Pancreatic & Salivary Gland function

• These study notes outline the autonomic and hormonal regulation of secretion from the gut, the composition and volume of alimentary secretions, the digestion and absorption of macronutrients, gastric motility and emptying and more

Enteric Nervous System

• Intrinsic innervation of GI tract allows autonomous function, despite CNS connection via ANS fibers
• The myenteric plexus (Auerbach's plexus), located between muscle layers, controls GI motility by innervating the muscle layers
• The submucous plexus (Meissner's plexus), located between the middle circular layer and the mucosa, controls GI secretions by innervating glandular epithelium, intestinal endocrine cells, and submucosal blood vessels
• Utilizes NTs like ACh, NAd, 5-hT, GABA, ATP, NO, CO along with various peptides

Autonomic Nervous System

• "Extrinsic" innervation of GI tract occurs through the PNS and SNS fibers
• PNS fibers (ACh) increase gut motility, sphincter relaxation, and GI secretions while originating from the sacrum that synapses onto fibers of the ENS
• SNS fibers (NAd) typically synapse onto cholinergic PNS fibers, inhibiting them presynaptically, and some terminate on GI smooth muscle, decreasing GI motility and blood vessels, causing vasoconstriction

Enteroendocrine System

• Biologically active peptides, secreted by nerve and gland cells ("Enteroendocrine cells") in the GI mucosa, act in a paracrine manner and/or enter the systemic circulation to regulate GI secretion and motility
• The system's two major peptide families include the Gastrin and CCK family, and the Secretin family, which includes secretin, enteroglucagon, GIP, VIP The other peptides include motilin, somatostatin, GRP, histamine, substance P, neurotensin

Hormone Specific Notes

• Secretin, sourced from S-cells in the duodenal mucosa acts as a peptide hormone, is released by acidic chyme and FA in the duodenum, and produces an alkaline intestinal environment by increasing HCO3- rich watery secretion and decreasing gastric acid secretion.
• CCK, sourced from I-cells in the duodenal mucosa and stimulated by FA/MAG and a.a./peptides, increases biliary and pancreatic enzyme secretion, decrease gastric acid secretion, and increases duodenal secretion of enterokinase Gastrin, released in response to gastric distension, peptides/a.a. in the stomach, and GRP, increases gastric HCl and pepsinogen secretion, has a +ve trophic effect on mucosa and increases motility

Further regulators

• Somatostatin, sourced from D-cells in the gastric gland released in response to gastric pH and other hormones, and inhibits most GI hormones, gastric/pancreatic secretion, emptying rate, gallbladder contraction, and intestinal nutrient absorption
• GRP, sourced from Vagal nerve endings, is released by vagal outflow, causes ↑ gastric acid secretion due to ↑ gastrin release
• GIP, sourced from K-cells in the duodenal mucosa that is released in response to glucose and FA in the duodenal chyme, reduces gastric emptying and tone while increasing somatostatin and insulin secretion
• VIP, sourced from ANS and ENS nerves, and released in response to distension of chyme, increases intestinal water/electrolyte and biliary/pancreatic secretions while decreasing gastric acid secretion/emptying/tone and inducing peripheral vasodilation

Saliva

• Three major pairs of salivary glands including the parotid (serous secretion), sublingual (mucous secretions) and submandibular (mixed serous-mucous secretions)
• Each gland consists of three types of acini (serous, mucous, mixed) that open into intercalated and striated ducts which then empty into excretory ducts

Functions of Saliva

• Functions as lubricant to soften food and aid taste sensation
• Aids in the chemical digestion of starch (salivary amylase) and fats (salivary lipase)
• Oral hygiene
• Maintenance of teeth and oral mucosa through mechanical washing, buffering pH changes, and providing growth factors

