Module 6: Digestive System I Flashcards
What is the primary function of the digestive system?
Move nutrients, water and electrolytes from the external environment into the body’s internal environment (lumen of GI into ECF)
Starting from the mouth and ending at the anus, what is the path that food follows as it is digested?
Mouth → esophagus → stomach → small intestine → large intestine → rectum → anus
Define digestion. Glandular organs provide _____ that aid digestion. What are these organs?
The mechanical and chemical breakdown of food into absorbable units. Secretions. Salivary glands, liver, gallbladder, pancreas.
What begins digestion?
Digestion begins in the mouth with mastication (chewing) and the addition of saliva.
What are the 3 pairs of salivary glands? What are their 4 roles?
Glands:
- Parotid
- Sublingual
- Submandibular
Functions:
- Moisten and lubricate food
- Amylase partially digests polysaccharides
- Dissolve some food molecules (taste)
- Lysozyme kills bacteria
- Immunoglobulins also exist in the mouth
What is the muscle distribution of the wall of the esophagus?
The top 1/3rd is skeletal muscle while the bottom 2/3rds is smooth muscle.
_____ _____ moves food down the esophagus.
Peristaltic waves.
What is chyme? What structure allows chyme to enter the GI tract?
Food, acid and enzymes mixed together. The pyloric sphincter (valve) permits small amounts of chyme to enter GI tract.
The majority of _____ and _____ takes place in the small intestine.
Digestion and absorption.
What are the 3 regions of the small intestine? What is their lengths and what are their roles?
-
Duodenum: first 25 cm
- Liver and pancreas release exocrine secretions into the duodenum
-
Jejunum and 3. Ileum: 260 cm
- Churn and mix food, causing mechanical and chemical (enzymatic) breakdown
What are the 4 regions of the large intestine and what direction do they run? What does the LI do and what is its length?
- Ascending colon → up
- Transverse colon → right
- Descending colon → down
- Sigmoid colon → left
Water and electrolytes are removed to create semisolid feces, which triggers a defecation reflex once the feces reaches the rectum. The LI is about 150 cm long and has a much larger diameter.
What comprises the enteric nervous system? Where are these found?
- Submucosal plexus: influences the epithelial layer (submucosa)
- Myenteric plexus: influences the muscle (muscularis externa)
What are the 4 main layers of the GI tract?
- Mucosa
- Submucosa
- Muscularis externa
- Serosa
What are the 3 layers of the mucosa?
- Epithelium
- Lamina propria
- Muscularis mucosae
Epithelium
- Types of cells (3)
- Junctions (leaky or tight?) in stomach, SI, and colon
- Lifespan
- Cells produced by:
- Include transporting epithelial cells (enterocytes in SI), endocrine and exocrine secretory cells
- Junctions very tight in stomach and colon (transcellular transport), leaky in small intestine (paracellular)
- Short lifespan (a few days)
- GI stem cells constantly producing new cells.
Lamina propria
- What does it control?
- Describe the structure
- Controls blood and lymph vessels
- Subepithelial mucosal tissue containing nerve fibres (stimulate secretion), small blood vessels and lymph vessels
Muscularis mucosae
- Role (where is it more important?)
- Structure
- Influences surface area (more important in small intestine)
- Thin layer of smooth muscle in the mucosa that can alter the surface area available for absorption
Describe the submucosa
Middle distensible layer containing larger vessels (lymph and blood) and the submucosal plexus; one of the major nerve networks in the enteric nervous system
Describe the muscularis externa. What does it contain?
2 or 3 layers of smooth muscle: circular decrease diameter, longitudinal shortens the tube. Contains myenteric plexus.
Describe the serosa
Outer covering of connective tissue that is a continuation of the peritoneal membrane, sheets of mesentery hold intestines in place.
Define secretion, absorption and motility in the context of digestive processes.
Secretion: movement of material from cells into lumen or ECF
- Can be movement from ECF to lumen, cells to lumen, or cells to ECF
- Secretion can also mean movement of water and ions from ECF to lumen
Absorption: movement of material from GI lumen to ECF
Motility: movement of material through the GI tract as a result of muscle contraction
What are the 3 main challenges that the digestive system faces? Describe these.
