NPB 101 Digestive System

derp

What is the major function of the digestive system?

Takes in food we eat, digests it, absorb nutrients from outside of the body

What happens to the complex meals we eat?

Meals are broken down into macronutrients that can be absorbed across mucosal lining of SI (where they are actually absorbed)

Where the do the macronutrients that were absorbed across mucosal lining of SI go?

Either go into the bloodstream or lateral vessels in the lymphatic system

What is considered as something outside of our body?

• Something in lumen of the tube

• Something in SI or colon

• Basically anything that is continuous with the outside

What is considered as something inside of our body?

Only things we consider inside out body is after we’ve absorbed it

What are the 4 major tissue layers?

• Serosa (outermost)

• Muscularis externa

• Submucosa

• Mucosa (innermost)

What is the serosa?

• Outermost

• Secretes serous fluid

  • Lubricates and allows to move across each other with no friction

• Continuous with mesentery throughout much of the tract

What is the muscularis externa?

• Contractile activity

• Major smooth muscle that we can’t actively control

• Has 2 layers

  • Inner circular layer

  • Outer longitudinal layer

• Myenteric plexus

  • Part of enteric nervous system

  • In between the muscle layers

What is the myenteric plexus?

• Part of enteric nervous system

• In between the muscle layers of the muscularis externa

What is the contractile activity of the muscularis externa?

• Contractile activity produces propulsive and mixing movements 

  • Ring like contraction propagating down tube that pushes food down = swallowing

  • Chaotic kneading movement that mixes things together achieved by inner and outer layers = small intestine

What is the inner circular layer?

• Contraction decreases diameter of the lumen

• Contract → constrict lumen

What is the outer longitudinal layer?

• Contraction shortens the tube

• Contract → shorten tube

What is the submucosa?

• Thick layer of connective tissue that allows for easy movement 

• Elastic

• Contains larger blood and lymph vessels where most blood supply in the tube runs through 

• Submocosal plexus nerve network

What is the submucosal plexus nerve network?

• Located between the muscularis externa and submucosa

• Part of enteric nervous system

What is the mucosa?

• Innermost

• Lines lumen: Highly folded surface increases absorptive area

• Muscularis mucosa (outermost)

• Lamina propria (middle)

• Epithelial layer (innermost)

What is the muscularis mucosa?

• Outermost layer

• Smooth muscle

• Smaller fibers that do local contractions

  • Nudge secretions to occur

  • Local movement to ensure secretions don’t stick but are gently pushed out

What is the lamina propria?

• Middle layer

• Loose connective tissue

• Small blood vessels, lymphatics, and enteric neurons

• Contains gut-associated lymphoid tissue (GALT)

  • Contains immune cells to respond quickly and escalate

What is the epithelial layer?

• Also called the mucous membrane

• Innermost layer

• Cells modified for secretion and absorption

• Contains exocrine gland cells

  • Secrete digestive juices, mucus, enzymes, water, into lumen to smooth it out

• Contains endocrine gland cells

  • Secrete GI hormones into capillaries

Why do we separate the gastrointestinal tract from the rest of the body?

• pH in our stomach can get as low as 2, that level is not compatible with life.

• Harsh enzymes that break down food could destroy our own body’s tissues; enzymes are synthesized as inactive forms - activated when reach the lumen.

• Microorganisms inhabit and enter the GI tract and could be lethal if enter the rest of the body.

What are the 4 basic digestive processes?

• Motility

• Secretion

• Digestion

• Absorption

What are the only digestive parts that are skeletal muscle = voluntary?

• Ingestion

• Chewing

• Swallowing

• Defection

What is motility?

• Begins with ingestion to which we have active control - decide what we bite, swallow, eat

• Muscular contraction that mix and  move forward contents of GI tract forward

  • If not mixed well, will not digest

What are the 2 types of motility movements?

• Propulsive movements (peristalsis)

• Mixing movements (segmentation)

What are propulsive movements (peristalsis)?

• Move movements forward through the digestive tract 

• Moves in 1 direction

• Ring-like contraction of muscularis externa that contracts and propagates 

  • Pushes bolus down tube and lands to top of stomach

• Occurs in esophagus and when pooping

What are mixing movements (segmentation)? 

