Continence Flashcards
Which structures control continence of the rectum?
Internal and external anal sphincters
What controls the internal anal sphincter? What type of muscle is it?
ANS - smooth muscle
What controls the external anal sphincter? What type of muscle is it?
Somatic fibres - Striated muscle
What are the key structures and nerve fibre types of the enteric nervous system?
2 plexi
— submucosal/Meissner’s plexus
— myenteric/Auberbach’s plexus
Contain mainly post-ganglionic sympathetic fibres and pre-ganglionic parasympathetic fibres
Where do the parasympathetic fibres innervating the rectum arise from? What do they do?
Pelvic splanchnic nerves, S1-S4
Promote defecation - relaxation of internal sphincter and stimulation of rectal motility
Where do the sympathetic fibres innervating the rectum arise from? What are their function?
L1-L3 passing through mesenteric and pelvic plexi
Promotes continence, by inhibiting rectal smooth muscle and contracting internal sphincter
What nerve innervates the external anal sphincter, where does it arise from and what does it do?
Pudendal nerve
Somatic fibres from S2-S4
Control the external anal sphincter
What stimulus initiates defecation? What reflexes does it stimulate?
Distension of the rectum stimulating stretch receptors (15mmHg of pressure)
Short reflex - through myenteric plexus causing increased local peristalsis
Long reflex - through spinal cord stimulation of somatic and parasympathetic fibres innervating anal sphincters (also increased peristalsis in large intestine moving more faecal material into rectum
How does the body deal with it not being socially acceptable to poo in public?
Contraction of external sphincter
— peristaltic contractions diminishes
Mechanism of action of parasympathetic fibres on internal anal sphincter?
Preganglionic fibres release ACh which bind to Muscarinic receptor
Stimulates the release of nitric oxide to act on SMCs of sphincter
NO stimulates guanylyl cyclase to convert GTP to cGMP which activates PKG which causes muscle relaxation
What role do parasympathetic fibres place in bladder continence?
Where do they originate?
Stimulate detrusor smooth muscle contraction
Pelvic nerves S2-S3
What role do parasympathetic fibres place in bladder continence?
Where do they originate?
Relax detrusor smooth muscle (B)
Stimulate urethral internal sphincter contraction (A)
Hypogastric nerve from L2
What role do somatic fibres place in bladder continence?
Where do they originate?
Stimulate external sphincter contraction
Pudendal nerve S2-S4
What stimulates the micturition reflex?
~200ml of urine
What type of drug would you administer for overactive bladder?
Muscarinic antagonist (e.g. oxybutynin)
