Unit 9: Digestion LO's Flashcards
Trace the path a piece of undigested food would travel from the mouth to the anus and name all the anatomical structures it might encounter.
- Mouth/Oral Cavity
- Pharynx
- Esophagus
- Stomach (passage time ~2.5-5 hours)
- Small Intestine
a. Duodenum – main site of chemical digestion
b. Jejunum
c. Ileum - Caecum
- Large intestine (colon)
- Rectum
- Anus
Name the four layers of the GI tract wall, identify key functions of each layer, and describe key modifications of the layers found in different GI sections.
- Mucosa
a. Mucosal Epithelium – lines inside surface of the tract
b. Lamina propria – holds the epithelium in place
c. Muscularis mucosae - contraction can help to dislodge food from the folds/crypts of the intestine - Submucosa - Contains the submucosal plexus, the first nerve network of the enteric nervous system.
- Muscularis externa - 2 layers of smooth muscle
- Contains the myenteric plexus (between the inner circular and outer longitudinal), the second nerve network of the enteric nervous system.
- While most of the tract consists of these 2-3 layers, of smooth muscle, the middle 1/3 of the esophagus contains both skeletal muscle and smooth muscle to allow for some voluntary control of swallowing. - Serosa
- Forms sheets of connective tissue that help to hold organs in place.
- Also provides support for blood vessels and nerves going to/from abdominal organs.
Diagram how slow wave potentials are created in the intestines and how this can lead to smooth muscle contraction.
Intrinsic control GI Motility:
- The smooth muscle cells of the digestive tract have a resting
membrane potential of -50 mV.
- Interstitial Cells of Cajal (ICC), act like pacemakers for the contraction of gastrointestinal smooth muscle.
- They spontaneously produce slow waves of depolarization that spread to adjacent smooth muscle cells through gap junctions.
- The cells slowly depolarize toward threshold (-40mV), and if threshold is reached, action potentials (sometimes referred to as spike potentials) and contraction will occur. The frequency of the spikes determines the strength of the muscle contraction. Frequency depends on the amount of depolarization. (Unlike cardiac pacemaker potentials, each slow wave depolarization does not always reach threshold, so the number of contractions produced through these pacemaker cells is variable.
- Depolarization of the Cajal cells can be stimulated by intrinsic factors such as stretch in the smooth muscle cells (which allows for greater entry of Ca++ during depolarization or extrinsic factors including, parasympathetic stimulation via acetylcholine (which opens ligand-gated Ca++ channels in the smooth muscle cell membrane). .
- Recall from BIOL2410 – Depolarization of smooth muscle cells triggers calcium induced calcium release from the sarcoplasmic reticulum. Ca++ then binds to calmodulin, which activates myosin light chain kinase and initiates contraction.
- Hyperpolarization of the cells of Cajal (which can occur as a result of stimulation by norepinephrine) results in no muscle contraction.
Distinguish and describe the different patterns of smooth muscle contraction observed in the GI tract.
- Peristalsis
- small, weak waves of contraction in which circular smooth muscle contracts upstream from the bolus of food, while muscle downstream (in front of bolus) relaxes. Result is bolus is pushed further along the tube.
- In small intestine.
- ~3-9 hours
- Primarily controlled by myenteric plexus - Segmental contraction - mixes
- Alternating contraction and relaxation of circular muscle in adjacent sections of the intestine.
- Mixes up and churns the ingested material, which ensures that it all gets exposed to the absorptive surface (material from the middle of the lumen gets moved to be in direct contact with the mucosa). And also ensures exposure to digestive enzymes (mixing distributes enzymes amongst the food particles for digestion). - MigratingMotorComplexes (mass movements)
- a larger/stronger wave of contraction that propels the substance over a longer distance (up to the entire length of the small and large intestines).
- These movements are a cleansing contraction that propel undigested material as well as pathogens and bacteria out of the small intestine and into the large intestine.
- Typically occur during or right after a meal (cleans out the tube to get it ready to process the incoming food).
- Part of the gastrocolic reflex (a long reflex arc, also known as the defecation reflex) which is initiated by distension of the stomach or duodenum when consuming a meal.
Diagram the location and the cellular mechanisms by which the GI tract secretes acid, bicarbonate, and NaCl.
- The Cephalic Phase - Begins with feedforward reflexes (long reflexes) that
prepare the stomach and intestines for digestion.
