Lecture 21 Flashcards
GI Tract
Subdivided into several functional units by muscular sphincters that regulate the orderly passage of contents from one part of the tract to another
What parts of the GI tract do not contain sphincters?
Small intestine (longest part of the GI tract), colon
Haustra
Sac-like segments subdivided by smooth muscle bands
What are some of the components of the upper GI system?
Mouth, sublingual salivary gland, submandibular salivary gland, parotid salivary gland, pharynx, trachea, esophagus
What are some of the components of the lower GI system?
Liver, gallbladder, stomach, pancreas, small intestines, large intestine (colon, cecum, rectum), anus
What type of cells in the GI tract are constantly replaced and have a rapid turnover?
Epithelial cells in the mucosal lining
Explain the structure of the lumen of the GI tract.
- Mucosa: Epithelium (endocrine cells, exocrine cells, mucous cells, ducts), lamina propria, muscularis mucosa
- Submucosa: major blood and lymphatic vessels, Submucosal nerve plexus (Meissner’s)
- Muscularis externa: circular muscle, myenteric nerve plexus (Auerbach’s), Longitudinal muscle
- Serosa: outside the abdominal cavity, the outermost layer is connective tissue (adventitia)
- The mucosal layer is composed of specialized squamous or glandular epithelium that overlies the lamina propria (loose CT and resident chronic inflammatory cells).
- Underlying the mucosa is a thin muscularis mucosa that separates it from the submucosa composed of adipose tissue, vessels, nerves.
- All these structures surrounded by a muscularis propria (externa) composed of two layers of smooth muscle
- GI tract has an intrinsic nervous system that controls peristalsis (rhythmic contractions that move food and fecal material through the GI tract), with both a submucosal and intermyenteric plexus of nerves and ganglia
What can cause an increase in motility and glandular secretory activity?
Stimulation of preganglionic parasympathetic (cholinergic) nerve fibers of the muscularis
What causes decreased motility?
Stimulation of postganglionic sympathetic (adrenergic) nerve fibers on the smooth muscle cells
What are the effects of the parasympathetic nervous system on the GI system?
Promotes digestion and absorption by stimulating GI secretions and peristalsis while inhibiting sphincter muscle contraction
What are the effects of the Sympathetic Nervous System on the GI system?
- Inhibits digestion and absorption
- Inhibit motility and secretion
- Stimulate sphincter tone and vasoconstriction
Explain the process of sympathetic innervation of the GI system.
- Brainstem of the spinal cord
- Preganglionic cholinergic neurons
- Sympathetic ganglia
- Postganglionic adrenergic and cholinergic neurons
- Longitudinal muscle layer, circular muscle layer, muscularis mucosae, blood vessels
- Secretory cells in mucosae
Explain the process of parasympathetic innervation of the GI tract.
Track 1:
1. Vagal nucleus / medulla
2. Vagus nerves preganglionic cholinergic
3. Vagovagal reflexes (ex: receptive relaxation of the stomach)
4. Myenteric plexus
5. Submucosal plexus
6. Secretory cells and endocrine cells in mucosae
Track 2:
1. Sacral cord
2. Pelvic and splanchnic nerves
3. Postganglionic cholinergic and adrenergic
4. Local reflexes (ex: gastrocolic reflex)
5. Endocrine cells and secretory cells in mucosae
Vagovagal Reflex
GI reflex circuits where afferent and efferent fibers of the vagus nerve coordinate responses to gut stimuli via the dorsal vagal complex in the brain
Astrocytes
send out a signal when they detect low blood glucose levels and thus trigger stomach contractions (not normally associated with communication: hindbrain glucoprivation)
Cephalic Phase
- Accounts for ~30% of total gastric secretory response
- CNS reflex that can be initiated by thinking about food or by seeing, smelling, and tasting it
- Stimuli transmitted through the vagus nerve
Explain the enzymatic digestion of ingested foods by salivary glands.
Amylase
Explain the enzymatic digestion of ingested foods by the stomach.
Gastric (acid), pepsinogen -> pepsin
Explain the enzymatic digestion of ingested foods by the liver.
Bile
Explain the enzymatic digestion of ingested foods by the pancreas.
- Amylase
- Trypsinogen -> Trypsin -> Other Zymogens -> Chymotrypsin and carboxypeptidases
- Lipase
Explain how pancreatic proteases are secreted.
In inactive zymogen form (Trypsinogen, chymotrypsinogen, and procarboxypeptidase) along with trypsin inhibitor (prevents premature activation)
Explain the enzymatic digestion of ingested foods by brush borders (SI Mucosa).
- Lactase, Maltase, Sucrase
- Enterokinase (Trypsinogen -> Trypsin)
- Amin-peptidases
Explain the process of absorption of nutrients.