-
Mass balance
-
We secrete a lot more than we actually ingest
- Ingest about 2 L/day, secrete 7 L/day (saliva, bile, etc.)
-
We secrete a lot more than we actually ingest
- Avoiding autodigestion
- Breaking food down into small enough molecules to be absorbed without digesting the cells of the GI tract (we are made of the same stuff we try to ingest)
-
Defense
- Absorbing water and nutrients while preventing bacteria, viruses and other pathogens from entering the body.
- Mechanisms: mucus, digestive enzymes, acid and the largest collection of lymphoid tissue (80% of lymphocytes are in small intestine)
What are the 3 fluid secretions of the digestive system?
- Water
- Digestive enzymes
- Mucus
What are the 2 roles of water as a fluid secretion in the digestive system?
- Transport ions from ECF to lumen
- Creates osmotic gradient for water movement
What 2 types of structures secrete digestive enzymes in the digestive system? Describe.
- Exocrine glands (salivary and pancreas)
- Epithelial cells in stomach and small intestine
- Proteins synthesized on rough ER and packaged in secretory vesicles until needed
- Once released some remain bound to apical membrane by lipid anchors
What is a zymogen? How is it useful?
An inactive form of an enzyme (pre-enzymes). Allow for the cell to stock pile without inflicting cellular damage.
What is the role of mucus in the digestive system? What cells produce mucus in the GI system and where are these cells found?
Viscous glycoprotein (mucins) secretions that helps protect GI cells and lubricate the contents.
Cells:
- Mucus cells: stomach and salivary glands
- Goblet cells: intestine
How do enzymatic secretions differ between the stomach and intestine? What is this due to?
- Stomach: enzymes prefer acidic conditions
- Intestine: enzymes prefer alkaline conditions
This difference is due to pH optimization.
What are the two purposes of motility?
- Moves food from mouth to anus
- Mechanically mixing food breaks it into uniformly small particles
What is the distribution of smooth and skeletal muscle in the GI system? What type of smooth muscle is in the GI system and how do these contract?
Muscles in the mouth and neck are skeletal, while muscles from the esophagus to the rectum are smooth. The smooth muscle in the small intestine is single unit (unitary) smooth muscle, connected by gap junctions, causing all of the cells to contract as a single unit.
GI motility determined by properties of smooth muscle and modified by chemical input from _____, _____ and _____
Nerves, hormones, paracrine signals.
What kind of nerve activity occurs at sphincters?
Tonic, usually contracted.
What kind of nerve activity occurs in the posterior region of stomach, small and large intestines?
Phasic cycles between contraction and relaxation.
Slow waves:
- What are they?
- What are they modified by?
- Where do they originate? Describe these cells.
- Slow waves are spontaneous depolarizations in GI smooth muscle, consisting of alternations of depolarization and repolarization
- Modified by chemical input from neurons, hormones and paracrine signals
- Likely originate in a network of cells known as the interstitial cells of Cajal (ICC) which contain many ion channels, allowing for spontaneous depolarization.
- Modified smooth muscle cells serving as the pacemaker for slow wave activity
- Slow waves begin spontaneously in ICC and spread to adjacent smooth muscle through gap junctions
- Different regions controlled by different ICC groups
- Typically, 3-12 waves/min
What are the three basic patterns of contraction that occur in the GI system leading to different types of movement? When do these occur? Where do these occur? What are their roles?
-
Migrating motor complex (motilin)
- Occurs between meals, largely after digestion has taken place
- Usually begins in the stomach, ends at the ileum
-
Takes around 90-120 minutes
- First 45 to 60 minutes: quiescent
- 20 to 30 minutes: period of infrequent peristaltic contractions
- 5 to 15 minutes in: cycle rapid forceful contractions
- The objective is to sweep food remnants and bacteria out of the upper GI tract and into the large intestine
-
Peristaltic contractions
- Forward movement
- Progressive wave of contraction of circular muscle behind a mass (bolus) of food, accompanied by relaxation in front of the bolus
- Moves at approximately 2-25cm/s
- Occurs in the esophagus, stomach, and intestines
-
Segmental contractions
- Churns and mixes ingested food
- Small segments alternatively contact and relax circular and longitudinal (no net forward movement)
- Occurs in the small and large intestine
_____ and _____ are the primary regulated functions of the GI system. _____ and _____ are not controlled.
Motility, secretion. Digestion, absorption.