A mixing movement of the muscularis externa that kneads and slowly moves contents by alternating local contractions

What are the 3 function of mixing movements (segmentation)?

• Aids digestion by mixing food with digestive juices 

• Facilitate absorption by exposing food to absorbing surfaces

• Forward movement (slow and non-linear)

  • Helps move stuff along but in a 2 steps forward 1 step forward 

What is secretion (exocrine)?

• Release substances such as digestive juices, enzymes, acids, buffers, electrolytes, water, and bicarb into the GI lumen to promote digestion

What is secretion (endocrine)?

• Hormones secreted into the blood that act as signals

• Don’t go into the GI lumen

What is digestion?

• Enzymes accomplish breakdown of structurally complex macronutrients into absorbable units

  • Pass through mucosal layer to be inside body for use and storage

What is absorption?

• The transfer of small absorbale units from the lumen into the blood or lymph

  • AA and carbs go into blood

  • Lipids go into lymphatic and get added to circulation - will eventually enter bloodstream

What are pacemaker cells/Interstitial Cells of Cajal)? 

• Basic electrical rhythm (BER)

  • 3-5 Hz in stomach

  • 8-11 Hz in SI

How are the depolarizations from ICCs spread?

The depolarizations spread through gap junctions to smooth muscle cells by the enteric nervous system

What do the oscillations from the ICCS do?

Oscillations from ICC have a steady rate and rarely reach threshold 

What occurs if an ICC reaches threshold?

An action potential will occur leading to a muscle contraction

What happens in the absence of food?

BER stays below threshold leading to no action potentials and muscle contractions - trigger the migrating motility complex

What is the migrating motility complex?

• On rare occurrences when depolarizations reach threshold that trigger contractions to “sweep” contents from stomach and small intestine into the large intestine 

  • Nothing lingers in the GI tract

  • Triggered by extrinsic factor motilin

What happens when there is food in the lumen?

Stretch and gastrin increase amplitude and frequency of the BER depolarizations leading to it reaching threshold more often, more muscle contractions

How can stretching cause more muscle contractions?

• When the stomach is filled, it stretches

• Stretch receptors send out a signal and influence BER to reach threshold. 

How is gastrin released?

• When stomach receptors recognize a stretch occurring

• When there is protein within the stomach

How and what intrinsic factors control the digestive system?

• Autonomous smooth muscle cells connected by gap junctions, forming a functional syncytium (single-unit muscle cell)

• Interstitial cells of Cajal (ICC) act as pacemaker cells and generate slow wave potentials

• Enteric nervous system (myenteric and submucosal plexus) form synapses on cells to further modulate activity

How and what extrinsic factors control the digestive system?

• Extrinsic nerves (from outside the digestive system) from sympathetic and parasympathetic branch can influence motility

  • Modify ENS activity 

  • Alter gastric hormone secretion

  • Act directly on smooth muscle, glands, and ICCs

What is chewing and swallowing?

• Skeletal / voluntary control - decide when to swallow

• Break down food

• Begin to mix

What is the palate?

• Roof of oral cavity

  • Separates oral cavity from nasal passage

  • Allows chewing and breathing to occur at the same time

What is the uvula?

• Soft tissue that hangs from rear of mouth that seals off nasal passage during swallowing

  • Is pushed backwards when swallow to prevent food from getting up to nasal passageway

What are the salivary glands?

• Composed of 3 layers

  • Sublingual

  • Submandibular

  • Parotid

• Secrete saliva

What does salivary gland saliva contain?

• Mucus

  • Moisten food and lubricate

• Lysozyme

  • Kill any bacteria we ingest

• Bicarbonate

  • Neutralize acids

• Amylase

  • Breaks down polysacc into disacc

What is salivary amylase?

It starts the process of carbohydrate (polysacc) digestion, starts in the mouth

What is swallowing?

• Has 2 phases

  • Oropharyngial phase

  • Esophageal phase

• Entire process of moving food from the mouth, through the pharynx and esophagus, to the stomach

• All-or-none reflex that is initiated when bolus voluntarily forced by tongue to rear of mouth into pharynx

• Can be initiated voluntarily but cannot be stopped once it has begun 

What is the oropharyngial phase?