- Food converted to chyme
- Chyme expelled into the duodenum
- Simple sugars and amino acids are absorbed into blood capillaries of the villi; digested fat products in chyme form into chylomicrons, which diffuse into lacteals (lymphatic system) and form chyle that ultimately enters venous circulation
- Unabsorbed food material moves along the jejunum and ileum and into the large intestines
What types of vitamins are passively absorbed?
- Water soluble (B,C)
- Fat soluble (A, D, E, K)
Vitamin B12
Large charged molecule; in order to be absorbed, it must first bind to intrinsic factor (produced by gastric parietal cells) which is then endocytosed in the ileum
What does a deficiency of intrinsic factor or vitamin B12 cause?
Pernicious anemia
Iron (Fe)
Dietary iron is released in relatively large amounts after the digestion of proteins such as myoglobin and hemoglobin (meats)
Where is Iron primarily absorbed?
- Duodenum and proximal jejunum
- Converted to ferrous (Fe2+) form for absorption
- Within cell, it is bound by one or more iron-binding proteins and delivered to the basolateral membrane, where it complexes with transferrin
- Then transported in plasma bound to transferrin and stored within cells as ferritin
What is caused by a deficiency of iron or defects iron transport and storage?
Iron deficiency anemia and anemia of chronic disease
What couples to allow sodium entry?
Organic solutes (hexoses, dipeptides, amino acids, water-soluble vitamins)
How are digested carbohydrates absorbed?
- Three forms: glucose, galactose, fructose
- Absorbed by the small intestine (uptake across the apical membrane into the epithelial cell (enterocyte) and their exit across the basolateral membrane into portal circulation)
1. Sodium-dependent glucose co-transporter (sodium-glucose linked transporter, SGLT1): found in apical membrane of enterocytes; transports glucose and galactose into the cell
2. Single transported (GLUT2): responsible for the movement of glucose, galactose, and fructose from the basolateral membrane of the enterocyte into portal circulation
How are digested proteins absorbed?
- Peptides and amino acids are co-transported into enterocytes with Na+
- Products of protein digestion are absorbed as small peptides (major) and free amino acids (minor) through cotransport with Na+ into enterocytes
- In the cytoplasm of enterocytes, the peptides are degraded to amino acids, which then enter the portal blood -> liver
How are digested lipids absorbed?
- Lipids -> fatty acids and monoglycerides
- Absorbed across intestinal mucosa
- Re-esterified to produce triglycerides
- Triglycerides packaged as chylomicrons and transported through the intestinal lymphatics to the thoracic duct -> left subclavian vein (not portal vein)
Splanchnic Circulation
Derived entirely from the celiac artery, the superior mesenteric artery, and the inferior mesenteric artery
Hepatic Portal Circulation
Venous blood from intestine, stomach, pancreas, and spleen reaches sinusoids of the liver by way of the portal vein
Enterohepatic Circulation
- Most of the bile salts entering intestinal tract via bile are absorbed in the ileum and returned via the portal vein to the liver
- Hepatocytes reclaim bile salts from hepatic portal vein (ileum -> liver)
- Bile salts are absorbed by specific sodium-coupled transporters in the ileum into the hepatic portal blood
- Liver -> Common bile duct -> gallbladder -> duodenum (bile salts) -> 5% lost in feces
What does impaired reabsorption of bile salts cause?
Malabsorption of fat (steatorrhea) and impaired absorption of the fat-soluble vitamins (vitamin A, D, E, K)
What does the integrated extrinsic and intrinsic (enteric) networks regulate?
Peristaltic contractions of the muscularis and movements of the muscularis mucosae
What happens to the contractions after most of the meal is absorbed?
The segmenting contractions in SI cease and are replaced by a pattern of peristaltic activity (migrating myoelectric complex (MMC))
Migrating Myoelectric Complex (MMC)
Moves undigested food to LI and prevents bacterial multiplication in SI
What initiates MMC?
Increase in plasma concentration of motilin
Megacolon/ Hirschsprung Disease
Autonomic smooth muscle ganglia do not develop -> aganglionic segment contracts but cannot relax
How can the GI tract dilate or become obstructed?
Obstruction may be anatomical or functional resulting in dilation of the lumen proximal to obstruction
How is the GI tract an important source of biologically active substances?
Each portion produces unique enzymes, hormones, and immunoglobulins
What is the function of the intestinal mucosa?
An interface and a barrier between the internal and external milieu that is maintained actively (requires energy) and should be kept intact
What makes the GI tract prone for hemorrhage or ischemia?
Abundant blood flow and superficial location of blood vessels in the mucosa
Dysphagia
Abnormal swallowing
Dyspepsia
Abnormal digestion
