RENAL - Physiology of Micturition Flashcards
Describe autonomic innervation of the bladder.
- 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
Describe voluntary control of the bladder.
Somatically mediated through pudendal nerve
- Arises from sacral spinal cord - innervates skeletal muscle of external sphincter - stimulates contraction
Describe how sensory information is conveyed in the urinary tract.
- 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
Describe what happens during bladder storage. PART 1
- 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
Describe what happens during bladder storage. PART 2
- 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
Describe the guarding reflex.
- Process by which sensory afferents detecting bladder distension stimulate nerves that prevent voiding
What is the usual threshold at which voiding occurs?
200 ml
Describe what happens during bladder voiding. PART 1
- 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
Describe what happens during bladder voiding. PART 2
- 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
Describe the control on the pontine micturition centre.
Under voluntary control via higher order inputs to the periaqueductal grey
Describe bladder control in infants.
- 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
What is the consequences of toilet training on the aforementioned spinal reflexes in infants?
Greater control from higher brain centres
- Spinal reflex overriden
- Voluntary control induced
Describe a clinical importance of this spinal reflex in later life.
- Pathway remerges following SCI when higher cortical control lost
What has been deduced from MRI studies regarding conscious control of micturition in humans? PART 1
- 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
What has been deduced from MRI studies regarding conscious control of micturition in humans? PART 2
- 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
What would be the consequence of an L3 lesion on control of micturition in humans? PART 1
- 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
What would be the consequence of an L3 lesion on control of micturition in humans? PART 2
- 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
What would be the consequence of an L3 lesion on control of micturition in humans? PART 3
- 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
What would be the consequence of an L3 lesion on control of micturition in humans? PART 4
- 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
What would happen to bladder control in a T9 lesion?
- 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
What would happen to bladder control in a S3 lesion?
- 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