• When food is still in the mouth entering pharynx

• Pharyngoesophageal sphincter closes and breathing resumes at end of this stage

What is the esophageal stage?

• Peristaltic (propulsive) waves more bolus into stomach 

  • Once bolus enters esophagus, peristalsis occurs

  • Moves from top of esophagus to top of stomach

  • No digestion, no enzymes added, no absorption

What is the stomach?

• J-shaped chamber between the esophagus and small intestine

• Thick layer of smooth muscle

• Connected to small intestine by pyloric sphincter

• Divided into 3 sections

  • Fundus

  • Body

  • Antrum

What is the fundus (stomach)?

Located above the gastroesophageal sphincter

What is the body (stomach)?

Middle portion

What is the antrum (stomach)?

Bottom portion

What is the pyloric sphincter?

• Key regulator of gastric emptying

• Usually a little open / closed

• Controls rate of gastric emptying by controlling how much sphincter to contract

What does the pyloric sphincter allow to come through into the small intestine?

• Not big chunks of food

  • Will stay in stomach to be further broken down, mixed, digested

• Liquified, digested well enough food called chyme

  • Liquid consistency = can pass through 

What does the stomach do?

• Store ingested food

  • Within body of the stomach

• Produce gastric secretions: HCl and enzymes that start chemical digestion of protein

• Gastric motility (gastric mixing and gastric emptying) converts pulverized food to chyme and emptying into small intestine

What is gastric-filling?

• Gastric volume can expand 20x fold during a meal

  • Expansion of gastric volume is vagally-mediated, process is called receptive relaxation

What is receptive relaxation?

Stomach senses it is getting filled up and will send signals to relax and accommodate that food

Where do gastric secretions come from?

• From two distinct areas

  • Oxyntic mucosa (fundus and body)

  • Pyloric gland area (antrum)

What is in the oxyntic mucosa (fundus and body)?

• 3 types of gastric exocrine secretory cells associated with gastric pits

  • Mucous cells

  • Chief cells

  • Parietal cells 

• 1 type of gastric endocrine secretory cell associated with gastric pits

  • Enterochromaffin- like (ECL) cells

What do (exocrine) mucous cells do?

Secrete watery mucus

What do (exocrine) chief cells do?

Secrete enzyme precursor pepsinogen

What do (exocrine) parietal cells do?

Secrete HCl (acid)

What do (endocrine) ECL cells do?

Secrete histamine, which activates parietal cells

What does HCl do?

• Activates pepsinogen (from Chief cells) into pepsin within the lumen

  • Once pepsis is activated, it can also activate other pepsinogen (autocatalyst)

• Denatures proteins

  • Doesn’t breakdown

  • Will open / unfold proteins to make it easier for enzymes to chop them

• Kills any possible microorganisms that escaped from lysosomes

Why are pepsinogen (inactivated) instead of pepsin (activated)?

• In order to protect the stomach lining / protective mucus layer

• Pepsin may digest your own cells if activated in the wrong area

How does the stomach survive despite the acid?

• Stomach lining has protective mucus layer that contains bicarbonate,  protecting against low pH 

What is in the pyloric gland area (antrum)?

• 2 types of gastric endocrine secretory cell associated with gastric pits

  • G cells

  • D cells

What do (endocrine) G cells do?

Secrete hormone gastrin into bloodstream

What does gastrin do?

• Stimulates

  • Parietal cells (HCl)

  • Chief cells (pepsinogen)

  • ECL cells (histamine)

• Increases gastric motility and promotes movement of leftover, undigested / unabsorbed material out of ileum into large intestine

  • Signals that more food is coming down, make sure no leftover food in the GI tract - sweep and make space of the next meal 

What do (endocrine) D cells do?

Secrete hormone somatostatin into bloodstream

What does somatostatin do?

• Inhibits

  • Parietal cells (HCl)

  • ECL cells (histamine)

• Helps stomach quiet down at the end of digesting a meal

What is gastric mixing?

Mix food with gastric secretions to produce chyme

What is gastric emptying?

• When chyme propelled towards pyloric sphincter, where small amount is pushed into duodenum

• In response to chyme, sphincter closes and remaining chyme is tumbled back into antrum

  • Retropulsion

  • Longer bits gets rejected back to antrum to be further broken down

What are the factors in the stomach that control gastric mixing and gastric emptying?

