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PHGY 210 – Digestion
Lecture01
Wednesday, March 22, 2006
Ann Wechsler
DIGESTION NTC
Dr. Ann Wechsler
Room 1135 McIntyre
398-4341
[email protected]
Lecture 01 – March 22th 2006
GIT role in homeostasis
The main reason that
systems fct the way they
do
is
to
maintain
homeostasis. It insures
balance. We will examine
the way the GIT maintains
a constant supply of
nutrients.
Provide nutrients
- We obtain food from the
external
environment
(food = large particles) 
digestive
system

converted into absorbable
molecules - in order to be
absorbed, food must be converted into smaller molecules (monosaccharide, amino acids,
lipids,…)
 they enter the internal environment (circulation) further processed  and they are
ultimately distributed to cells, which use to provide the energy (ATP) and raw materials for
growth & repair and permits function & regulation of this latter  which are all derived from
food
So, this is the way the GIT contributes to homeostasis.
* We will examine what becomes of food within the digestive system; how is the food converted
into absorbable molecules and how do they enter the internal environment?
GI structure
External environment  earthworm
 internal environment
GI tract: it is a tube, which extends
from the anterior to the posterior end,
and it communicates w/ the exterior
environment at both ends.
The lumen (central cavity of the GIT)
is a continuation of the external
environment.
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
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PHGY 210 – Digestion
Lecture01
Wednesday, March 22, 2006
Ann Wechsler
There are 2 lines of development.
1- Growth (enormous elongation and expansion of the tube)
2- Differentiation (diff. regions of the tube become specialized to perform diff. digestive
functions)
* The tubular nature as well as the communication w/ the external environment is always
maintained throughout these 2 steps. These two elements are conserved thru growth and
differentiation
a) Tubular nature
b) Communication w/ external environment
Growth 1 – Elongation of the GI tube
In mammals (including humans), the tube is
much longer than the trunk and even the
height of the individual
The GI tract has a length of 4.5m in an
adult human (1.5 m)
In the cadaver, the length of the adult GI tract
is 9m. It is only half that length in a living
adult b/c the muscular elements in the wall of
the GIT are always in partial contraction,
making the tube shorter.
Growth 2 – of the internal surface area of
the tube
Lumen – Central cavity
* (continuation of the external environment)
The internal lining has many folds and outpushings, inpushings.
The result: internal surface area is 600 X greater than the
external surface area of the tube, reaching up to 200-250 m2
(about the size of tennis court). It allows for efficient
absorption.
* Surface area is a major component in allowing efficient
diffusion.
Different
iation
Instead of having a simple tube, there are
specialized cells that differentiate so that
they can perform different fcts of the
digestive system efficiently. We therefore
have a sequence of organs (mouth,
esophagus, stomach, small intestine,
large intestine, colon, rectum, and anus).
Notice the maintenance of the
connection w/ exterior and of the
tubular nature.
There are 3 major accessory structures
associated w/ GIT (from a functional and
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
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PHGY 210 – Digestion
Lecture01
Wednesday, March 22, 2006
Ann Wechsler
developmental point of view):
Salivary glands – release the product into the oral cavity
Liver – largest organ in the body; performs large # of functions, including release of digestive
secretions.
Pancreas – endocrine and exocrine gland
Anatomy of the GI tract wall – Vander’s p.581
If we took a cross-section b/w mid-esophagus and terminal portion of the large intestine, you
find same typical 4 layers that make up the wall. There are quantitative and qualitative changes
that occur.
If you start from the outside of the tube towards the
lumen
serosa - Thin but tough layer of connective tissue.
It surrounds the tube. In the abdominal tract, it is
continuous w/ the membrane lining the abdominal
cavity.
muscularis externa  layer of muscle (consists of
2 layers of muscle)
1- longitudinal fibers - outer layer of
longitudal fibres = parallel to the tube)
(when it contracts, the GIT shortens),
2- Circular fibers (innermost, wound around
the tube, at right angles to the tube, making the
lumen narrow) inner layer circular fibres
(when these fibers contracts, lumen
narrows)
- musculature in oral cavity, pharynx, upper 1/3 esophagus
and external anal sphincter are made up of striated (or
skeletal muscles)
The rest of the GIT musculature is smooth muscle. There is
no skeletal type muscle in this area. Smooth muscle has
specific properties (to be discussed later on)
submucosa
–
loose
connective tissue, housing
neuronal network (nerve
ganglia
and
fibers),
lymphatics, blood vessels
mucosa - innermost, which
consists of 3 layers muscularis
mucosae
–
muscular; outermost layer
of the mucosa; wherever the
musculature of the GIT is
smooth in the muscularis
externa, you will also find
smooth muscularis mucosae
- lamina propria (loose
connective tissue) –
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
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PHGY 210 – Digestion
Lecture01
-
Wednesday, March 22, 2006
Ann Wechsler
Connective tissue. Contains immune cells – these cells play important role against
invasion of microorganisms from external environment.
epithelial layer (secretory – exocrine and endocrine & absorptive cells) – critical
role in secretion (enzymes, into lumen) & it also contains cells where absorption
takes place.