(think of the roles of the ENS [myenteric: movement, submucosal: secretion])
How is GI function regulated neurally? What are the two types of reflexes? Describe briefly.
Submucosal and myenteric plexuses form the ENS (100-500 million neurons), where neurons synapse with each other, smooth muscles, glands and epithelial cells.
- Short reflexes integrated entirely within the ENS, allowing ENS to work without the CNS
- Long reflexes integrated within the CNS
What are the 3 types of GI peptides that are used to regulate GI function? What distance do each work at?
- Hormones (long distance)
- Neuropeptides (long distance)
- Cytokines (short distance)
What 5 similarities does the ENS share with the CNS?
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Intrinsic neurons: entirely within the ENS
- Comparable to interneurons within CNS
-
Neurotransmitters and neuromodulators: about 30 neurotransmitters many identical to CNS
- Serotonin, ACh, VIP, nitrous oxide
- Glial support cells: similar to astrocytes
-
Diffusion barrier: like blood-brain barrier
- Prevents substances from affecting neurons
- Integrating center: can function autonomously
What are the two types of ENS reflexes? Describe them.
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Short reflexes: originate in the ENS and are carried out entirely within wall of the gut (local stimuli)
- Myenteric plexus: motility
- Submucosal plexus: secretion from GI secretory cells
-
Long reflexes: integrated in the CNS. Some originate outside the GI tract, but others originate in the ENS.
- Sensory info also sent to CNS (long reflexes)
- ENS receives information from the CNS via autonomic neurons
In long reflexes, parasympathetic input _____ GI function, sympathetic _____
Enhances (rest and digest), inhibits (fight or flight)
If a long reflex begins in the brain it is a _____ reflex (feedforward and emotional). What is the role of this type of reflex? What are the stimuli?
Cephalic reflex. It prepares body for eating. Stimuli include the sight, thought, smell of food
GI peptides (_____, _____, and _____), can act as _____ or _____, which excite or inhibit _____ and _____.
Hormones, neuropeptides and cytokines. Hormones or paracrine signals. Motility and secretion.
What 3 places are GI peptides primarily secreted into? What do these act on?
- Lumen → act on apical membrane receptors
- ECF → act on neighboring cells
- Outside the GI tract → brain, pancreas and liver primarily, causing many signals
What are the 3 hormone families and what are the members?
- Gastrin family
- Gastrin
- Cholecystokinin
- Secretin family
- Secretin
- Vasoactive intestinal peptide
- Gastric inhibitory peptide (glucose-dependent insulinotropic peptide)
- Other
- Motilin
Integrated function within the digestive system is often described in three phases, what are they?
- Cephalic/oral phase: digestive processes occurring before food enters the stomach (mouth and esophagus).
- Gastric phase: digestive processes in the stomach.
- Intestinal phase: digestive processes in the intestines (small and large)
Describe the cephalic phase.
- When does it begin? When is it reinforced?
- What kind of reflex is it?
- What part of the nervous system is primarily involved and what kind of output does it give?
- Begins before food enters the mouth and are reinforced once food enters the GI tract
- Long reflexes beginning in the brain (cephalic reflex)
- Increased parasympathetic output from medulla to salivary glands (facial and glossopharyngeal) and to the enteric nervous system (vagus nerve (CN X): vagal reflex)
When does chemical and mechanical digestion begin? Describe how this process causes chemical and mechanical digestion. What are the 4 functions of this process?
Chemical and mechanical digestion begin in the mouth
- Mechanical digestion begins with mastication (chewing) of food by teeth, joined by chemical digestion caused by a flood of saliva from the three pairs of salivary glands (parotid, submandibular and sublingual)
Four functions:
- Soften and moisten food: makes it easier to swallow
- Digestion of carbohydrates (amylase): stops once you reach stomach
- Dissolve foods (taste): solid to liquid
- Defense (lysozyme and immunoglobulins)
Saliva
- What kind of secretion is saliva?
