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What is the taste we’re most sensitive to?
- taste buds are composed of groups of between 50 & 150 columnar taste receptor cells bundled together
roughly 1 in 4 people is a that is several times more sensitive to bitter & other tastes
What is one type of regulators of flow in the GI tract?
- rope off (compartmentalize) different digestive areas of the GI tract
- they ensure there’s unidirectional movement of digestive material & meter what “moves on” in the digestive process
- they relax as food is swallowed → tighten up again as food passes
Components of the Total Liquid Environment of the GI Tract
- Saliva: secrete ~1 L/day
- Liquid Ingestion: 2.3 L
- Gastric Juices: 2 L
- Bile: 1 L
- Pancreatic juice: 2 L
- Intestinal juice: 1 L
- TOTAL: 9.3 L
- the small intestine (SI) reabsorbs 8.3 L, while the large intestine (colon) reabsorbs 0.9 L
- a total of 9.2 L is reabsorbed, meaning that only 0.1 L (100 mL) is excreted in feces
Why do we need all this fluid?
- it’s important for the digestive process
- digestion would be SLOWED w/o the liquid environment b/c enzymes are more mobile in a fluid environment
- w/o water, they’d get stuck in “sludge” & couldn’t work as efficiently
*What are the 7 sphincters in the GI tract?
- a. Upper Esophageal (UES): top part of the esophagus
- b. Lower Esophageal (LES): end of the esophagus, beginning of stomach
- c. Pyloric: end of the stomach, beginning of duodenum
- d. Oddi: duodenum
- e. Ileocecal: end of ilium, beginning of colon
- f. Internal Anal
- g. Externa Anal
What is a cause of heart burn?
- it can happen because the Lower Esophageal Sphincter (LES) doesn’t constrict fully after food has passed from the esophagus into the stomach
- stomach acid can splash up into the esophagus, causing burning pain
- unlike the esophageal sphincters that allow all swallowed material to pass through, THIS is a metering one
- it can restrict or selectively allow small amounts of material to pass into the SI
- allows stomach digestion to occur fully & limits SI epithelial cell exposure to the stomach’s highly acidic & destructive environment
- (the Ileocecal Sphincter at the entrance to the colon also meters digestive contents)
What is the ONLY sphincter that food does not move through, only goes by?
- the Oddi Sphincter
- it controls the flow of bile (from the liver) & pancreatic enzymes + fluid into the SI
- lies in the duodenum of the SI JUST after the stomach empties into the SI
Over which sphincter do you have some control?
- you have SOME control over the External Anal Sphincter; can relax it during defecation
- the rest of them are all controlled by the autonomic n.s.
Sphincters, Muscle Type, Function
- esophagus has 2
- 3 more in the middle
- anal has 2
Stages of Swallowing
1. tongue rises, closing the oropharynx
2. soft palate rises, shutting off the nasopharynx* so food doesn’t go into/come out your nose
3. hyoid rises & moves forward
4. esophagus anterior wall is drawn forward by the hyoid
5. esophageal lumen is pulled open
- 6. epiglottis tilts backward to shut off the glottis* so food doesn’t go into the airway
Gut & the Autonomic Nervous System
- made up of the enteric, sympathetic, & parasympathetic nervous systems
- SNS stimulation CONTRACTS GI sphincters & BVs and INHIBITS GI secretion & motor activity
- in contrast, PSNS stimulates these digestive activities
Gut Enteric Nervous System
- GUIDES the functions of the GI tract - is the MAJOR controlling factor, even though there are SNS & PSNS innervations as well
- can be seen along the entire GI tract from the esophagus → the colon
Enteric Nervous System Functions
- it controls movement of food along GI tract
- regulates temperature
- sensory regulation of enzyme secretion
- + it senses pH & the concentrations of amino acids, fats, & carbohydrates
What neurotransmitters are secreted by Enteric neurons?
- Acetylcholine (excitatory when food is present): stimulates SM muscle contraction, ↑ intestinal secretions + release of enteric hormones, & dilates BVs
- Norepinephrine: from extrinsic sympathetic neurons is almost always INHIBITORY & has the opposite effect of ACh
- might see in STOMACH after a large meal slowing things down
What are the 2 orientations of smooth muscle in the GI tract?
- circular & longitudinal
- circular muscles SQUEEZE, longitudinal muscles CONTRACT
- intestinal layers, outermost layer → lumen
What are the 2 plexuses that make up the enteric nervous system?
- 1. Myenteric
- 2. Submucous
- both of these are embedded in the digestive tract wall & extend from esophagus → anus (these are the main components of the Enteric NS, control the “brain” of the GI tract)
- is buried in the Submucosa (closer to the lumen)
- its principal role is in sensing the environment within the lumen, regulating gastrointestinal blood flow, & controlling epithelial cell function
- it feeds the information it senses up into the myenteric plexus
- located between the longitudinal & circular layers of muscle in the Tunica Muscularis
- it exerts control primarily over digestive tract MOTILITY
What 3 types of neurons can be found in the Enteric plexuses (submucosal & myenteric)?
1. Motor: control GI motility & secretion
2. Sensory: receive info from sensory receptors in the mucosa & muscle (includes mechanical, thermal, osmotic, & chemical stimuli)
- 3. Interneurons: integrate info from sensory neurons & transmit it to ("programming") enteric Motor neurons
- most of these are multipolar
Where is the only place in the GI tract where there is epithelial stratification?
- in the Esophagus
- everywhere else in the GI tract there is a single layer of epithelial cells that are just like any other epithelium
- there is also Striated Muscle in the 1st 3rd of the esophagus - throughout the rest of the GI tract there’s only smooth muscle
Order of Relaxation/Contraction of GI Tract
- when material moves through the GI tract, the tract in front of it RELAXES (eg. by Nitric Oxide, NO)
- behind the material muscles CONTRACT so the stuff is propelled through/keeps moving forward
- muscle tone relaxation (+NO) happens PRIOR to contraction (+ ACh)
- [usually you think of ACh functioning in the PSNS to relax BVs, but in the gut you have to REVERSE your thinking & remember that the PSNS STIMULATES GI function therefore ACh → CONTRACTION]
How does contraction activity vary in the stomach from the Lower Esophageal Sphincter to the Pyloric valve?
- activity/stomach muscle contractions get MORE intense as food moves down the stomach length-wise
- STOMACHINTES PICTURE
Stomach Contraction Morphology
- Proximal Stomach (near LES): large, lazy contractions
- as you move down the stomach contractions become more NUMEROUS & sharp
- tracing in SI shows a LOT of activity & mixing happening
- stomach shows a progressive increase in the # of contractions seen as you move from its beginning to end
Which leaves the stomach faster, liquids or solids?
- LIQUIDS - they leave the stomach much faster than solids
- eg. to get a lot of energy or electrolyte into your system it will happen more quickly if given in liquid form (takes longer to get into SI & be absorbed if solid)
- takes 30 minutes for 1/2 of liquid to be gone, & takes 90 minutes for 1/2 of solid to leave the stomach
How is movement in the large intestine DIFFERENT from that in the small intestine?
- there are mostly MASS (large volume*) movements in the colon as opposed to the small sharp movements that occur in the SI
- large movements happen 3-5x a day