* There may quantitative and qualitative differences as you go down the GIT, but all these layers
are typically recognized.
GIT functions
ROLE OF GIT: To convey food along the GIT, allowing it to be disrupted into small
molecules/particles, so that they may be absorbed into circulation.
* Absorption is the raison d’être of the GIT in all its complexities
3 principal activities of the GI tract:
1. Motility (Muscular activity in the wall = contractile activity):
- Propulsion (allows food to move along the tract)
- Physical breakdown (disruption of large masses into smaller
particles)
2. Secretion (Glandular activity of the epithelial cells + liver and pancreatic
cells):
Chemical breakdown – production of fluids containing enzymes, produced by
glands that permit chemical breakdown of food into small molecules.
The term digestion refers to the chemical breakdown of the food into
progressively smaller molecules
3. Absorption – Transfer of molecules from the lumen of the tract into the
circulation, where it will be distributed to the cells of the body.
Digestive / Absorbtive efficiency (Vander’s, pp.588-590)
Almost everything we eat is digested and ultimately
absorbed:
* This efficiency results from the fact that the 3
activities of the GI tract is highly integrated by
NEURAL and HORMONAL activity.
Enteric innervation (ENS)
The GI tract has its own innervation (ENS), such that it can fct independently from the CNS or
ANS.
This innervation allows for the integration and regulation of all the aforementioned activities.
GIT is capable of initiating activity
Activities of muscular and secretory elements (within a particular organ): initiates,
programs, regulates, coordinates activities of muscular and secretory elements within
particular organs.
GI tract wall innervation
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
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PHGY 210 – Digestion
Lecture01
Wednesday, March 22, 2006
Ann Wechsler
The ENS manifests itself as a huge number of neurons and interconnected fibers found in
ganglia, which are organized into 2 plexuses.
* Plexus: integrated collection of ganglia
1- Submucosal plexus – in the submucosa
2- Myenteric plexus - b/w the circular and longitudinal muscle
Structurally they are different; but for our purposes, the plexuses are considered as 1:
~ enteric plexus/innervation - a functional unit that comprises the submucosal and myenteric
plexus. This is b/c there are too many connections b/w the two to distinguish them one from the
other, from a functional stand.
Within the enteric innervation, we find all the elements of the reflex arc.
Sensory neurons (purple): can bring info from receptors (located in either the mucosa →
chemoreceptors, osmoreceptors or mechanoreceptors [stretch receptors] of the muscle).
Effector neurons (red): can activate secretory cells in the mucosa, or muscular cells in either
layer of the smooth muscle.
Large # of interneurons: integration of activity in a given region of the GIT.
If you stimulate at one point:
1- Activation a particular sensory fiber
2- This sensory fiber activates an effector fiber - Cause activation of a muscle cell.
* B/c of the presence of interneurons, there is also activation of an effector
neuron that may cause contraction in the longitudinal fibers (for example).
** It can activate enteric effector neurons which may cause secretion at another
point. So, you can cause secretion and contraction at a slight distance.
Whereas many of the effector neurons are excitatory (cause contraction or secretion by mucosal
cells), many of the enteric nerve fibers are inhibitory (orange).
* The predominant innervation of the circular layer of muscle is inhibitory.
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
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PHGY 210 – Digestion
Lecture01
Wednesday, March 22, 2006
Ann Wechsler
Summary:
-
ENS = 2 diff. plexuses (anatomically distinct, behave as a functional unit)
Include all the elements required for reflex arcs - Sensory neurons, effector neurons
and interneurons.
- ENS consists of ganglion cells and their processes. They synapse w/ smooth muscle
cells, endocrine and exocrine cells and other ganglion cells.