- How much do we produce per day?
- What is the composition?
- What kind of cells are involved and what do each produce/do?
- What kind of control is it under?
- Exocrine
- About 1.5 L
-
Solutes
- 99.5% water
- 0.5% solutes
- Na+, Cl- absorbed in small amounts
- K+, HCO3-, PO4- secreted
- Amylase, lysozyme, mucus, IgA
-
Cells: bundles called acini
- Myoepithelial cell: contract to squeeze out saliva
- Mucous cell: secretes mucus
- Serous cell: produces watery solution
- Sympathetic control
The glands that secrete saliva are not identical, what are the differences in secretions?
- Parotid: watery solution with amylase → serous cell
- Submandibular: similar to parotid plus some mucus → serous and mucous cells
- Sublingual: mainly mucus → mucous cell
Define deglutition. What are the 3 parts of it?
Deglutition: a reflex that pushes a bolus of food or liquid into the esophagus
Part 1
-
Pressure against soft palate and back of mouth (voluntary [tongue]) activate sensory neurons going to the medulla
- Starts voluntary, becomes involuntary
- Medulla (swallowing center): somatic motor outputs to pharynx and upper esophagus and autonomic outputs to the lower esophagus
- Soft palate closes off nasopharynx
- Contraction moves larynx up and forward, glottis closes trachea
- Upper esophageal sphincter relaxes and opens
Part 2
- Epiglottis covers airways (breathing is inhibited) as bolus passes
Part 3
- Food moves into the esophagus propelled by peristaltic waves and aided by gravity (helps but isn’t required [keg stand])
- Peristaltic waves are driven by reflex from medulla signalled by pressure on esophagus
- Lower esophageal sphincter tension relaxes
GERD
- What is it?
- What is it caused by?
- How is it treated?
- Gastroesophageal reflux disease (heartburn)
- Caused by lower amounts of contraction within the sphincter or high pressure within the stomach
- Treated using a proton pump inhibitor to decrease the acidity of the acid washing back up
What are the three general functions of the stomach?
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Storage: stores food and regulates its passage into the small intestine
- Otherwise food would proceed rapidly through intestines and little breakdown would occur
- Digestion: chemical and mechanical digestion into chyme
-
Defense: destroys bacteria and other pathogens in food and pathogens trapped in airway mucus (mucocilliary escalator)
- Mucus is pushed up and over the wall between the esophagus and trachea
How much food, drink and saliva enters the stomach each day? Does the stomach do much absorption of these substances?
About 3.5 L. No, that is mostly done by the intestines.
What is receptive relaxation?
Upon swallowing food, parasympathetic neurons to the ENS cause the fundus of the stomach to relax
_____ of the stomach enhances motility. What kind of motility occurs and what is the result of this?
Distension. Weak peristaltic waves (15-25 seconds) that increase in force proceeding down to the antrum moves chyme toward pylorus (propulsion) and then larger particles are moved back to the body (retropulsion). This mixes food with acid and digestive enzymes, small amounts of chyme squeeze through pylorus (to the duodenum)
What are and what is the role of the gastric glands? What are they not to be confused with?
Invaginations in the stomach that provide substances necessary for digestion. Do not confuse with rugae.
G cells
- What do they do?
- What stimulates them (3)?
- What is their direct action?
- What is their indirect action?
- Release gastrin.
- Responds to amino acids, peptides and distension (short reflex [gastrin releasing peptide])
- Parasympathetic neurons in the ENS cause stimulation of gastrin production during cephalic phase (long reflex), stimulating gastric acid secretion directly from parietal cells
- Stimulates histamine release from enterochromaffin-like cells (ECL), which then stimulates parietal cells
Parietal cells
- What do they do?
- What stimulates them (3)?
- What is the cellular response to these stimuli?
