• List the 5 GI fxns

• Which is the most important?

1. Digestion: physical (chewing) & chemical (salivary enzymes) processing of food
2. *Absorption: Net movement from gut lumen to body
3. Secretion: lubricate & solubilize food
4. Motility: mass movement & mixing of food
5. Elimination: of non-digestible or toxic gut contents

* most important

• the GI tract is a muscular tube w/ an __ at the interface btw the lumen & the tissue
• High local blood flow is used to sweep away __ & provide substrates for secretion

• the GI tract is a muscular tube w/ an epithelial layer at the interface btw the lumen & the tissue
• High local blood flow* is used to sweep away absorbed material & provide substrates for secretion

*the need of a para symp system

• swallowing & __ moves food through the UES & LES under coordination by __

• swallowing & peristalsis moves food through the UES & LES under coordination by CNS

*area = upper esophagus

** if you have a nerve problem won't be able to swallow

• the stomach is a __ organ
• it uses 2 strong reagents: __ & __
• food particles no bigger than __ can pass through the pyloric sphincter in order for this to happen you need the 2 reagents above and this process __
• the stomach also secretes __ which aids in the absorption of this vitamin

• the stomach is a digestive* organ
• it uses 2 strong reagents: HCL & pepsin
• food particles no bigger than 1mm can pass through the pyloric sphincter; in order for this to happen you need the 2 reagents above and antral peristalsis
• the stomach also secretes intrinsic factor** which aids in the absorption of Vit B12 (cobalamin)

*not absorptive

**after gastric bypass, person requires I.F. to absorb vit B12

• In which part of the sm. intestine is gastric acid neutralized in the lumen by HCO₃^-?
• From where is HCO₃^- secreted?
• What happens if we didn't have HCO₃^- secreted?

• the duodenum is the part of the sm. intestine where gastric acid is neutralized in the lumen by HCO₃^-
• HCO₃^- is secreted from the duodenum & the pancreas
• If we didn't have HCO₃^- secreted we'd likely die of dehydration

• Does digestion continue in the duodenum?
• This process happens for the 1st time due to this portion of the gut being leaky

• Yes, digestion continues in the duodenum due to pancreatic enzymes & surface enzymes in the enterocytes that line the GI tract
• Osmolarity* of the gut contents rapidly adjust H₂O content in this leaky portion of the gut

*gain or lose H₂O

• What part of the GI tract absorbs the majority of salts, H₂O, aas & CH₂Os?
•  Can the sm. intestine absorb fat w/o help?
  
  • If no, what does it need?
• Does the body need to continuously make bile salts?

• The jejunum & ileum absorb the majority of salts, H₂O, aas & CH₂Os?
• No, to absorb fat the liver & gallbladder are recruited for synthesis, storage & secretion of bile salts
• No, the majority of luminal bile salts are taken up at the terminal ileum for use again later

• Is the body done w/ absorption by the time chyme reaches the colon? 
• Non-absorbed protein + CH₂O → SCFAs (for absorption); what aids the conversion?

• No, when chyme reaches the colon salt & H₂O must still be conserved (i.e. body's requirements for NaCl & H₂O > what is absorbed by sm. intestine
• *Non-absorbed protein + CH₂O → SCFAs (for absorption); bacteria aids the conversion

*also absorption of vit K (aldosterone sensitive)

• Name the 3 levels of ctrl for GI fxn

1. Coordination by hormones
2. Ctrl of motility by the Enteric Nervous System (ENS = intrinsic)
3. CNS ctrl = extrinsic

Nervous system ctrl is the major regulator of gut motility

ENS

• Which one is btw the outer longitudinal & middle circular m. layers?
• Which one is btw the middle circular layer & the mucosa?
• What are the 4 types of neurons that these plexi contain?

• Myenteric (Auerbach's) plexus is btw the outer longitudinal & middle circular m. layers
• Submucous (Meissner's) plexus is btw the middle circular layer & the mucosa

1. motor neurons → smooth m.
2. secretory neurons → endocrine & exocrine secretion in the mucosa
3. sensory neurons
4. interneurons

*10^⁸ neurons never leave the GI tract

• What distinguishes the proximal & distal ends of the GI tract from the middle portion?
• ANS = __ + __

• The proximal & distal ends have striated (skeletal) m. w/ somatic (voluntary) innervation vs. smooth m. w/ autonomic innervation
• ANS = ENS + CNS

Innervation

• PNS = __ + pelvic nn. 
• SNS synapses w/__ & post-syn end at __

• PNS = vagus n. + pelvic nn. [preganglionic nn. synapse w/ ENS]
• SNS synapses w/prevertebral ganglia (celiac, SM, IM) & post-syn end at ENS

• *PNS & SNS can also directly synapse on muscle or epithelial cells

Reflex arcs

• SNS & PNS have __ neurons that project to the brain & SC
• What do these neurons provide?
• __ monitor distention or contraction
• __ monitor the composition of intestinal contents

• SNS & PNS have afferent (sensory) neurons that project to the brain & SC
• These neurons provide sensory input in reflex arcs
  
  • PNS: vago-vagal → receptive relaxation [to vagus n.]
  • SNS: enteroenteric → intestino-intestinal  [to celiac plexus]
  • ENS → peristalsis
• mechanoreceptors monitor distention or contraction
• chemoreceptors monitor the composition of intestinal contents

Motility Patterns

1. __ major mode of unidirectional propulsion
2. __ promotes mixing & can limit transit
3. __ blocks transit
4. __ provides for storage

