GI system and metabolism week 1

Question 1

energy input

energy output

Answer 1

energy input: derived from food intake

energy output:

• internal work: cellular processes necessary to sustain life, heat production, temp regulation-the “metabolic cost of living”
• external work: work performed by skeletal muscle

Question 2

Define neutral, positive, and negative energy balance.

Where is energy derived from in negative energy balance?

Answer 2

Question 3

The GI system contributes to the maintenance of homeostasis by transferring ____, ____, and _____ from the external environment to the internal environment in order to do what 2 things?

Answer 3

The GI system contributes to the maintenance of homeostasis by transferring nutreints (carbs, lipids, protines, vitamines, nutritional cofactors), electrolytes, and water from the external environment to the internal environment in order to maintain constancy of the internal environment and distribute energy to storage and utilization sites within the organism.

Question 4

What are 5 overall functions of the GI system? List and define.

Answer 4

1. Digestion and absorption of dietary nutrients and calories
2. Excretion of waste material. For example: nondigested/nonabsorbed food products, colonic bacteria, heavy metals (iron, copper), several organic anions and cations (e.g. drugs).
3. Fluid and electrolyte balance
4. Immune function (mucosal immune system GALT = gut-associated lymphoid tissue). Important for protection against microbial pathogens, permits immunologic tolerance to potentially immunogenic dietary substances and bacteria that reside in the GI tract (colon).
5. Non-immunologic defense: secretions, peristalsis, epithelial cell permeability barrier.

Question 5

What are the 4 basic digestive processes? (just list)

Answer 5

1. Motility
2. Secretion
3. Digestion
4. Absorption

Question 6

Define motility. What 2 types of propulsion are there?

Answer 6

motility: muscular contractions that mix and move the contents of the digestive tract (normally orthograde propulsion; retrograde propulsion occurs, e.g. vomiting)

Question 7

What are the two types of secretion by the GI system? What do they consist of?

Answer 7

exocrine glands: secrete digestive juices, consisting of water, electrolytes, and specific organic constituents important for digestive process (enzymes, bile salts, mucus)
endocrine glands: hormones for regulation of the GI system

Question 8

What is digestion? What are the 3 categories of energy rich foodstuffs?

Answer 8

digestion: degradation of structurally complex foodstuffs by digestive enzymes.
3 categories of energy-rich foodstuffs: carbohydrates, proteins and lipids

Question 9

absorption

Answer 9

absorbable units as a result of the digestive process are transported along with water, vitamins andelectrolytes from the lumen of the GI tract into the blood and lymph

Question 10

What are the primary and accessory digestive organs?

Answer 10

I. Primary digestive organs
• digestive tract: continuous tube, about 30 feet long, consisting of:
• mouth
• pharynx
• esophagus
• stomach
• small intestine (duodenum, jejunum, ileum)
• large intestine (cecum, appendix, colon, rectum)
• anal canal

II. Accessory digestive organs
• salivary glands
• exocrine pancreas
• biliary system: gallbladder and liver
• endocrine glands/cells

Question 11

What are sphincters? What are their functions?

What sphincters are located in the GI tract?

Answer 11

Sphincters: specialized circular muscle structures (smooth muscle or skeletal muscle) that separate compartments of the GI tract and control flow of luminal content between compartments (in parentheses times taken by first part of a meal to reach various parts of GI tract after leaving mouth):

mouth/pharynx

• pharyngoesophageal sphincter (upper esophageal sphincter)

esophagus

• gastroesophageal sphincter (lower esophageal sphincter; 3 seconds)

stomach

• pyloric sphincter/valve
• small intestine with chole-docho duodenal sphincter (sphincter Oddi); barrier towards biliary system and pancreas; 1-5 minutes; latter part of a meal takes 3-5 hours to leave stomach)
• ileocecal sphincter/valve (4.5 hours)

large intestine (cecum 5.5 hours, transverse colon 6.5 hours, descending colon 9.5 hours, sigmoid colon 12-24 hours)

• internal anal sphincter
• external anal sphincter

attached is slide 19 of notes

Question 12

What are the 4 layers of the wall of the GI tract? (just list)

Answer 12

1. mucosa
2. submucosa
3. muscularis externa
4. serosa

Question 13

What is found in the mucosa? What are the functions of its components?