- Sight, smell and thought of food stimulates the long vagal reflex: - The Gastric Phase - Initiated by presence of food stomach – distension/ stretch (stimulates baroreceptors) and presence of amino acids in food (stimulates chemoreceptors)
- The Intestinal Phase - Chyme (mixture of gastric juice and partially digested food from stomach) enters small intestine. Gastric emptying into intestine is regulated so that 1) acid can be neutralized in the small intestine; 2) tonicity does not overwhelm small intestine, 3) so that there is adequate time for digestion and. absorption of nutrients.
Contrast long reflexes, short reflexes, and reflexes involving GI peptides.
Involves reflex responses that are integrated in both the enteric nervous system (short reflexes) and the central nervous system (long reflexes). This includes any reflex involving the brain (cephalic reflexes).
clearly:
- SHORT reflexes originate in the ENS & are integrated there
- LONG reflexes may originate in the ENS or outside it but are integrated in the CNS
Describe the anatomy and physiology of the ENS and compare it with the CNS.
Enteric NS – “brain” of the gut; contains as many neurons as the spinal cord and provides local intrinsic control of motility and secretion. Involved in short reflexes.
Identify the three families of GI peptide hormones and give examples of each.
GI hormones are divided into the gastrin gamily (gastrin, cholecystokinin (CCK), secretin family (secretin, gastric inhibitory peptide, glucagon-like peptide-1), & hormones that do not fit into either of those 2 families (motilin)
Explain how pH can be used to predict the location where a particular digestive enzyme might be most active.
Enzymes function best in a restricted range of pH. Stomach enzymes must be active at acidic pH; salivary and intestinal enzymes work bet in alkaline pH. ?
List the steps of the deglutition (swallowing) reflex.
- Tongue pushes bolus against soft palate & back of mouth, triggering swallowing reflex
- Breathing is inhibited as the bolus passes the closed airway
- Food moves downward into the esophagus, propelled by peristaltic waves & aided by gravity
Diagram the chemical digestion and absorption mechanisms for carbohydrates, proteins, fats, and nucleic acids,
- Protein – pepsinogen secreted by chief cells is activated by HCl, which converts it to pepsin. Pepsin (an endopeptidase) enzymatically breaks peptide bonds. It digests 10-20% of ingested proteins while in the stomach. Products of pepsin digestion are shorter polypeptides.
- Fats (Lipids) – minimal digestion by gastric lipase (<10%). Enzymatically breaks down triglycerides into monoglycerides and fatty acids.
Describe the absorption of vitamins, minerals, ions and water.
The small intestine is the site of most absorption of nutrients (see notes section J for mechanisms of absorption for the different nutrients). ?
Map the key processes and control pathways associated with the three phases of gastric secretion (cephalic, gastric, intestinal). Include diagrams of cellular mechanisms and all enzymes, hormones, paracrines, ions, or other substances that are involved. Link the events of the different phases to one another.
Cephalic Phase - Begins with feedforward reflexes (long reflexes) that prepare the stomach and intestines for digestion.
- Sight, smell and thought of food stimulates the long vagal reflex:
Gastric Phase - Initiated by presence of food stomach – distension/ stretch (stimulates baroreceptors) and presence of amino acids in food (stimulates chemoreceptors)
Intestinal Phase - Events of the intestinal phase are initiated in response to duodenal stretch and the presence of H+, amino acids and lipids in the chyme. The latter are detected by chemoreceptors that extend into the lumen from the submucosal plexus.
Compare and contrast small and large intestines in terms of anatomy, digestion, absorption, secretion, and motility.
Small Intestine - where most digestion takes place
- 3 sections: duodenum, jejunum & ileum
- digestion is carried out by intestinal enzymes, aided by exocrine secretions from 2 accessary glandular organs: the pancreas & the liver
- secretions from these 2 organs enter the initial section of the duodenum through ducts
- a tonically contracted sphincter keeps pancreatic fluid & bile from entering the small intestine except during a meal
Large Intestine - digestion finishes in SI, & nearly all digested nutrients & secreted fluids are absorbed there, leaving about 1.5 liters of chyme per day to pass into the large intestine
- in the colon - the proximal section of the large intestine - watery chyme becomes semisolid feces as water & electrolytes are absorbed out of the chyme & into the ECF
Describe the GALT and explain how M cells provide information about the contents of the lumen.
for defense from invaders, the GI tract contains the largest collection of lymphoid tissue in the body, the gut-associated lymphoid tissue (GALT)
M cells (microfold cells) in small intestine sample gut contents and endocytose any antigens. Antigens are transported to Peyer’s patches (in contact with basolateral membrane of M cells) and are presented to dendritic cells, macrophages and B and T lymphocytes that will initiate the immune response.