• Volume of chyme

• Fluidity of chyme

How does volume of chyme control gastric mixing and gastric emptying in the stomach?

• Stomach distention stimulates

  • Stretch receptors on the smooth muscle

  • Enteric and parasympathetic nervous system

  • Gastrin to increase motility

How does fluidity of chyme control gastric mixing and gastric emptying in the stomach?

Liquids don’t require extensive mixing and churning

What are the factors in the duodenum that control gastric emptying?

• Factors in duodenum that signal stomach to slow down, because they can’t keep up

  • Fat

  • Acid

  • Hypertonicity

  • Distention

How does fat control gastric emptying into the duodenum?

• Fat is only digested and absorbed within the small intestine

• Can only absorb at a certain rate

  • Will take longer to absorb if there are more fats

How does acid control gastric emptying into the duodenum?

• Highly acidic chyme from stomach is neutralized from sodium bicarbonate in the duodenum

• Un-neutralized acid in the duodenum inhibits gastric emptying

  • Duodenum not protected by acid

  • Pancreas secretes bicarbonate and takes a a while to occur

  • If too much un-neutralized acid comes into duodenum - pH drops and will signal slower gastric emptying

How does hypertonicity control gastric emptying into the duodenum?

• Higher tonicity (more concentrated solutes) - higher concentration of undigested food and digestive products

• Increased osmolarity in duodenum indicates a back-up of nutrients and delays gastric emptying

  • Basically have lots of nutrients to be absorbed, but duodenum has a set absorption rate 

How does distention control gastric emptying into the duodenum?

• Too much chyme in the duodenum inhibits gastric emptying

  • Contents in duodenum physically stretch which activates stretch receptors to signal stomach to slow down gastric emptying 

What are factors that regulate gastric motility?

• Neural responses

• Hormonal responses

What are the neural responses that regulate gastric motility?

• Responses that are mediated through both intrinsic (short reflex) and autonomic nerves (long reflex)

  • Collectively called enterogastric reflex

What are the hormonal responses that regulate gastric motility?

• Cholecystokinin (CCK)

• Secretin

What is Cholecystokinin (CCK)?

• Released by I-cells in response to fat in the duodenum

• Inhibits antrum contractions and induces contraction of the pyloric sphincter

  • Slows down gastric emptying

What is secretin?

• Released by S-cells in response to un-neutralized acid in the duodenum

• Slows down gastric emptying

What are the controls of gastric secretion?

• 3 phases

  • Cephalic (excitatory)

  • Gastric (excitatory)

  • Intestinal (inhibitory)

What is the cephalic phase of gastric secretion?

• Stimuli in the head

  • Seeing, smelling, tasting, thinking about food

• Pre-activation of stomach by signals that have not yet reached the stomach

• Vagus nerve conveys the signal

What is the gastric phase of gastric secretion?

• Stimuli in the stomach

  • Peptide fragments 

  • Distention

• Activated more potently due to signals from stomach activate the stomach

What is the intestinal phase of gastrin secretion?

• The loss of signals that trigger gastric secretion are gone due to removal of protein, no distention, as stomach empties

• Local release of D-cells secreting somatostatin

• The 4 signals in the duodenum that signal stomach to slow down gastric emptying

What is the pancreas?

A mixed gland that contains both endocrine and exocrine tissue

What does the exocrine pancreas contain?

• Duct cells

• Acinar cells

What are pancreatic duct cells?

Release sodium bicarbonate into duodenum to neutralize acidic chyme 

What are pancreatic acinar cells?

• Release digestive enzymes into duodenum

  • Pancreatic amylase

  • Pancreatic lipase

  • Proteolytic enzymes (secreted as inactive forms)

What is pancreatic amylase?

Breaks down polysaccharides into disaccharides

What is pancreatic lipase?

Breaks down triglyceride into monoglycerides and free fatty acids

What are proteolytic enzymes (inactive forms)?

• Are enzymes that breakdown proteins - cleave bonds between amino acids

  • Trypsinogen

  • Chymotrypsinogen

  • Procarboxypeptidase

What is trypsinogen?

Converted to active form trypsin by enteropeptidase in the luminal (brush border) membrane of small intestine