Answer 13

Mucosa
• epithelial cell layer: protection, secretion and absorption
• lamina propria: connective tissue cell layer, lymphoid tissue, glands (e.g. mucus)
• muscularis mucosae: smooth muscle cell layer, inner surface folding (folds, ridges, valleys –> increased surface area)
• exocrine cells: e.g. HCl secreting parietal (=oxyntic) cells in stomach
• endocrine/paracrine cells: gastrointestinal hormones

Question 14

What is contained in the submucosa?

Answer 14

Submucosa
• connective tissue; large blood vessels, submucosal glands, lymphoid tissue
• submucosal nerve plexus (Meissner’s plexus)

Question 15

What is the function of the muscularis externa? What are the two layers? What is contained within the muscularis externa? (be specific)

Answer 15

Muscularis externa:
• contraction of muscularis externa mix and propel contents in aboral direction
• smooth muscle cell layer separated into:
inner circular layer
outer longitudinal layer
• myenteric nerve plexus (Auerbach’s plexus) located between circular and longitudinal muscle layer

Question 16

What is the serosa? What type of cells are contained there? What is its function?

Answer 16

Serosa
• connective tissue covering digestive tract covered with layer of squamous mesothelial cells; secretion of serous liquid (lubrification, prevents friction)

Question 17

Describe the 3 basic integrative regulatory systems of GI function (course objective).

Answer 17

GI motility is regulated by specific properties of smooth muscle cells, nervous system, and hormones.

Question 18

What is the effector organ of GI motility? What are the membrane potential oscillations of GI smooth muscles called?

What is the name of the pacemaker cells of GI smooth muscle and what is their function?

How is pacemaker activity controlled?

Answer 18

Intestinal smooth muscle cells are the effector organ of GI motility.

• Gastrointestinal smooth muscle cells are electrically coupled and their resting membrane potential oscillates with a rhythm characteristic of each segment of the GI tract.
• The membrane potential oscillations (called slow waves) originate from intestinal pacemaker cells interstitial cells of Cajal) and control the timing and force of contractions of GI smooth muscle.
• Pacemaker activity is autonomous, but can be modulated by neural and hormonal activity.

Question 19

Describe the branches of neural regulation of the GI tract.

Answer 19

Question 20

Where in the GI tract is there somatic innervation?

Answer 20

Somatic innervation (voluntary control)

• oral phase of the swallowing act
• tonus of the external anal sphincter

Question 21

What is the enteric nervous system (ENS)? What are its functions?

Are the fibers myelinated?

What 2 major plexuses are in the ENS?

Answer 21

Enteric nervous system = miniature nervous system in the wall of the GI tract. It is a major network of ganglia and interconnecting neurons (about 108 neurons!) with nonmyelinated nerve fibers (nerve plexuses). It is a semi-autonomous nervous system (“the little brain of the gut”) that controls the motor and secretory activities of the GI system, but receives sympathetic and parasympathetic input. The enteric nervous system is a subsystem of the ANS.

2 major plexuses
• myenteric plexus (Auerbach’s plexus)
• submucosal plexus (Meissner’s plexus)

Question 22

What type of fibers (pre or postganglionic) and what NTs are secreted through sympathetic innervation of the GI tract?

Where are cell bodies of these neurons located? Where do the fibers terminate?

What are the effects of sympathetic efferent innervation of the GI tract?

Answer 22

Sympathetic efferent innervation
• Primarily via postganglionic adrenergic fibers with cell bodies in prevertebral and paravertebral plexuses that terminate in submucosal and myenteric plexus.
• Typically inhibitory effect on synaptic transmission in the enteric plexuses.
• Effects of sympathetic activity
- vasoconstriction of gastrointestinal blood vessels
- inhibition of glandular function
- muscularis externa: inhibition of motor activity
- contraction of muscularis mucosae and certain sphincters.

Question 23

What nerves provide parasympathetic innervation to the GI tract? What NT is released?

Where do these fibers terminate?

What are the effects of parasympathetic innervation of the GI tract?

Answer 23

Parasympathetic efferent innervation
• Vagus nerve (upper gastrointestinal tract to transverse colon) and parasympathetic fibers of pelvic nerves. Predominantly cholinergic fibers that terminate on ganglion cells of intramural plexuses.

