GI 1

Question 1

What are the functions and goal of the GI tract

Answer 1

Goal is to intake, breakdown and digest food such that we can extract nutrients from it
Other functions that lead to this goal are ingestion, secretion (as in of digestive enzymes), motility, digestion, absorption and defecation

Question 2

Digestive and absorptive capacity of the digestive system

Answer 2

Really good, overall very efficient, can digest and reabsorb potentially
99% of all carbs
95% of all fats
92% of all proteins

Question 3

GI structure (generally speaking)

Answer 3

It’s tubular in nature, meaning its one long cohesive thing that links to external points, mouth and anus
The lumen is technically extracellular environment because its a tube through the body, anything in the tube has not officially entered the body, acts as a barrier in this way

Question 4

Length of GI tract in humans

Answer 4

Living: about 4.5 m, so longer than we are tall fs
Cadaver: up to 10m, because when you’re dead theres much less muscle tone contributing to the tenseness of the tract, leading to it being looser

Question 5

What can be said about the surface area of the GI tract

Answer 5

The external part of the tract has a lower surface area than the internal part, up to 600x larger, due to all the folds and microvilli and rugae used to maximize the surface area available for absorption

Question 6

What are the layers of the wall of the GI tract

Answer 6

From lumen to external layer:
Mucosa: innermost, in contact with pseudo external environment, epithelial lining
Submucosa
Muscularis Externa: thickness here varies, usually 2 but 3 in the stomach
Serosa: outermost serous lining

Question 7

Serosa

Answer 7

Thin layer, outermost, of connective tissue of the GIT, its tough and protective
In some parts of the digestive tract it’ll be continuous with the mesentery or the peritoneum, anchoring the GIT in place

Question 8

Muscularis externa

Answer 8

Second outermost layer of the GIT
Very important for motility and mixing, muscle obviously so moves food via different mechanisms
2 layers of muscle: the outer one is the longitudinal layer (muscle cell is in cross section in a cross section) and the inner is circular (muscle cell is longitudinal section in a cross section, wrapping around the tube) (this is about how the muscle fibres run)

Question 9

How does the muscularis externa differ from location to location of GIT

Answer 9

Can differ in type of muscle as well as layer number (theres 1 exception)
In the upper 1/3 of the esophagus for example theres striated muscle, aka voluntary, as well as in the external anal sphincter
The rest is smooth
They will differ in control and such

Question 10

Submucosa

Answer 10

Loose layer of CT, lymphatics and blood vessels will be present, not crazy important

Question 11

Mucosa

Answer 11

Mucosa itself has 3 layers
Muscularis mucosae: a thin layer of smooth muscle pertaining to the mucosa, smooth muscle everywhere in GIT
Lamina propria: interior to MM, loose CT, lymphatics and blood vessels here too, feeds the
Epithelial layer: epithelial cells as well as glandular cells that make up the GIT

Question 12

ENS

Answer 12

Enteric nervous system, refers to a subset of the nervous system that governs the GIT
These neurons reside within the walls of the GIT
It’s considered to be independent from the CNS, albeit they work together, but it can operate without input from it
Has all neurons required to functional autonomously and have reflexes, etc
Integrative in the sense that what happens in point A of GIT will be relayed to point B

Question 13

What does the ENS do

Answer 13

Initiates, programs, regulates and coordinate activities of the muscular, secretory and absorptive components of the GIT

Question 14

Describe the GI wall innervation (layer not function)

Answer 14

Serosa on the outer most layer-> longitudinal layer of muscle (part of muscularis externa)-> the myentery plexus-> the circular layer of muscle (also part of the muscularis externa)-> Submucosal plexus-> then the submucosa and mucosa

Question 15

Plexus

Answer 15

A connection of nerve cell bodies
2 in the GI tract, the Myenteric and the Submucosal, they have different functions but to us they will function as a unit of the ENS

Question 16

Sensory neurons in the GI tract innervation

Answer 16

Sensory neurons have receptors that branch into either the muscle or mucosa, includes but is not limited to stretch receptors, chemoreceptors, osmoreceptors

Question 17

Motor/effector neurons in the GI tract innervation

Answer 17

They activate the secretory and muscle cells in the mucosa and muscle layers respectively

Question 18

Interneurons in the GI tract innervation

Answer 18

Connect different parts of the GI tract to contribute to the integrated nature of the system

Question 19

How do the plexuses differ in function (broadly)

Answer 19

The Myenteric plexus has the circular layer of muscle on one side (inner) and the longitudinal layer on its outside, it primarily innervates these and therefore is in charge of gastric movements like peristalsis
-this isnt to say it does not innervate these mucosa, its just a much lesser reach, would be secondary, not prominent
The Submucosal plexus is next to teh mucosa and circular layer of muscle, so its reach is also to both, but primarily teh mucosa for controlling secretion
-again, it does innervate the circular layer, but its secondary

Question 20

How does an excitatory effector neuron do its thing

Answer 20

Reaches its target, say a muscle cell, releases ACh, it will bind to muscarinic receptors on the muscle cell, this leads to the depolarization of the muscle cell and leads to contraction
Note: if it had reached a secretory cell instead, same idea, result will be a secretion though

Question 21

How do inhibitory effector neurons do their thing

Answer 21

Reach their target, either muscle or secretory cell, release NANC (non-adrenergic non-cholinergic) peptides, don’t need to know which, an example is NO, it will hyperpolarize the cell such that it does not secrete or contract

Question 22

What can muscarinic receptors be blocked by

Answer 22

Atropine, will prevent the blocking of Ach

Question 23

Short enteric reflexes

Answer 23

Aka intramural reflexes,occurs independent of CNS
Basically reflexes that are able to occur simply due to the players already present in the GI wall
This includes chemo, osmo and mechanoreceptors, the interneurons and the effector neurons (which can modulate the secretory and muscular cells

Question 24

What’s unique about ANS control over the ENS

Answer 24

When the ANS overrides and wants something done, it doesn’t directly tell the tissues, it tells the neurons of the ENS so they can go on and tell the effector tissues

Question 25

Explain the control of the parasympathetic NS on the neurons of the ENS

Answer 25

Pre-ganglionic neurons comes in, goes all the way to the gut wall, finds the ENS neuron, releases Ach, ENS neuron has nicotinic receptors, the ENS neuron will then also secrete ACh and the effector cells will have muscarinic receptors Net result is an increase in activity, makes sense because digestion increases when youre chilling

Question 26

Explain the control of the sympathetic NS on the neurons of the ENS

Answer 26

Pre gang neuron synapses on post gang, releases Ach, post gang catches it with nicotinic receptor, it then reaches out to teh ENS neuron, releases NA (noradrenaline), net result is a decrease in activity, adrenergic receptors

Question 27

What effect do para and sympathetic NS have on BV in the GIT

Answer 27

Para: leads to dilation of BV, makes sense because it’ll be most active during parasympathetic conditions Sym: leads to constriction of BV, makes sense because well want to reroute blood to more pressing locations