Micturition and defecation Flashcards
What are the divisions of the NS?
Central nervous system:
- Brain
- Spinal cors
Peripheral NS:
- Sensory
- Motor:
>Somatic/voluntary
>Autonomic : Sympathetic and
parasympathetic
Where does sympathetic control come from for micturition and defecation?
Thoracolumbar outflow
Lumbar splanchnic > hypogastric > pelvic nerves
Where does parasympathetic control come from for micturition and defecation?
Craniosacral outflow
Sacral spinal nerves > pelvic splanchnic nerves > pelvic nerves
What are the features of the three sectors of ganglionic transmission?
Pre-ganglionic (symp and parasymp)
- ACh onto ganglionic cells
- Nicotinic ganglion cell receptor
Postganglionic (sympathetic)
- Mainly norepinephrine and epinephrine and cholinergic for eccrine sweat glands
- Junctional receptor
- Effect depends on receptor
Post-ganglionic (parasmypathetic)
- ACh onto ganglionic cells
- Ganglionic cell receptor - muscurinic
What nerves provide sympathetic and parasympathetic and somatic and afferent innervation?
Sympathetic nerves: (thoracolumbar outflow) Lumbar splanchnic > hypogastric > inferior hypogastric plexus > pelvic nerves
Parasympathetic nerves: (sacral outflow) Pelvic splanchnic > IHP > pelvic nerves
Somatic: Pedundal nerve
Afferent: follow autonomic nerves, mainly parasympathetic nerves
What nerves and receptors are found in the urinary system from bladder and below?
Bladder:
Pelvic nerve - ACh - M3 receptor
Hypogastric nerve - NA - B3 receptor
Urethra:
Hypogastric nerve - NA - a1 receptor
External urethral sphincter:
Pedundal nerve - ACh - Nicotinic receptor
What function do sympathetic, parasympathetic, somatic and afferent nerves have in the urinary system?
Sympathetic:
- Relaxes detrusor muscle
- Contracts internal urethral sphincter
Parasympathetic:
- Contracts detrusor muscle
- Contracts urethral muscle
Somatic:
- Contracts external urethral sphincter
- Contracts levator ani
Afferent:
- Sensory - stretch, pain, temperature
What is the guarding reflex?
Initiated by distension of bladder during filling - activates stretch -sensitive mechanoreceptors in the bladder wall which generate afferent signals to sacral spinal cord where pudendal motorneuron efferents are activated
What happens in the guarding reflex?
-Low level vesical afferent firing during storage
- Stimulates sympathetic outflow in hypogastric nerve. Causes bladder to relax and internal urethral sphincter to contract
- Vesical afferent fibres increase pudendal outflow to external urethral sphincter
- The pontine storage centre may also increase external urethral sphincter activity
What is the micturition reflex, how does it work?
- Bladder stretch causes intense afferent firing, which activates spinobulbospinal reflex
- Afferent fibres may activates in periaqueductal grey (PAG)
- Stimulates parasympathetic outflow (bladder and urethral smooth muscle contracts)
- Inhibition of sympathetic and pudendal outflow (somatic)
What other voluntary factors bring about voiding of the bladder?
- Laryngeal cavity closed
>Air retained in thorax
> Fixed diaphragm - Contraction of abdominal wall
> Increase in intra-abdominal pressure
What is the effect on micturition with a spinal cord injury above sacrum?
Automatic bladder
- Micturition is initailly blocked, urinary catheterisation needed
- Alternatively micturition reflex established
- Loss of bladder sensation. Emptying controlled by reflex
What is the effect on micturition with a spinal cord injury at the sacrum or below?
Atonic bladder
Loss of sensory input
Loss of micturition reflex leads to overflow incontinence
What is the enteric NS?
Associated with digestive system
Controls peristaltic activity, secretion, transfer of ion and water etc.
Intrinsic neurons in the wall approx. same number as the spinal cord
What are the layers of the GIT and plexi in the GIT?
-Mucosa (3 layers)
- Submucosa
- Muscularis - longitudinal and circular
Two intramural plexuses:
- Myenteric plexus - between longitudinal and circular muscle
- Submucosal plexus - deep to circular muscle
What does the myenteric and submucosal plexuses supply?
Smooth muscle
Gland and epithelium
Endocrine function
Blood vessels
What are the preganglionic, ganglionic and postganglionic fibres of the parasympathetic innervation of the GIT?
