GI motility

Question 1

Swallowing

Answer 1

• Primary mediators are mouth and pharynx, sucking/chewing are first step of digestion (voluntary)
• Swallowing is coordinated by medulla (swallowing reflex)
• 3 phases of swallowing: buccal, pharyngeal, esophageal
• Buccal phase: voluntary (oral cavity muscles), tongue forces bolus of food toward pharynx
• Pharyngeal phase: involuntary (muscles in pharynx), soft palate and uvula fold upward to cover nasopharynx, epiglottis folds over larynx
• Esophageal phase: involuntary (esophageal muscles), upper esophageal sphincter (near pharynx) opens, lower esophageal sphincter opens when food reaches it

Question 2

Esophagus in swallowing

Answer 2

• Opening of UES initiates the peristalsis
• Primary peristalsis (during swallowing): moves food down to keep esophagus empty
• Secondary peristalsis: to keep reflux in the stomach (associated w/ esophageal distension)
• LES relaxes 1-2 sec after swallowing and remains open for 6-8 sec
• Esophageal resting tone is stimulated by Ach from ENS, swallow-induced relaxation is via vagal inhibition of the ENS

Question 3

Stomach motility 1

Answer 3

• Receptive relaxation of foods (to receive food) is initiated by swallowing via vagal inhibition of motor neurons in ENS
• Peristalsis in stomach is initiated by interstitial cells of cajal (ICC, pacemaker cells)
• Mild peristalsis in fundus/body to propel food to antrum, where stronger peristalsis breaks down the food

Question 4

Stomach motility 2

Answer 4

• ICCs: frequency of contraction is determined by the frequency of AP bursts (“slow waves”), amplitude of contraction is determined by number of APs in each burst + neural/hormonal input
• Factors affecting ICC control over peristalsis: other neural control (ENS), hormones
• ENS signals and hormones slowdown delivery of stomach contents to duodenum, allowing for adequate digestion and absorption time in duodenum

Question 5

Motility patterns of the SI 1

Answer 5

• Fed patten (digestive state): segmentation waves (non-propagative) of low-moderate amplitude (frequency set by ICC)
• Serves to mix nutrients, does not occlude lumen, lasts 2-4 hrs post prandial
• Migrating motor complex (MCC): cyclical activity of propagative waves (3 phases)
• Keeps material moving in between meals to prevent bacterial buildup

Question 6

Motility patterns of the SI 2

Answer 6

• Phase 1 (inter digestive state): resting phase (to recover strength), quiescent to infrequent low amplitude non-propagative contraction
• Phase 2: increasing activity, similar to fed state peristalsis but lasts only 30-60 min
• Phase 3 (clustered contractions): highest amplitude of SI contractions (10-20 min duration every 1-1.5 hr), propagating and propulsive house keeping contractions that are inhibited by fed state

Question 7

Colonic motility

Answer 7

• Also has both segmental activity (mixing, non-propagative) and propagative activity
• Segmental activity: majority of daily motor activity (increases w/ age)
• Low amplitude propagative contractions (LAPCs): poorly studied, but are propagative (60x/day)
• Believed to help transport liquid and gaseous colonic contents
• High amplitude propagative contractions (HAPCs): infrequent (0-6x/day) but are seen more in children and less in adults
• Associated w/ defecatory stimulus, diurnal and nocturnal patters directly related to physiologic events (sleep, activity, eating)
• HAPCs are exhibited most during waking hours, almost all activity is absent during sleep

Question 8

Mass movement of the colon

Answer 8

• The presence of food in stomach and chyme in duodenum stimulates mass movements (HAPCs?) in colon
• Mass movements are integrated by the enteric plexus, and propel the contents of the colon toward the rectum
• The presence of feces in rectum stimulates PsNS and local reflexes that result in defecation

Question 9

Gastrocolic response

Answer 9

• Reproducible physiologic response of colonic activity w/in 1-3 min following a meal
• More active at younger age
• Chiefly segmental contractions, but LAPCs and HAPCs also increase
• Post prandial colonic motility is influenced by caloric content (more calories more movement) and meal composition (more movement from fats/carbs, less from protein)
• Cephalic phase of digestion also stimulates colonic activity

Question 10

Rectal motor complex

Answer 10

• Independent from SI and colonic activity

- Rise in anal canal pressure, to avoid rectal stasis and preserve nocturnal fecal content (esp active during sleep)

Question 11

Defectation

Answer 11

• 2 phases: involuntary and voluntary
• Involuntary: colonic contents transported to rectum, increased rectum dissension and pressure, relaxation of internal anal sphincter
• Voluntary: increased intra-ab pressure, pelvic floor muscles relax and descent (straightening of anorectal angle), relaxation of external anal sphincter
• Increase of HAPCs often precedes defectation

Question 12

Other anal reflexes

Answer 12

• Anal cough reflex: EAS contracts w/ coughing, sneezing, standing
• Cutaneoanal reflex: afferent and efferent reflex arch using pudenedal nerves (S4) to tighten EAS when cutaneous stimulation is applied