Saliva Content

• Contains H₂O and electrolytes at 99% and proteins at 1%, and has a basal rate of 0.5 mL/min which can increase to 5 mL/min with stimulation
• It is slightly hypotonic with ↑ K+ (15 mmol/L) and ↑ HCO3 (50 mmol/L), and ↓ Na⁺ (50 mmol/L) and ↓ Cl (15 mmol/L) compared to plasma
• Salivary content, tonicity, and pH vary with secretory flow rate; increased salivary rate increases sodium, chloride and basic pH Decreased salivary rate leads to an increase in potassium and acidic pH, which will require more time for modification

Saliva Regulation and Content

• Contains digestive proteins, immunological proteins, mucin, and growth factors, and it is regulated through the ANS where the PNS and SNS release large and small amounts of saliva, respectively with varying mucous content
• Salivary secretion induced by ANS occurs at cephalic, oral, oesophageal, gastric and intestinal phases of digestion

Gastric Juice

• Contains H₂O and electrolytes at >99.5%, solid material at < 0.5%, which includes digestive enzymes, mucous in alkaline fluid, intrinsic factor
• The volume of gastric juice typically produced is at a rate of 2 - 2.5 L/day

Gastric Juice Characteristics

• Slightly hyperosmotic with high H⁺ (150-170 mmol/L), Cľ (190 mmol/L), K⁺ (10 mmol/L), and low Na⁺ (2-4 mmol/L) compared to plasma
• Varies based on flow rate, as lower secretion rates increase/decrease sodium and potassium content

Phases of Gastric Juice Secretion

• A 50% secretion is in the cephalic phase which is initiated by thought, sight, taste and smell of food and is mediated via vagal outflow
• A 50% secretion is in the gastric phase, intitiated by entry of food into stomach, and mediated by local/vago that release Gastrin, increasing gastric acid secretion
• A <1% secretion is in the intestinal phase intitiated by chyme entering the duodenum with the help of neurotransmitters

Gastric Juice Content - HCl

• Parietal oxyntic cells in the Fundus and Body of the Stomach are the Source
• Functions of HCl, creates an acidic gastric luminal environment for protein digestion, defense against micro-organisms, facilitates iron absorption in duodenum and stimulates biliiary/pancreatic juice

Increasing Stomach HCl

• Factors increasing HCl secretion include ACh from CN X outflow, secretin from G-cells due to gastric distension and peptides, histamine, secretin from EL cells in response to gastrin
• Phases that increase HCl include the Cephalic phase, Gastric phase, and Intestinal phase in combination
• The source - cheif peptic cells located in at the base of the gastric glands Pepsinogen, the source itself, undergoes autocatalytic cleavage to become pepsin and secretes digestive-enzymes in response to the gastric pH It is regulated through several different phases and secretions

Intrinsic Factor(IF)

• Intrinsic factor - Produced by parietal (oxyntic) cells in the fundus and body of the stomach, and is secreted under stimulatory conditions(same ad when stimulating HCl production)
• Glycoprotein that facilitates Vitamin B12 (cobalamin) absorption, in particular B12 with R-Proteins

Alkaline-Rich Mucous Fluid

• Source - Mucous cells within mucous glands:
  • Mucous and water that produce viscose with help of alkaline
  • Creates the gastric mucosal barrier to prevent destruction by HCl

Gastric Lipase and Amylase

• Source - Cheif (peptic) cells in the body and fundus which help digestion of fats and CHO

Other Stomach Hormones

• G-cells are the source for gastrin, and aid in the stimulation of parietal cells to secrete HCl
• Enterochromaffin (ECL) cells source histamine
• Regulation of secretion – Histamine is released by ECL degranulation in response to → (i) Castrin and (ii) vaga, where the source is the stomach itself D-cells, inhibit the gastric acid secretion, source is located in gastric cells

Pancreatic Juice

• Exocrine secretions of pancreas, acinar and ductal cells
• 1.5L/day - volume
• Organic component: digestive enzymes
• Inorganic component: Electrolytes and H2O

