUS (alpha-1 receptors)
4. ) Sensory Afferents - sensitive to stretch receptors in the detrusor to stimulate sympathetic efferents
   - enter the spinal cord between S2-S4 then moves up to synpase at the sympathetic ganglion between T10-L2
5. ) Somatic Innveration - via the pudendal nerve (S2-S4)
   - voluntary contraction of the EUS (nACh receptors)
6. ) Inhibitory Projections - found in the spinal cord
   - inhibits the parasympathetic ganglion
   - can be damaged in spinal cord injuries –> overactive micturition reflex –> urinary incontinence

Question 3

4 structures involved in the voiding of urine

Sensory Afferents
Parasympathetic Efferents
M Centre
Cortex Neurones

Answer 3

1. ) Sensory Afferents - stimulates parasympa efferents
   - stimulates the M centre during a full bladder (+ve FB)
2. ) Parasympathetic Efferents - originate between S2-S4
   - travel to the bladder via the pelvic nerve
   - contraction of the detrusor muscle (M3 receptors)
   - damage to sacral cord (e.g cauda equina syndrome) leads to overactive storage –> overfill incontinence
3. ) Somatic Motor Fibres - pudendal nerve (S2-S4)
   - voluntary relaxation of the EUS (nACh receptors)
4. ) M Centre - micturition centre
   - also stimulates the parasympathetic ganglion
   - inhibits Onuf’s nucleus which inhbits sympathetic activity
   - inhibits the L centre
5. ) Cortex Neurones - helps regulation and provides more control of voiding and storage of urine
   - can stimulate or inhibit the M cenre

Question 4

5 features of the strucutre of the bladder

Epithelium
Bladder Sections
Smooth Muscle Layers
Internal Urethral Sphincter
External Urethral Sphincter

Answer 4

1.) Epithelium - the bladder lumen is lined with transitional epithelium to allow distension

2. ) Bladder Sections
   - main portion is the body which collects urine
   - funnel shaped extension is the neck that connects with the urethra
3. ) Smooth Muscle Layers - 3 layers
   - inner circular, outer longitudinal, detrusor muscle
4. ) Internal Urethral Sphincter
   - made up of smooth muscle so involuntary
   - also prevents retrograde ejaculation in men
5. ) External Urethral Sphincter
   - made up of skeletal (striated) muscle so can be controlled voluntarily

Question 5

3 features of the pressure-volume graph of the bladder

Storage Phase
Max Storage
Voiding Phase

Answer 5

1. ) Storage Phase - increase in volume only produces slight increase in pressure
   - this is because, as the bladder fills, the walls distend to maintain the intra-vesicular pressure
2. ) Max Storage - roughly 500ml
   - small increase in volume after 400ml produces large increases in pressure because the bladder can no longer expand
3. ) Voiding Phase - graph fluctuates downwards
   - due to rhythmic contractions to release urine