- Produce gastric acid (HCl)
- Stimulated by gastrin from G-cells, histamine from enterochromaffin-like cells (ECL) or ACh from ENS neurons via long and short reflexes
- Stimuli causes exocytosis of vesicles containing apical H+/K+ ATPase, Cl- and K+ transporters, causing them to insert into the membrane
What are the 5 steps of gastric acid production by parietal cells?
Steps:
- Free H+ actively transported across apical membrane
- Water dissociates to H+ and OH- freeing up more H+ to be actively transported
- OH- combines with CO2 via CA to generate HCO3-
- HCO3- exchanged for Cl- at basolateral membrane
- Cl- diffuses across apical membrane through open channels following electrochemical gradient
What are the 5 functions of gastric acid secretion?
- Stimulates release of pepsinogen from chief cells, cleaves pepsinogen to pepsin (digests proteins)
- Denatures proteins making it easier for pepsin to function
- Kills bacteria and other ingested microorganisms
- Inactivates amylase from saliva (amylase cannot function in acidic conditions, is reactivated in the intestines)
- Stimulates D cells to release somatostatin, inhibiting further acid secretion
What are cephalic phase stimuli? What process does this initiate? What 3 cells does this process trigger and what effect do they cause?
Include the thought, sight, smell of food. Initiates long reflex. Cephalic phase stimuli enters the brain, increasing enteric neural activity.
Activates:
- G-cells: gastrin secretion, causing increased histamine (ECL activation) and acid secretion
- ECL cells: activated by G cells, secrete histamine leading to acid secretion
- Parietal cell: acid secretion
When HCl concentration becomes too high in the stomach, what cells become activated? Describe their process to reset HCl levels.
D cells release somatostatin when HCl becomes too high, which decreases acid secretion by inhibiting parietal cells.
Describe the short reflex in gastric acid secretion, starting with the gastric phase stimuli.
Gastric phase stimuli (distension, amino acids and peptides) activate G cells which then activate ECL and parietal cells.
Chief cells
- What do they do?
- What are they activated by?
- What cell do they act in tandem with?
- Release pepsinogen, which along with HCl and intrinsic factor is converted to pepsin, which degrades proteins.
- Stimulated by acid secretion via short reflex
- Parietal cells
Parietal cells
- What do they do?
- What are they activated by?
- What cell do they act in tandem with?
- Release HCl and intrinsic factor to cleave pepsinogen to pepsin
- Stimulated by acid secretion via short reflex
- Chief cells
What kind of macromolecule digestion primarily occurs in the stomach? Which occurs in the intestine?
Protein in stomach, fat in intestine.
What 3 cells are involved in paracrine secretion? What do each of them release and what does this do?
-
ECL: histamine
- Activates H2 receptors on parietal cells to stimulate HCl secretion
-
Parietal cell: intrinsic factor
- Forms a complex with Vitamin B12 so it can be absorbed
-
D cell: somatostatin
- Negative feedback for acid secretion (G cells, parietal cells and ECL cells)
- Inhibits pepsinogen release
What is the role of mucus secretion in the stomach? What comprises the mucus layer? What stimulates secretion?
Mucus and HCO3- secreted from mucous cells protect the cells of the stomach from being dissolved by the acid. Mucus secretion stimulated by parasympathetic input and irritation. HCO3- by parasympathetic input and H+
What is the mucus-bicarbonate barrier?
The mucus-bicarbonate layer is a layer of mucus and bicarbonate that coats the inside of the stomach and acts as a protective sheet between the cells and the gastric juice. Bicarbonate acts as a buffer that will neutralize any acid absorbed in the mucus layer
Peptic ulcer
- What is it?
- What 3 things is it caused by?
- Describe the primary cause
- A sore or break in the lining of the stomach or duodenum
-
Causes:
- Excessive acid production (gastrin secreting tumors)
- Nonsteroidal anti-inflammatory drugs (NSAIDS)
-
Helicobacter pylori (primary)
- The bacteria create an ammonium and bicarbonate buffer zone (urease enzyme) around the cell allowing it to survive in the low pH environment
- They have pH chemoreceptors that direct them towards higher pH environments (mucus layer)
- Once at the mucus layer, they can break it down, imbed, and damage the epithelial cells (high ammonia causes damage to the epithelium)