Preganglionic:
Vagus: foregut and midgut
Pelvic splanchnic: hindgut
Ganglion:
Sits on both intramural plexuses
Postganglionic:
ENS plexus
What are the preganglionic, ganglionic and postganglionic fibres of the sympathetic innervation of the GIT?
Preganglionic:
Lateral horn of spinal cord T1-L2
Ganglionic:
Prevertebral splanchnic ganglia - coeliac, superior and inferior mesenteric
Postganglionic:
ENS plexus
What are the different role of the sympathetic vs parasympathetic innervation of the GIT?
Sym - inhibitory effect on digestion
Para - stimulatory effect on digestion
What do sensory fibres detect via what, and where do these fibres run?
Pain: via para or sympathetic fibres
Distention: via parasympathetic
Run through posterior grey horn of spinal cord via the dorsal nerve roots
What are the overall stages of the defectaion reflex?
Initiation: sensory
- Stretch receptors activated in distended sigmoid colon and rectum
Involuntary: autonomic
- Intrinsic and extrinsic reflexes cause peristalsis and relaxation of internal anal sphincter
Volunatry: somatic
- Relaxation of the external anal sphincter and puborectalis
What is the motility of the colon - how does it work, what does it result in?
Peristalsis of colon:
- Slower - haustral contractions
- Faster - mass movements
Faeces accumulate in sigmoid colon and rectum
Sigmoid colon and rectum distends which activates stretch receptors
What are the anatomical features of the anal cana?
Internal anal sphincter
- Smooth muscle
- Autonomic control
External anal sphincter
- Striated muscle
- Voluntary control
- Pudendal nerve S2-S4
What is the effect of sympathetic activation on the anal canal?
Rectal smooth muscle relaxed - no peristalsis
Internal sphincter under tonic contraction
What are the different defecation reflex pathways?
Urge to defecate first appears when stretch receptors are activated
Intrinsic pathway activates myenteric plexus and imitates local peristalsis
Extrinsic pathway activates parasympathetic input >stimulates peristalsis and internal anal sphincter relaxation
If willed - voluntary relaxation of external anal sphincter and puborectalis via the pedendal nerve and defecation occurs, or will be delayed
What is the intrinsic - myenteric - reflex?
Involuntary
Stimulus:
Faeces build up in sigmoid colon and rectum causes distention
Receptors:
Stretch receptors in rectal wall
Pathway:
Stretch receptors cause local activation of myenteric plexus which causes smooth muscles to contract in the sigmoid colon and rectum
Response:
Weak peristalsis causing faeces to travel distally towards rectum
What is the extrinsic - parasympathetic - reflex?
Involuntary
Stimulus:
Faeces build up in the sigmoid colon and rectum causes distention
Receptors:
Stretch receptors in the GI wall activate sensory afferents
Pathways:
Sensory afferents terminate in spinal cord and activate parasympathetic nerves
Responses:
Strong peristalsis causing faeces to travel distally towards rectum and relaxation of internal anal sphincter
What is the voluntary control of defecation?
Inhibition of the external anal sphincter via the pudendal nerve S2-S4, causes relaxation of the striated muscle of the sphincter
Puborectalis (levator ani) relaxes
Valsalva manoeuvre
- Close the glottis
- Deep inhalation
- Contraction of abdominal muscles
What does the purborectalis do?
Levator ani forms the pelvic floor
Puborectalis one of three parts of the levator ani
Pedendal nerve S2-S4
Ano-rectal junction
Tonic contraction
What can cause faecal incontinence?
Spinal cord injury: loss of sensory afferents, loss of voluntary motor function
Short arc reflex intact
Long arc (parasymp) reflex may/not be intact
Results in loss of voluntary control
- Childbirth
- Nerve injury/trauma/surgery
- Cauda Equina syndrome
Where does reflex bowel damage originate from and what are the features?
Damage above T12/L1 - UMN
- Loss of bowel sensation
- Loss of descending control from brain
- Defecation reflex intact
- Sphincters have some tone
Where does flaccid bowel damage originate from and what are the features?
Damage below T12/L1 - LMN
- Loss of bowel sensation
- Loss of descending control from brain
- Loss of defecation reflex
- Loss of reflex tone of sphincter
