2025 Physiology Exam 3 Flashcards
Lectures 12-16: GI
Regulation of GI Physiology
GI peptides
Nerves
Smooth muscle
Gastrointestinal Peptides/Modulators
GI Hormones
Four steps are required to establish existence of GI hormone:
Physiological release
- Effects independent of nervous system
- Isolated substance has physiological effect.
- Chemical identification and synthesis
Gastrin—Distribution and Release
Know… (for each hormone)
Job
Stimulates HCl from cells in the stomach
Where released
Antrum of Stomach - just before pylorus
Duodenum
The Stimuli
Inhibition - acid reaches set point (Negative Feedback Loop)
Gastrin—Physiological Effects
Cholecystokinin
Know its…
Job (on slide 7)
Where Released
Stimuli
CCK—Physiological Effects
Job(s)
CCK—Physiological Effects (Flow Chart)
Secretin
Know
Where Released
Stimuli
Effects
Stops the process of HCl secretion essentially
*** Look into this more past the Gastrin inhibition
Glucose-Dependent Insulinotropic Peptide (GIP)
Stimuli/release
Released from K-cells of duodenum and proximal jejunum
All major foodstuffs—fat must be hydrolyzed.
Oral glucose but not i.v. glucose
Physiological effects
Stimulates insulin release (also called glucose-dependent insulinotrophic peptide—GIP)
Inhibits gastric acid secretion (enterogastrone)
Motilin
Stimuli/release
Released from M-cells of duodenum and proximal jejunum during fasting at 100 min intervals
Release is under neural control (acid and fat can also cause small amounts to be released)
Physiological effects
Stimulates upper GI motility
Accounts for the migrating motility complex, “housekeeping contractions”
Distribution of GI Hormones
Releasers of GI Hormones
Physiological Actions ofGI Hormones
Paracrines
Somatostatin (peptide)
Found in gastric/duodenal mucosa and pancreas
Release—stimulated by acid, inhibited by Ach
Inhibits release of all gut hormones
Directly inhibits parietal cell acid secretion
Mediates acid-induced inhibition of gastrin release
Histamine
Gastrin and Ach cause release from cells in stomach
Stimulates acid secretion.
Histamine H2 receptor blockers lower Acid secretion
Cimetidine (Tagamet), Ranitidine (Zantac)
Enteric Nervous System (ENS)
Neural Control of GI Tract
Enteric Nervous System (ENS) Visual
ENS—Myenteric Plexus
ENS—Submucosal Plexus
Parasympathetic Innervation
Excitatory for GI Function
… so you get Rest and Digest
Come out of Cranium and Sacrum
Long Preganglionic Fibers
Short Postganglionic Fibers
Sympathetic Innervation
Inhibitory for GI Function
Come out of Thoracic and Lumber Regions
Short Preganglionic Fibers
Long Postganglionic Fibers
*** Slide wrong with the Long Pre
Neurotransmitters (Neurocrines)
PNS - Parasympathetic
SNS - Sympathetic
*** Know the length for each between Pre and Post ganglions
Sensory Afferent Neurons
Autonomic Nervous System = EFFERENT MOTOR SYSTEM
Afferent is SENSORY
Gastrointestinal Smooth Muscle
Unitary (single-unit) smooth muscle
Slow waves
Spike potentials
Muscle contractions
Unitary (Single Unit) Smooth Muscle
Syncronized Cells acting like Tissue
Gastrointestinal Movements
Peristalsis
Rhythmic segmentation
Tonic contraction
Propulsive Movements - Peristalsis
Stimuli that initiate peristalsis
Distention - orad contraction with downstream receptive relaxation = “Law of the Gut”
Irritation of gut epithelium
Parasympathetic nervous system
Function
Myenteric plexus required
Atropine (blocks Ach receptors) -peristalsis
Congenital absence of plexus - no peristalsis
Motility
Chewing and swallowing
Esophageal motility
Gastric motility
Small intestinal motility
Large intestinal motility
Chewing (Mastication)
Purpose of chewing
Breaks cells—breaks apart indigestible cellulose
Increases surface area—decreases particle size
Mixes food with saliva
Begins digestion of starches (-amylase, lingual lipase)
Lubricates food for swallowing
Swallowing (Deglutination)
Three stages
Voluntary—initiates swallowing process
Pharyngeal—passage of food through pharynx into esophagus
Esophageal—passage of food from pharnyx to stomach
Nervous Control of Esophageal Phase
By Vagus Nerve
Gastric Motility
pH of stomach is ~1
Regulation of Gastric Emptying
Chyme must enter duodenum at proper rate.
pH must be optimal (~7) for enzyme function (pancreas neutralizes the high stomach acid with bicarbonate)
Slow enough for nutrient absorption.
Immediately after meal—emptying does not occur before onset of gastric contractions.
