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PHGY 210 Study Guide - Somatostatin, Lactase, Taste


Department
Physiology
Course Code
PHGY 210
Professor
Ann Wechsler

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SUMMARY: DIGESTION
General information about the GIT:
- Main role in homeostasis: provide appropriate amount and type of nutrients to body.
-Main properties:
oTubular nature
oCommunication with external environment:
The lumen is considered as being part of the external environment
-2 types of evolutionary development:
oGrowth:
In length (the human GIT is 3x the height of an average man)
Growth of the internal surface (inner surface is 600x larger than the outer
surface because of convolutions). Role is to increase efficiency of
diffusion.
oDifferentiation:
The GIT is not a simple structure; it is a sequence of interconnected
organs
However, if you take a cross section anywhere, you will get the same 4
typical layers making the wall of the GIT.
-4 layers of the gut wall (from outside to lumen):
oSerosa: thin but though layer of tissue
oMuscularis externa:
Longitudinal layer: muscles fibers parallel to the tube responsible for the
shortening of the GIT.
Circular layer: muscle fibers at right angle with the long axis of the gut.
They cause constriction of the GIT when they contract.
Note: the GIT is composed of smooth muscle everywhere except
in mouth, pharynx, upper 1/3 of the esophagus and anus, which
have striated muscle.
oSubmucosa: loose connective tissue. Characterized by having a lot of nerves,
lymphatics and blood vessels.
oMucosa:
Muscularis mucosa: smooth muscle
Lamina propria: connective tissue with immune properties
Epithelial cells: secretory and absorptive cells
-GIT functions:
oMotility:
Propulsion
Physical breakdown
Contractile activity of the muscle of the GIT
oSecretion:
Chemical breakdown
oAbsorption:
Transfer to circulation
Raison d’être of the GIT
Digestive/absorptive efficiency:
Carbohydrates: 99%
Fat: 95%
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Protein: 92%
The enteric nervous system:
- Unique to the GIT
- Independent, integrative nervous system
- Initiates, programs regulates and coordinates activities of muscular and secretory
elements of the GIT
Gut wall innervation:
Purple: sensory neurons
Red: excitatory neurons (release
ACh on muscarinic receptors of
the muscle and secretory cells)
Orange: inhibitory neurons
(release NANC)
You also have interneurons in the
plexuses
We will consider the submucosal
plexus and the myenteric plexus as
one functional unit.
All elements of a reflex arc are
present
Note 1: predominant innervation to the circular muscle layer is inhibitory
Note 2: muscarinic receptors are blocked by atropine
Sphincters :
Name Anatomy Properties Roles Hormonal
regulation
Neural regulation
UES
(upper
esophageal
sphincter)
Corresponds
to the
cricopharyng
eus muscle
Normally
closed
Relaxed
during
deglutition
Prevents
from
entering
esophagus,
where
pressure is
lower than
in the
pharynx
Closure: impulses
from the CNS
mediated by the
vagus, releasing
ACh on nicotinic
receptors causing
muscle contraction
Relaxation:
cessation of
impulses during
deglutition
LES
(lower
esophageal
sphincter)
Anatomicall
y
insignificant,
but
functionally
very
important
Consists of
Closure is
myogenic: it
is an
intrinsic
physiologic
sphincter
Assisted by
presence of
Prevent
reflux of
gastric juice
in the
esophagus
Small
influence on
resting tone
Note that
gastrin does
NOT play a
role in
regulating
Mall influence on
resting tone
Relaxation:
neurogenic.
Caused by local
release of NANC by
enteric inhibitory
2

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lower 4 cm
of esophagus
Inferior 2 cm
are in the
abdominal
cavity
intraabdomi
nal segment
the LES
under
physiologica
l conditions.
Estrogen
may lower
pressure
within the
sphincter
neurons
Pyloric
sphincter
Anatomicall
y well
developed
(massive
muscle)
Very narrow
lumen that
behaves as
sphincter
Functionally
insignificant
Open at rest
Closed by
antral
peristalsis
- -
Ileocecal
valve
Between SI
and colon
Prevents
bacteria of
colon from
entering in
SI
- -
Spincter of
Oddi
At the
intersection
of common
bile duct and
duodenal
lumen, near
pancreatic
duct
When
closed, bile
is stored in
gallbladder
Control flow
of bile in
duodenal
lumen. - -
Autonomic innervation:
Parasympathetic Sympathetic
Synapses Preganglionic: synapses only
on the enteric neuron
Postganglionic: synapses first
outside of the wall, and the
postganglionic fiber than
synapses on the enteric
neuron
Neurotransmitter ACh First synapse: ACh
In GIT wall: NA (NE)
Receptor Nicotinic cholinergic
receptors
First synapse: nicotinic
cholinergic receptors
In the GIT wall:
adrenoreceptors
Origin Medulla Spinal cord
Overall effect Excitatory Inhibitory
Also innervates smooth
muscle in blood vessel in the
wall, causing vasoconstriction
3
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