1-2. overview of GI + regulation; cephalic/oral/esophageal phases

what is the difference between microbiome and microbiota?

• microbiome: refers to microbes + their genomes

• microbiota: refers to just microbes (w/o genomes)

what are enterocytes?

• epithelial cells in intestinal villi

• most abundant cell

• functions in absorption

what are enteroendocrine cells?

• “taste buds” of the gut

• continuously sample lumen

what are paneth cells?

anti-microbial cells found in small intestine crypts

what drives absorption in the small intestine?

Na⁺ (major), H₂O (follows Na⁺), nutrients, ions

what drives secretion in the small intestine?

Cl^- (major), H₂O, ions

what types of regulatory mechanisms are present in the GI tract?

• neural

  • extrinsic

  • intrinsic

• hormonal

  • endocrine

  • paracrine

local & distant communication

what are the major endocrine mediators in the GI tract?

• gastrin

• choleocystokinin (CCK)

• secretin

what are the major paracrine mediators in the GI tract?

• somatostatin

• histamine

what are the main neurotransmitters of the parasympathetic nervous system?

• acetylcholine

• peptides

what are the main neurotransmitters of the sympathetic nervous system?

• norepinephrine

• peptides

cephalic phase

sensory information / anticipation (idea of food) prepares GI tract to begin food processing and digestion without food touching the oral cavity

oral phase

• food contacts the oral cavity and prepares GI tract to being food processing and digestion

• presence of food is the stimulus

what are the functions of saliva?

• lubrication of food to facilitate chewing & swallowing

• antibacterial substances: lactoferrin & lysozyme (disrupt bacterial cell walls); immunoglobulin A (IgA)

• initial enzymatic digestion of food (lipase, amylase; not essential)

• oral hygiene, grooming, thermoregulation

• taste sensation (solubilizes food molecules to facilitate contact with taste buds)

ptyalism

hypersalivation

xerostomia

dry mouth

how is deglutition (swallowing) initiated?

• presence of a bolus in pharynx → sensory nerves send information to brain → output through cranial nerves and vagus n. to open sphincters and initiate peristaltic contractions

• initiation of swallowing is voluntary

• once swallowing is initiated, moves into involuntary phase (peristalsis, sphincter relaxing/contracting)

what is the difference between primary vs. secondary peristaltic contractions?

• primary peristaltic contractions are initiated by swallowing

• secondary peristaltic contractions are initiated by esophageal distention (ex. if food gets stuck in esophagus)

dysphagia

• difficulty in swallowing

  • oral: disorders of tongue

  • pharyngeal: dysfunctional pharyngeal motility (nerves or muscles)

  • esophageal: cricopharyngeal (failure of UES to relax); achalasia (failure of LES to relax)

achalasia

failure to relax in any segment of GI tract, typically lower esophageal sphincter