Functions of Pancreatic Juice

• Digestion, major source of digestive enzymes that helps digest food(Carbs, Protiens, etc.)
• Neutralizes the PH created by HCL by increasing alkalinity which is necessary for digestion

Pancreatic Secretion

• Acinar Cells: Synthesis of organic component
• Ductal Cells: Help secrete inorganic components

Control of Pancreatic Secretions

• Cephalic Phase: Initiated when thinking, tasting, smelling, etc. This phase helps innervate directly by the Acinar Cells
• Gastric Phase: Stimulated by food in stomach and helps G Cells create acinar cell secretions
• Intestinal Phases: Triggerred by Chyme in the Duodenum increasing secretin and CCK and releases acinar stimulation

Biliary Secretion

• Liver : Produces bile, creates and mixes the concentrate
• Bile : Independent 97%, 3% dependent, consists largely of elecrtrolytes, such as H2O

Secretion volume

• Around 1 - 0.2 L perday and help contract when you eat foods, like fatty meals or any type of meal eaten really

Galbladder Functions

• Concentrate Bile: Absorb H2O
• Store the concentrated bile
• Lowers alkalinity to help aid with digestion
• Secretes mucus: promotes intestinal flow

The Bile itself

• Biles roles are - Enhancing digestion, to excrute the bad stuff, neturalize ph, and to reduce the likelyhood of any issues

Contol of Bile

• Increase bile with increase with S-Cell in smaller intestine with amino acids

Small Intestine Secretions

• 2. Liters a Day
• Crypt of liebkhum, loacted in the the walls of the small intestine and create various secretions
• Goblet Cells: Help lubricate the intestine with a mucus with help with digestion

SI - Regulation of Secretion

• Control via, local stimuli, controls, and other stimuli's, that help innervate it, increasing and promoting the lubrication needed

Digestion - CHO

• The GI system digests starch, which is plants that have branched glucose, glycogen that have glucose in them themselves
• The mouth will create amylase, so if the food doesn't get digested in that small amount of time it will continue to be digested a little when it reaches the intestines

CARB Absorption

• Glucose through a small amount of transport
• Galactose absorbed with help similar methods to aid glucose transport
• Fructose - passively helped by a trans cell

Protein Digestion & Absorption

• Protein is digested through several main sources from the foods we eat
• Luminal digestion with help of 10-15% of ingested proteins to digest it
• The pancreas with help give many enzymes as zymogens Then transport them out by Trypsin

Luminal Regulation

• Helps release into small digestive enzymes and help transfer out enzymes to aid peptic
• Within small intestines they start to degrade with help of H and help give electrical support

Digestion of Fats

• Are helped with Lipase in both the mouth and Stomach to take in some, but most is done and completed by the liver and gall bladder

Small and Medium Fatty Acids

• Are absorbed without any transport or help
• Longer chain needs help, so it gets a micelle cell to help transport them

Main GI Electrolytes

• Handles 8-9 liters a day
• 2 - 2.5 Intake of H2O

Absorption

• The volume is high in the small intestine more than anything which is the majority and the electrolytes also get carried into the digestive tract

Vitamin Absorption

• Fats needs help with electrolytes but B12 just has a chain method
• Receptors bind together

Apsorption of Fe

• There is hemo or non heme that will decide on how quickly it with get metabolized will be with iron and acids in the upper intestines

Regulation of absorbing Iron

• Iron content is a lot, but is helped with all to aid a healthy body to absorb iron.

The Stomach Functions

• Helps storage food
• Mixing food
• Antimicrobial Protection
• Empty content -
• Nutrient - Aiding in B12

Gastic Mortility

• Muscle Layer - Helps digest food, Receptive - Helping the fundis to create contractions which are needed

Gastric Motility Cont.

• Gastric emptying is a complex function that is coordinated in chime
• All are regerated for adequate digestion of gastric contents before that
• Volume and how fast food intake is helped by the body being in good conditions