Small Intestinal Motility
Small intestinal motility contributes to digestion and absorption by:
Mixing chyme—With digestive enzymes and other secretions
Circulation of chyme—To achieve optimal exposure to mucosa
Propulsion of chyme—In an aboral direction
Control of Small Intestinal Motility
Whether spike potentials and hence contractions occur depends upon neural and hormonal input.
Nervous factors (PNS—stimulates/SNS—inhibits)
Peristaltic reflex (Law of the Gut)—Mediated by ENS
Intestino-intestinal reflex—Severe distention inhibits bowel Extrinsic nerves
Gastroileal reflex—Meal stimulates. Ileocecal sphincter relaxes, ileal peristalsis increases. (gastrin, CCK, extrinsic nerves, ??).
Ileocecal Junction
Absorption and Storage Function
Job is the absorption of water
Intrinsic Defecation Reflex
Rectal distention initiates afferent signals that spread through myenteric plexus to descending and sigmoid colon, and rectum. This causes contractions that force feces toward anus.
Control of Secretions
Daily Secretion of Intestinal Juices
Mucus Composition—Properties
Buffering - neutralize an acid
Saliva
lipase breaks down fats
Other one???
Majority the Parotid
Functions of Saliva
Lubrication and binding
Solubilizes dry food
Initiates starch digestion
Oral hygiene: Flow of saliva decreases during sleep allowing bacteria to build up in mouth
Functions of Stomach
Short-term storage reservoir
Secretion of intrinsic factor
Chemical and enzymatic digestion is initiated, particularly of proteins (proteins only)
Liquefaction of food
Slowly released into the small intestine for further processing.
Gastric Secretions
Gastric (Oxyntic) Gland
Pyloric Gland
Gastric Acid
Three major functions:
Bacteriostatic
Converts pepsinogen to pepsin
Begins protein digestion (with pepsin)
Pepsinogen
Pepsinogen is an inactive, secreted form of pepsin .
Acid converts pepsinogen to pepsin.
Pepsin (35 kDa) converts more pepsinogen to pepsin.
- Proteolytic enzyme
- Optimal pH 1.8–3.5
- Reversibly inactivated >pH 5.0
- Irreversibly inactivated >pH 7–8
Pepsinogen Secretion
Two signals stimulate secretion of pepsinogen.
Vagal stimulation as mediated by acetylcholine
Direct response to gastric acid
Rennin (Chymosin)
Proteolytic enzyme–Causes milk to curdle in stomach
Milk retained in stomach and released more slowly
Rennin secretion–Maximal first few days after birth. Replaced by secretion of pepsin as major gastric protease
Secreted as inactive proenzyme (prochymosin) that is activated on exposure to acid
Gastric Intrinsic Factor
Regulation of Gastric Secretion
Role of Vagus in Gastric Secretion
Phases of Gastric Secretion
What Is the Gastric Mucosal Barrier?
Integrity of Mucosal Barrier
Pancreas
Youtube this more!!!
Enzymes for Protein Digestion
Enzymes for Carbohydrate Digestion
Why Doesn’t the Pancreas Digest Itself?
Only active once it leaves the Pancreas
Bicarbonate Neutralizes Acid Chyme
Distribution of Secretin
Helps with enzyme Release???
Secretions of Small Intestine
Secretions of Large Intestine
Large intestine also contains crypts of Lieberkühn but there are no villi or enzymes.
Crypts mainly secrete alkaline mucus
Mucus secretion increased by parasympathetic stimulation
Liver Secretion and Gallbladder Emptying
Basis for Digestion—Hydrolysis
Digestion involves the breakdown or hydrolysis (addition of water) of nutrients to smaller molecules that can be absorbed in small intestine.
Carbohydrates—Monosaccharides
Proteins—Small peptides and amino acids
Fats—2-monoglycerides and fatty acids
Types of Digestion
Luminal or cavital digestion:
Occurs in lumen of GI tract
Enzymes from salivary glands, stomach, pancreas
Pancreatic enzymes can do all EXCEPT …
Membrane or contact digestion:
Enzymes on brush border of enterocytes
Digestive Enzymes
Anatomical Basis for Absorption
Sites of Absorption
Digestion of Carbohydrates
Starch digestion:
Begins with a-amylase in saliva (5% digestion in mouth, up to 40% in stomach)
Continues in small intestine with pancreatic amylase
Final digestion occurs at brush border.
Lactose and sucrose—Digestion only occurs at brush border.
Digestion of Proteins
Digestion of Proteins Flow Chart
Assimilation of Lipids– Overall Scheme
Large Lipids to Lymph System through the Lacteals
Chylomicrons—Life Cycle
What Exactly Is Dietary Fiber?
Fluid Entering and Exiting the Gut
Water Movement in Small Intestine
Water moves into or out of gut lumen by diffusion in accordance with osmotic forces.
Hypotonic chyme—Water is absorbed
Hypertonic chyme—Water enters intestine
Chyme is isotonic.
