Motility Of The SI And LI Flashcards
What is small intestinal bacterial overgrowth (SIBO)?
A condition of colonic bacteria over abundance in the SI
MMC is important for cleansing mechanisms in the SI of debris and prevention of SIBO
Bacterial overgrowth can also disturb small bowel motility, causing nausea, anorexia and bloating
What are the digestive and absorptive functions of the SI?
Mix the chyme with digestive enzymes and pancreatic secretions
Expose nutrients to the intestinal mucosa for absorption
Propel the unabsorbed chyme along the SI to the LI
What are the two types of contractions that occur in the SI?
Segmentation and peristaltic contractions
What are segmentation contractions?
Generates back and forth movements to mix chyme but produces no forward propulsive movement along the SI
Contraction splits the chyme sending it in both orad and caudad directions and relaxation allows them to merge back together
What are peristaltic contractions?
Propel chyme along the SI toward the LI
Circular and longitudinal muscles work in opposition to complement each other’s actions (one contracts and the other relaxes)
Are reciprocally innervated
Describe the electrical activity of the ICCs and smooth muscle cells in the SI
Slow wave activity is always present whether contractions are occurring or not
Slow waves themselves do not initiate contractions in the SI (unlike in the stomach)
Spike potentials (AP) are necessary for muscle contraction to occur
Slow wave frequency sets the maximum frequency of contractions
What is the slow wave frequency gradient in the small intestine?
Duodenum 12 cycles/min
Jejunum 10 cycles/min
Ileum 8 cycles/min
Describe the regulation of peristaltic contractions in the SI
Chemical/mechanical stimulation of the mucosa is detected by ECL of the intestinal mucosa which releases serotonin (5HT) —> 5HT binds to receptors on IPANs which initiate the peristaltic reflex in that segment of SI
Behind the food bolus excitatory transmitters (ACh, substance P, NPY) are released in circular muscle and and inhibited in longitudinal muscle—> this segment narrows and lengthens
In front of the bolus inhibitory pathways (VIP, NO) are activated in circular muscle while excitatory pathways are activated in longitudinal muscle —> segment of SI widens and shortens
The myenteric plexus mainly regulates what in the SI?
The relaxation and contraction of the intestinal wall
The submucosal plexus senses what in the SI?
The lumen environment
Describe the neural input that controlled contractions of the intestines
Peristaltic reflex mediated by ENS
In general parasympathetic NS stimulates and sympathetic NS inhibits contractions
Describe the hormonal control of contractions of the intestines
Serotonin stimulates contractions, certain prostaglandins can stimulate contractions,
Epinephrine released from adrenal glands inhibits contractions
Gastrin, CCK, motilin and insulin tend to stimulate contractions
Secretin and glucagon tend to inhibit contractions
Describe the vomiting reflex
Coordinated by the medulla
Nerve impulses are transmitted by vagus and sympathetic afferents to multiple brain stem nuclei
What are the events of the vomiting reflex?
Reverse peristalsis in the SI —> stomach and pylorus relaxation —> forced inspiration to increase abdominal pressure -> movement of the larynx -> LES relaxation -> glottis closes —> forceful expulsion of gastric contents
What allows for flow of contents from the ileum into the colon?
Distention of the ileum causing relaxation of the ileocecal sphincter/junction
What prevents passage of contents from the colon to ileum?
Distention of the colon causing contraction of the ileocecal sphincter
What are the muscle layers of the LI?
Longitudinal which contain teniae coli (3 flat bands of longitudinal fibers that run from cecum to rectum)
Circular fibers which are continuous from the cecum to the anal canal
What is the internal anal sphincter composed of?
Smooth muscle
What is the external anal sphincter composed of?
Striated muscle
What are haustra?
Small pouches that give the LI its segmented appearance
Not fixed, appear and disappear
What innervates the internal anal sphincter?
Pelvic splanchnic N
What innervates the external anal sphincter?
Somatic pudenadl nerves
Describe the parasympathetic innervation of the LI
Vagus nerve: cecum, ascending and transverse colon Pelvic nerves(sacral portion of the SC (S-4)): descending and sigmoid colon, rectum
Describe the sympathetic innervation of the LI (T10-12)
Superior mesenteric ganglion (proximal regions)
Inferior mesenteric ganglion (distal regions)
Hypogastric plexus (distal rectum and anal canal)
What are mass movements?
Motility occurring in the colon over large distances
1-3 times/day
Stimulate defecation reflex
A final mass movement propels the fecal content into the rectum
Motility in the large intestine is key for what?
The absorption of water and vitamins + the conversion of digested food into feces
Poor motility in the LI causes what?
Greater absorption and hard feces in transverse colon cause constipation
Excess motility in the LI causes what?
Less absorption and diarrhea or loose feces
Describe motility of the rectum and anal canal
Rectum fills intermittently (mass movements + segmentation contractions)
As it fills with feces SM wall of the rectum contracts and internal anal sphincter relaxes (rectosphinteric reflex)
The external anal sphincter is tonically closed (under voluntary control)
Describe the neural control of the rectosphinteric reflex and defecation
Controlled partially by the ENS
Reinforced by activity of neurons within the SC
What causes a loss of voluntary control of defecation?
Sensation of rectal distention and voluntary control of the external AS are mediated by pathways within the SC that lead to the cerebral cortex
Destruction of these pathways causes a loss of voluntary control of defection
What is the cause of Hirschsprung’s disease?
Ganglion cells absent from segment of colon
What is the result of a loss of ganglion cells in Hirschsprung’s disease?
VIP levels low —> SM constriction/loss of coordinated movement —> colon contents accumulate (colon equivalent of achalasia)
Present at brith and it causes difficulty in passing stool (congenital megacolon)
What is the presentation of Hirschsprung’s disease?
In an affected newborn it is characterized by failure to pass meconium
Other sx in newborn/infant include poor feeding, jaundice and vomiting
In order children sx include constipation, swollen belly and malnutrition
What is the Tx for Hirschsprung’s disease?
Surgical resection of colon segment lacking ganglia
What is the vago-vagal reflex?
Generally stimulatory
Increases motility, secretomotor, vasodilator activities
Vagus carries both afferents(75%) and efferent (25%)
What is the intestino-intestine reflex?
Depends on extrinsic neural connections
Inhibitory
If an area of the bowel is grossly distended contractile activity in the rest of the bowel is inhibited
What is the gastroileal reflex (gastroenteric)?
Gastric distention relaxes ileocecal sphincter
What is the gastro and duodeno-colic reflex?
Distention of stomach/duodenum initiates mass movements
Transmitted by way of the ANS
What is the defecation reflex (rectosphinteric)?
Rectal distention initiates defecation
When the rectum is distended by feces the internal sphincter relaxes
