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MBBS Year 2 - Physiology > RENAL - Physiology of Micturition > Flashcards

RENAL - Physiology of Micturition Flashcards

1
Q

Describe autonomic innervation of the bladder.

A
  • SYMPATHETIC innervate detrusor muscle (inhibition of its contraction) and smooth muscle of internal sphincter - arise from T11-L2 through hypogastric nerve
  • PARASYMPATHETIC - from sacral spinal cord through pelvic nerve - stimulation of detrusor muscle contraction
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2
Q

Describe voluntary control of the bladder.

A

Somatically mediated through pudendal nerve
- Arises from sacral spinal cord - innervates skeletal muscle of external sphincter - stimulates contraction

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3
Q

Describe how sensory information is conveyed in the urinary tract.

A
  • Afferent information pertaining to bladder fullness are conveyed through hypogastric and pelvic nerves
  • Information from the urethra and bladder neck conveyed through pudendal and hypogastric nerve respectively
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4
Q

Describe what happens during bladder storage. PART 1

A
  • Activation of low threshold mechanoreceptors by low levels of distention
  • Travel through pelvic nerve to sacral spinal cord
  • Afferent input stimulates sympathetic efferents via the inferior mesenteric ganglion and hypogastric nerve to prevent detrusor muscle contraction
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5
Q

Describe what happens during bladder storage. PART 2

A
  • Occurs through stimulation of beta 3 noradrenergic receptors - expressed on parasympathetic terminals. Stimulation hyperpolarises membrane - prevents ACh release from terminals
  • Sympathetic stimulation prevents bladder contraction (opposite for PNS)
  • Sympathetic efferents also stimulate the internal sphincter to contract (in males) via alpha 1 receptors
  • Sensory afferents stimulate the pudendal nerve to contract the external sphincter
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6
Q

Describe the guarding reflex.

A
  • Process by which sensory afferents detecting bladder distension stimulate nerves that prevent voiding
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7
Q

What is the usual threshold at which voiding occurs?

A

200 ml

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8
Q

Describe what happens during bladder voiding. PART 1

A
  • Upon filling, stimulation of high threshold mechanoreceptors - greater thresholds for activation - greater levels of distension for activation
  • Ascending projections terminate in an area of the midbrain called the periaqueductal grey
  • This acts to regulate the activity of the pontine micturition centre
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9
Q

Describe what happens during bladder voiding. PART 2

A
  • Descending projections from the pontine micturition centre inhibit sympathetic outflow
  • They also inhibit pudendal outflow, leading to relaxation of the external sphincter and activates parasympathetic nerves.
  • Parasympathetic stimulation leads to release of acetylcholine onto M3 muscarinic receptors. This in turn leads to an increase in intracellular calcium and a subsequent contraction of the smooth muscle
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10
Q

Describe the control on the pontine micturition centre.

A

Under voluntary control via higher order inputs to the periaqueductal grey

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11
Q

Describe bladder control in infants.

A
  • Filling targets mechanoreceptors - initiate spinal reflexes to control voiding (no HIGHER control of micturition)
  • Sensory afferents inhibit sympathetic outflow
  • They inhibit pudendal outflow to the external sphincter
  • Excite parasympathetic fibres to initiate detrusor contraction
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12
Q

What is the consequences of toilet training on the aforementioned spinal reflexes in infants?

A

Greater control from higher brain centres
- Spinal reflex overriden
- Voluntary control induced

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13
Q

Describe a clinical importance of this spinal reflex in later life.

A
  • Pathway remerges following SCI when higher cortical control lost
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14
Q

What has been deduced from MRI studies regarding conscious control of micturition in humans? PART 1

A
  • Afferent information from the bladder ascends the spinal cord and interfaces directly with neurons in the periaqueductal grey.
  • PAG then sends projections to the pontine micturition centre which, when activated, promotes bladder voiding
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15
Q

What has been deduced from MRI studies regarding conscious control of micturition in humans? PART 2

A
  • Sensory afferents also activate the anterior cingulate and insula cortex i.e areas involved in self-awareness and decision making
  • Higher order brain regions send inhibitory projections to the PAG, which in turn inhibits the PMC thereby suppressing voiding
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16
Q

What would be the consequence of an L3 lesion on control of micturition in humans? PART 1

A
  • No ascending information can reach the brain to inform it how full the bladder is (as sensory information enters the spinal cord below the lesion)
  • Information cannot influence activity of storage mechanism
  • No conscious control can affect voiding reflex - descending information cannot cross lesion and affect nerves below it
17
Q

What would be the consequence of an L3 lesion on control of micturition in humans? PART 2

A
  • Information on bladder volume can still be passed to the parasympathetic nerves and the pudendal nerves to void the bladder
  • Reflex now becomes involuntary – in essence the infantile reflex of bladder voiding remerges
  • Voiding still occurs - patient will be incontinent and may require intermittent catheterisation to ensure complete voiding
18
Q

What would be the consequence of an L3 lesion on control of micturition in humans? PART 3

A
  • Sympathetic nerves from T11-L2 also control the internal sphincter
  • Sensory information from the bladder controls the activity of this nerve to coordinate relaxation of the internal sphincter and contraction of the bladder
  • In supra-sacral lesions, sensory information can no longer control activity of internal sphincter
19
Q

What would be the consequence of an L3 lesion on control of micturition in humans? PART 4

A
  • If the bladder contracts because of the sacral reflex, it may contract against a constricted internal sphincter – this is known as detrusor-sphincter dyssynergia (DSD)
  • Subsequent rise in intra-bladder pressure can cause urine to go back into the ureters and back into the kidneys causing hydronephrosis
20
Q

What would happen to bladder control in a T9 lesion?

A
  • If a patient has a spinal cord lesion above T11 then all that is lost is voluntary control of bladder function as no ascending or descending signals from the brain can control voiding
  • However the entire voiding reflex remains intact
21
Q

What would happen to bladder control in a S3 lesion?

A
  • Pathways which send sensory information to the brain and to the parasympathetics and somatic nerves within the sacral region have been cut
  • Nervous system now has no way of knowing how full the bladder is, nor any means of controlling its voiding.
  • Absolute loss of control of the bladder will occur - constant catheterisation needed

MBBS Year 2 - Physiology (22 decks)

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