Micturition Flashcards
What is the proper name of the internal sphincter of the bladder?
Internal urethral sphincter. Made of smooth muscle under autonomic control.
What is the proper name of the outer layer of striated skeletal muscle around the internal urethral sphincter?
Urethralis muscle, which is under somatic control.
Which nerve innervates the urethralis muscle of the bladder?
The urethralis m. is innervated somatically (voluntary control) by the pudendal nerve, which arises from the sacral region (S1-3 in small animals, S3-5 in large animals).
The pudendal nerve also innervates the muscle around the perineal region, ie., the groin.
Which nerve innervates smooth muscle that makes up the internal urethral sphincter of the bladder?
Sympathetic nerve fibres of the hypogastric nerve provide adrenergic innervation (autonomic, stimulatory) to the smooth muscle of the internal urethral sphinchter.
Like all sympathetic nerves, the hypogastric starts out as pre-ganglionic fibres coming from the lumbar region (L1-L4) that synapse at the caudal mesenteric ganglia, then proceed post-ganglionically as the hypogastric nerves.
NB the hypogastric nerve also innervates the detrusor muscle of the bladder.
The hypogastric nerves – sympathetic – provide adrenergic innervatation to both the detrusor muscle of the bladder as well as the smooth muscle of the internal urethralis sphincter.
When excited, as during fight or flight, the detrusor muscle relaxes while the internal urethralis sphincter constricts, both actions preventing micturition.
How is this reflected in the adrenergic receptors for each muscle?
The detrusor muscle, a meshwork of muscle fibres throughout the bladder, has adrenergic ß-2 receptors, which in general cause smooth-muscle relaxation in the GIT, urinary tract, reproductive/sexual tract and bronchioles when they bind to noradrenalin during fight-or-flight situations.
When you’re fighting or being chased, you don’t necessarily want to digest your food, urinate or give birth, but you do want to facilitate breathing.
The internal urethral sphincter, on the other hand, has the adrenergic alpha-1 receptors, which cause smooth-muscle contraction in the sphincters, blood vessels (vasoconstriction), uterus when bound to catecholamines.
Where does the bladder get its parasympathetic innervation to contract the detrusor muscle during a relaxing situation to micturate?
Parasympathetic innervation to the detrusor muscles comes from the **post-ganglionic fibres of the pelvic nerves. **
The pelvic nerves are formed by pre-ganglionic fibres arising from the sacral spinal cord. These fibres synpase at the pelvic plexus and ganglia within the bladder wall.
What is the sensory innervation involved in micturition?
Two types of sensory innervation:
- Stretch receptors in the bladder wall detect when the bladder is filling up or full and relay signal via pelvic nerves (parasympthatic) to sacral segments of spinal cord.
- Afferent sensory fibres in the urethra detect flow, pain and distension, relaying signals via the pudendal nerve to sacral segments of spinal cord.
The pudendal nerve carries sensory AND motor fibres, somatic AND sympathetic fibres.
Where is the micturition centre in the brain?
Pons
During the storage phase of micturition, what type of nerve system is dominant?
In storage phase, sympathetic system dominates:
- the internal urethral sphincter is constricted (alpha-1 receptors bind to adrenalin, constrict smooth muscle)
- detrusor muscle in bladder is relaxed (beta-2 receptors bind to adrenalin, relax smooth muscle)
- direct inhibition of the pelvic nerves (parasympathetic) by the hypogastric nerves (sympathetic)
- external urethralis muscle maintains constant tone due to pudendal nerve (voluntary)
When bladder capacity is approached, stretch receptors in the bladder send impulses via the pelvic nerves to the sacral spinal cord and then to the pons (micturition centre) where integration occurs.
What type of nerve system dominates during the emptying phase of micturition?
The parasympathetic system dominates during the emptying phase:
The micturition centre sends impulses to the sacral region and simultaneously:
- inhibits the pudendal nerve (somatic) leading to relaxation of the urethralis muscle
- stimulates the sacral parasympathetic neurons to cause contraction of the detrusor m.
- This in turn inhibits adrenergic tone to the urethral sphincter
The coordination of detrusor contraction and sphincter relaxation allows for complete voiding
Where is the “voluntary house training” coordinated in the brain?
Cerebral cortex
Describe the micturition reflex & its voluntary control.
http://msjensen.cehd.umn.edu/1135/Links/Animations/Flash/0041-swf_micturition_re.swf
The micturition reflex involves impulses travelling from the bladder to the sacral region of the spinal cord and vice versa.
It is coordinated by neurons in the spinal cord and can be influenced by signals from the brain.
When the urinary bladder becomes stretched, there’s an increase in action potentials carried from the bladder wall to the sacral regions of the spinal cord.
In response, parasympathetic neurons from the sacral region of the spinal cord to the wall of the bladder are activated, causing the detrusor muscle in the bladder to contract (cholinergic receptors).
The sensory signals from the bladder to the sacral region also stimulate ascending pathways to the pons (micturition-reflex centre) & cerebrum (coordinates voluntary response), resulting in conscious desire to urinate.
If urination isn’t convenient, then the brain (cerebral cortex) sends signals down the spinal cord inhibt the micturition reflex.
Impulses carried via somatic motor neurons (pudendal nerve) keep the external urethralis muscle contracted, also preventing urination.
When urination is desired, the brain sends signals to stimulate the micturition reflex. The brain also decreases action potentials to the somatic motor neurons, causing the external urethralis muscle to relax.
Name drug classes that can be used to treat incontinence by targeting parts of the micturition reflex.
Incontinence can occur because the parasympathetic system is overriding the sympathetic uncontrollably. Acetylcholine (Ach) is the main neurotransmitter (muscarinic) of the parasympathetic system, which causes for example the detrusor muscle to contract.
Ach-blockers can therefore be used:
Antimuscarinics –block Ach receptors on detrusor muscle, thus reduce bladder contractions
Botulinum - toxin from spore-forming Clostridium botulinum bacteria (Gram-positive rod); inhibits Ach release for up to six months after injection
ß3-adrenoreceptor agonists: smooth-muscle relaxant, normally stimulated by noradrenalin (sympathetic); causes expansion of bladder during storage stage but doesn’t affect emptying contraction
Tramadol - opioid that works on central nervous system & increases bladder capacity