Typically excitatory effects

• Stimulation of motor (smooth muscle cells) and secretory activity.

Question 24

Note that there is typically no direct connection btwn the CNS and GI. The CNS exerts its effects through innervation of the enteric NS. An exception is blood vessels within the GI tract.

Question 25

What types of neurons do the myenteric plexus (Auerbach’s plexus) contain? What are some NTs/chemical secreted that have excitatory and inhibitory effects and on what structure within the GI tract are these released?

Answer 25

Myenteric Plexus (Auerbach’s plexus) contains:
• motor neurons: projecting to longitudinal and circular muscle layer of muscularis externa. Excitatory (acetylcholine, substance P), inhibitory (vasoactive intestinal peptide = VIP, nitric oxide = NO)
• sensory neurons (mainly distension (stretch)-sensitive neurons)
• interneurons

Question 26

What types of neurons does the submucosal plexus (Meissner’s plexus) contain? What structures within the GI tract does it exert its effects?

Answer 26

Submucosal Plexus (Meissner’s Plexus) contains:
• secretomotor neurons to glandular, endocrine and epithelial cells
• sensory neurons (chemical, mechanical)
• interneurons
• vasodilator neurons

Question 27

What are enteric reflexes?

What are short range reflexes? What are long-range reflexes?

Answer 27

Integration of neuronal control of GI function: Enteric reflexes
Enteric reflexes play a major role in controlling motility, secretory and absorptive functions of the GI tract.
• Short-range (intrinsic) reflexes: elements of the reflex arch are located within the intestinal wall.
• Long-range reflexes: extrinsic autonomic pathways between gut and CNS are involved. For example, activity in the stomach effects activity in the distal colon.

Question 28

From where are GI hormones released? How are they transported?

What are their effects?

What are stimuli for release of GI hormones?

Answer 28

Gastrointestinal hormones: Endocrine gland cells in the mucosa release hormones into the blood stream. Hormones are transported by the blood to other regions of the GI tract and exert inhibitory and excitatory effects on smooth muscle cells and exocrine gland cells. Stimuli for release are local changes in lumenal content of the GI tract or through nerve stimulation from the enteric nervous system.

Question 29

What are the 4 major GI hormones? (just list)

Answer 29

1. gastrin
2. secretin
3. cholecystokinin (CCK)
4. gastric inhibitory peptide (GIP)

Question 30

What are the functions of gastrin?

Answer 30

• stimulates secretion by parietal cells (HCL)
• is trophic to stomach and small intestinal mucosa

Question 31

What are the functions of secretin?

Answer 31

• stimulates aqueous NaHCO3 by pancreatic duct cells
• stimulates aqueous NaHCO3-rich bile by liver
• inhibits gastric acid secretion
• is trophic to exocrine pancreas

Question 32

What are the functions of cholecystokinin (CCK)?

Answer 32

• stimulates digestive enzyme secretion by pancreatic acinar cells
• stimulates pancreatic NaHCO3 secretion
• causes gallbladder contraction
• inhibits gastric emptying (pyloric constriction)
• is trophic to exocrine pancreas

Question 33

What are the functions of gastric inhibitory peptide (GIP)?

Answer 33

• inhibits gastric acid secretion
• stimulates insulin secretion by endocrine pancreas

Question 34

What cells release paracrine agonists? How do they reach target cells? Name examples of paracrine agonists.

Answer 34

Paracrine agonists are secreted by paracrine cells in the vicinity of target cells (e.g.histamine released by enterochromaffin-like cells in the stomach wall) and reach target cells by diffusion.
• Paracrine agonists released by cells of the gastrointestinal immune system (antibodies, inflammatory mediators).

Question 35

How much of the mass of the GI tract does the GI immune system compromise? What are some of its functions?

Answer 35

GI immune system

• half of the mass of immune cells in the body are in the GI tract
• antibody secretion to specific food antigens
• immunologic defense against pathogenic microorganisms.

Question 36

Name 3 pancreatic hormones. What pancreatic cells release hormones? What are these hormones responsible for?

Answer 36

Pancreatic hormones (insulin, glucagon, somatostatin) are produced and secreted (endocrine pancreatic secretion) by the islets of Langerhans. These hormones are essential for the regulation of metabolism.