BIOL 373 Unit 6 pt. 1

Absorption

Transport nutrients, water, ions, vitamins across epithelium

Pathway of absorption

Lumen → epithelium → interstitial fluid → blood

Why do barriers need to exist in the gastrointestinal function? + problems that arise

Aim is to digest macromolecules, but not the macromolecules that make itself up. Problems that arise when barrier is not followed are peptic, and duodenal ulcers

Why is an extensive system of entry required for GI function?

GI lining largest area of contact between internal and external environment.

Protection of pathogens is mediated by (5)

Epithelial barrier, mucus, digestive enzymes, acid, and Gut-Associated Lymphoid Tissue (GALT)

What is GALT?

Gut-Associated Lymphoid Tissue (GALT) “mucosa” - gentler immune system that understands the difference between proteins that need to be digested and pathogen proteins.

What does the GI need to balance

Water input/output - balance between secretion (exocrine) and reabsorption

Stomach gross anatomy

stomach entry from esophagus
pylorus exit to small intestine
characteristic folds (rugae), mucosal surface!

Characteristics of a mucosal surface

Mucosa - consists of epithelial lining, connective tissue (lamina propia, and muscle layer (muscularis mucosae)
Submucosa - on the outside of the muscle layer
Two more layers of smooth muscle (circular and longitudinal)
Serosa - epithelial layer, basically a bag
Neurons go through and in between layers
First neural plexus is submucosal plexus, and second is myenteric plexus (runs through circular and longitudinal muscle)

Stomach microanatomy (differences from typical mucosal surfaces)

In addition to circular and longitudinal muscle, it has diagonal muscle (oblique muscle) because the stomach is a bag rather than a tube and must contract in more dimensions.
Epithelial lined gastric glands - exocrine secretion, secreting out to a surface. Rugae are typical of bag-like organs that need to expand.

Small intestine zones

Duodenum, jejunum, ileum → in that order

Small intestine gross anatomy

Circular and longitudinal muscle, and a thin layer of muscle (mucosa and submucosa)

Has PLICA - folds, unlike rugae, these do not disappear as the intestine expands or shrinks. Intestinal villi.
Peyer’s patch - only evidence of an immune system in the small intestine, very numerous.

3 mechanisms to increase surface area

Intestine (pleca), villus (cross epithelial barrier and picked up by circulatory system or the lacteal), epithelium (at cellular level, microvilli)

4 functions of the Gi tract

Motility, secretion, and digestion

Patterns of gut motility

Peristalsis - moving food from mouth to anus (contract circular muscle to pinch bolus forward)
Segmental contraction - mixing

Single-unit smooth muscle

Gut muscle is single-unit smooth muscle, interconnected by gap junctions, contracts as a single unit, autonomic neuron dumps neurotransmitter and bathes smooth muscle in depolarization.

Two types of gut contraction

Tonic (minutes to hours) - smooth muscle sphincters, keeps food from moving backwards.
Phasic (few seconds):
between meals - migrating motor complexes to sweep tract, stomach to small intestine
during/after meals - peristaltic and segmental contractions

Slow wave potentials

Slow waves creep up but don’t reach threshold, no contraction. If they do, the ultimate amplitude is how long it stays to polarize. Force and duration is influenced by amplitude and frequency of action potentials.
Similar to pacemaker potentials

Process of action potential durign an above threshold slow wave

Opening of voltage-gated Ca channels → action potentials → contraction

Amplitude and duration of contraction is influenced by

neurotransmitters, hormones, and paracrine factors

Slow wave frequency is set by…

pacemaker cells called interstitial cells of Cajal, found between smooth muscle layers.

Does slow wave frequency differ between duodenum and stomach?

Yes duodenum has more frequency than stomach.

How are water and ions secreted into the lumen?

Ions transported via membrane transporters.
Water follows osmotic gradient.
Water and ions can sometimes pass between cells via paracellular routes.

Membrane transporters involved in secretion of ions

Na+/K+-ATPase, NKCC cotransporter, Cl-/HCO3- exchanger, Na+/H+ exchanger (NHE), H+/K+-ATPase

Ion channels involved in secretion of ions

ENac, K+ channels, Cl- channels (including CFTR channels)