Lecture 32

Question 1

explain the GI tract

Answer 1

• main role is to transfer nutrients and water from food into the body
• a 4.5m continuous tube from mouth to anus
  -> lumen is part of the external environment
• accessory glands and organs are functionally associated with GI tract but material does not pass through them
  -> salivary glands
  -> liver
  -> gallbladder
  -> pancreas

Question 2

what are the functions of the GI tracts organs

Answer 2

• oral cavity = digestion
• esophagus = passage to stomach
• stomach = continued digestion
• small intestine
  -> duodenum, jejunum, ileum
  -> most digestion and absorption occurs here
• large intestine
  -> colon absorbs water and electrolytes, production of feces
• rectum = holds feces until defecation through anus

Question 3

what are the sphincters of the GI tract

Answer 3

• serve as one way valves
• tonically constricted to separate adjacent organs
• upper esophageal sphincter
• lower esophageal sphincter = separate stomach from esophagus
• pyloric sphincter = separates stomach from small intestine
• ileocecal sphincter = separate small intestine from large intestine
• internal and external anal sphincters

Question 4

what are the 4 tissue layers of the wall of the GI tract through the stomach and intestines

Answer 4

1. mucosa (innermost layer)
2. submucosa
3. muscularis externa -> circular muscle
   -> longitudinal muscle
4. serosa (outermost layer)
   - mesentery - sticks the intestines together

Question 5

what is the mucosa comprised of

Answer 5

• innermost later
• epithelium
  -> lines the lumen of the GI tract
  -> single layer of epithelial cells
  (transporting cells, water, ions, digestive products)
  (secretory cells, enzymes, mucus, hormones)
  (stem cells)
• lamina propria
  -> loose connective tissue
  -> contains glands, lymph vessels, blood supply, nerve fibers
• muscularis mucosae
  -> thin layer of smooth muscle separates from submucosa
  -> can contract to later the surface area of the mucosa for absorption

Question 6

what is the submucosa comprised of

Answer 6

• loose connective tissue
• contains nerve trunks, blood supply and lymph vessels
  -> intestine has submucosal plexus (part of the enteric nervous system)

Question 7

what is the muscularis externa composed of

Answer 7

• muscularis externa is outer wall of GI tract
• 2 layers of smooth muscle cells
  -> inner circular layers (constrict lumen)
  -> outer longitudinal layers (shorten tract)
  -> stomach also has additional oblique layer just below submucosa
  -> the myenteric plexus (another part of the enteric nervous system) is located between two muscle layers

Question 8

what is the serosa comprised of

Answer 8

• outer layer covering the entire GI tract and is continuous with peritoneal membrane that lines the abdominal cavity
  -> comprised of secretory epithelium and connective tissues layers
  -> lubricates, protects, suspends

Question 9

describe the anatomy of the GI tract wall (stomach vs small intestine)

Answer 9

• small intestine has more myenteric plexus (submucosal plexus)
• small intestine has microvilli on its surface

Question 10

what are the 4 basic GI processes

Answer 10

• secretion = movement of material from cells into lumen or ECF
• digestion = chemical and mechanical breakdown of food into absorbable units
• absorption = movement of material from GI lumen to ECF
• motility = movement of material through the GI tract as a results of muscle contraction

Question 11

what are peristaltic contractions

Answer 11

• propel contents forward
• circular muscles contract just behind the bolus to push it forward

Question 12

what are segmental contractions

Answer 12

• mixing contents to break up food and increase exposure to enzymes
• short segments of intestine alternately contract and relax

Question 13

what does the GI system secrete

Answer 13

• Na+, K+, Cl-, HCO3-, H+ and water follows by osmosis
• digestive enzymes
• mucus
• bile
• total fluid input and removed = 9L

Question 14

explain the enteric system

Answer 14

• a self sufficient nervous system
  -> contains a complete reflex circuit and can operate without input from CNS or PNS
• myenteric plexus and submucosal plexus
• regulates motility and secretions throughout the GI tract
• modulated by the autonomic nervous system

Question 15

what are the 3 regulations of GI function

Answer 15

• short reflexes
• long reflexes
• GI peptides

Question 16

explain short reflexes

Answer 16

• integrated in the enteric nervous system and occurs entirely in the gut wall
  -> sensory receptors send information to enteric nervous system via interneurons

Question 17

explain long reflexes

Answer 17

• integrated within the CNS
• may originate in or outside of the GI tract
  -> feedforward and emotional reflexes are initiated and integrated entirely outside the GI tract -> called cephalic reflexes

Question 18

explain GI peptides

Answer 18

• may be secreted in both short and long reflexes, or independently
• signaling molecules that alter GI secretion and motility and eating related behaviors

Question 19

what is the pathway for neural regulation of GI function - short reflexes

Answer 19

local stimuli = distension, presence of food, osmolarity, acid
enteric nervous system
-> sensory receptors and neurons
-> interneurons
-> myenteric and submucosal plexuses
-> GI effectors
1. smooth muscle
2. secretory cells
-> response
1. altered motility
2. altered secretions

• after local stimuli
  -> secretory cells of the stomach and small intestine
  -> Gi peptides
  -> Gi effectors
• Gi peptides can be hormones, paracrine, or NT

Question 20

what is the pathway for neural regulation of GI functions - long reflexes

Answer 20

• look at slide

Question 21

autonomic regulation of GI function

Answer 21

• SNS -> inhibits GI motility and secretions
• PNS -> stimulates GI motility and secretions