1. peristalsis: major mode of unidirectional propulsion
   1. sites: esophagus, stomach, sm. & lg. intestine
   2. stomach only: non-propulsive, causes mixing
2. segmental contraction promotes mixing & can limit transit
   1. sites: sm. & lg. intestine
3. tonic contraction: blocks transit
   1. sites: GI sphincters
4. Inhibition of contraction (compliance): provides for storage

Sphincters

• are composed of this muscle type & use this motility pattern
• what type(s) of stimulation cause them to open?
• list the sphinters from top to bottom

• sphincters are composed of circular m. that is tonically contracted
• the sphincters open in response to neural & hormonal stimulation
• UES, LES, pyloric, sphincter of Oddi (flow from common bile duct), ileo-cecal, int & ext anal

• T or F, the entire process of swallowing is voluntary?
• the __ relaxes & mm of the pharynx contract starting the __ wave

• F, initiation of swallowing is voluntary but once past the pharynx it's involuntary (coordinated via swallowing ctr in medulla)
• the UES (inhibitory NT) relaxes & mm of the pharynx contract starting the peristaltic wave

Esophageal transit

• What promotes transit in the esophagus?
• What does tonic contraction of the UES prevent?
• the LES?
• How is smooth m. in the esoph different from the rest of the GI tract?

• Peristalsis is the only esophageal motility pattern coordinated w/ tonically contracted Eso Sphincters to promote transit
• Tonic contraction of the UES prevents air flow into the esophagus during respiration
• Tonic contraction of the LES prevents reflux of acid from the stomach into the esophagus
• Esophageal smooth m. has no spontaneous contraction (i.e. it needs a stimulus)

*esophagus: top 1/3 striated m.; bottom 2/3 smooth m.

• When the excitatory interneuron is on what happens in the receiving segment?
• When the inhibitory interneuron is on what happens in the propulsive segment?

• Excitatory interneuron is on: longitudinal m. is contracted (excitary motor n. is on) & circular m. is relaxed (inhibitory motor neuron is on)
• Inhibitory interneuron is on: Longitudinal m. is relaxed (off) & circular m. is contracted (off)

• In the lower part of the esophagus, the LES relaxes such that a __ is tightly formed around the food
• Pt presents w/ esophageal blockage, caused by tight LES & weak peristalsis, dx?

• In the lower part of the esophagus, the LES relaxes such that a vestibule is tightly formed around the food
• Achalasia: esophageal blockage, tight LES, weak peristalsis

• smooth m. cells of the gut spontaneously contract; they depolarize cyclically emitting oscillating currents called __
• sometimes spikes aka __ are imposed on the depolarization phase of slow waves & produce contractions

• smooth m. cells of the gut spontaneously contract; they depolarize cyclically emitting oscillating currents called slow waves (propagate down the canal)
• sometimes spikes aka action potentials are imposed on the depolarization phase of slow waves & produce contractions

Gastric Physical Digestion

• What 3 things are going on in the stomach during digestion?

1. the proximal corpus & fundus relax during feeding to allow ↑ reservoir capacity
2. the antrum provides a grinding & mixing fxn via antral peristalsis against tonically contracted pyloric sphincter
3. motility ctrlled by mechanoreceptors & ENS (intrinsic) innervation

• What anatomical feature prevents particles >1mm in dia from leaving the stomach?

• the pyloric sphincter prevents particles >1mm in dia from leaving the stomach

*Dextrose will have a M-M curve, liver will take longer, plastic spheres will never empty on a % emptied (y-axis) vs. hours (x-axis) graph

• What are the 3 phases of muscle contraction for the fasting motility pattern?
• Is Phase III regulated or random?
• What hormone is involved?

1. Phase I: quiescent (no contraction)
2. Phase II: random contractions & spike activity
3. Phase III: migrating myoelectric complex (MMC); muscle contractions begin in antrum, propogate, marked electrical spike activity

• regulated: sweeps residual material out of sm. intestine every 90 minutes
• MMC correlates w/ peak blood levels of hormone motilin

• How is the fed pattern different from the fasting pattern?
• What is purpose of the ileocecal sphincter?
• Can it be stimlated from both sides?

• the fed pattern consists of irregular spike & motor activity induced by meals → segmental contractions mixed w/ short peristaltic waves (promotes mixing & slow transit of intestinal contents)
• the ileocecal sphincter preserves unidirectional emptying from the ileum to the cecum & restricts reflux of colonic bacteria into the ileum
• No, the ileocecal sphincter can only be stimulated from the proximal side

Colon

• Segmentation (haustral contractions) impedes transit & facilitates __
• What causes evacuation or emptying of the colon?

• Segmentation (haustral contractions) impedes transit & facilitates absorption
• Evacuation is caused by high amplitude peristaltic waves → mass propulsion [1-3x/day postprandial, prior to defecation, heard w/ stethoscope]

Anal canal

• IAS - smooth m & autonomic
• EAS - striated & somatic
• Tonic contraction of __ preserves continence
• Distention of the rectum by intraluminal content triggers this reflex
• What purpose does this reflex serve?
• What is the role of the sensory receptors?

• Tonic contraction of IAS preserves continence
• the rectoanal inhibitory reflex is  a reflex relaxation of the IAS & simultaneous contraction of EAS
• this reflex serves to preserve continence while providing brief access of rectal contents to the sensory nn. in the anal canal
• Sensory receptors in the anal canal discriminate if luminal contents are solids, liquids, or gases (ok to fart)

*Further contraction of EAS can inhibit more proximal colonic motility until evacuation becomes convenient