- Some enteric neurons are excitatory (release mostly Ach – acting on muscarinic
receptors on both smooth muscle and secretory cells)
- Some are inhibitory (release NANC ~ non adrenergic, non cholinergic transmitters)
* Also present are enteric sensory fibers w/ cell bodies in plexuses
Enteric neurons and transmitters
ACh (Muscarinic receptors)  blocked by atropine - the enteric innervation involves
excitatory enteric neurons that release ACh, acting on the muscarinic receptors on the muscle
(contract) or on secretory cell.
NANC  Non-adrenergic non-cholinergic - The muscle cell may also be innervated by an
inhibitory enteric neurons. It releases NANC (non-adrenergic, non-cholinergic
neurotransmitter). Nitric Oxide (NO) is a very common inhibitory neurotransmitter; there is also
peptide. There might even be some Purine like ATP that can also act like a neurotransmitter for
these neurons in the gut.
Short /intramural reflexes
The enteric innervation has all
the elements for the reflex arc:
These are known as
short/intramural reflexes.
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
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PHGY 210 – Digestion
Lecture01
Wednesday, March 22, 2006
Ann Wechsler
Stimulus

 Activation of chemoreceptors, osmoreceptors & mechanoreceptors
 Sensory fibers activate nerve plexus
 Efferent enteric neurons smooth muscle or gland

Response
* Although the gut can fct when separated from the CNS and ANS, there is normally an intact
ANS and CNS
Autonomic innervation of GIT – Vander’s pp.199-202
The gut wall receives innervation from
CNS via the ANS
a) Parasympathetic (preganglionic):
Synapse ONLY w/ enteric neurons (not
with muscle or anything else) by
releasing ACh (acts on nicotinic receptors)
b) Sympathetic (postglanglionic) – There
is a synapse within a ganglia OUTSIDE
the gut wall. The postganglionic cell
synapse also w/ the enteric neurons only.
Autonomic innervation of the gut wall
(diagram not to memorize)
Sympathetic
Emerges form the spinal cord. There are synapses
and the ganglia outside the wall of the GI tract
= postganglionic fibers innervate it
Parasympathetic
Vagus X (main parasympathetic nerve that reaches
the wall as preganglionic fibers) – esophagus,
stomach, small intestine & colon
Pelvic nerves – takes over the parasympathetic
function at colon & rectum.
Elements present in the ENS: sensory, effector and interneurons.
*There are also many sensory fibers sending CONSTANTLY info from the gut to the CNS
(medulla or the spinal cord).
Parasympathetic innervation = Preganglionic fibers that synapse w/ enteric neurons on
plexuses and release ACh acting on nicotinic receptors .
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
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PHGY 210 – Digestion
Lecture01
Wednesday, March 22, 2006
Ann Wechsler
Sympathetic innervation = Postganglionic fibers (presence of a synapse in a ganglia
outside the gut wall) that release NA (noradrenaline) in the vicinity of enteric neurons.
* Basis for long reflexes – Stimulation of a receptor causes a lot of integration.
There are effector fibers that can change the activity of the enteric neurons, either via
sympathetic or parasympathetic innervation.
Enteric Neurons & Autonomic Innervation - of muscle or glandular cells
Parasympathetic (EXCITATORY):
1 – Preganglionic neurons releasing ACh acting on nicotinic receptor of enteric neurons
2 – The enteric neurons release ACh acting on muscarinic receptors present on effector
cells
*Parasympathetic can also act on inhibitory interneurons. In this case, it activates these neurons
to release NANC transmitters to the receptor cells.
Sympathetic (INHIBITORY): Postganglionic cells releasing NA
Effects: a) on excitatory enteric neurons (decreases the excitation of enteric neurons and, thus,
diminish the contractile activity of the muscle)
b) on inhibitory enteric neuron (diminish the inhibition of effector cells by enteric
inhibitory cells. Thus, the effector cells will be more active = EXCITATION)
* When you stimulate via ANS, you can
stimulate long lengths of the GIT
Long reflexes:
Receptors on gut wall send info to the CNS
that react to these stimulus by sending efferent
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
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PHGY 210 – Digestion
Lecture01
Wednesday, March 22, 2006
Ann Wechsler
info to the GI plexuses via the ANS (Sympathetic/Parasympathetic)
* Many emotional states, as well as sensory (visual, sound, taste) also modulate or modify the
activity of the enteric neurons via ANS.
ANS role on the activity of ENS
Allows for integrated activity over longer distances along gut
Permits long and extrinsic reflexes.
In general – Parasympathetic: Excitatory (may also excite inhibitory neurons)
- Sympathetic: Inhibitory (may also inhibit inhibitory neurons)
Hormonal regulation of gut activity – Vander’s pp.589-590
Gut is regarded as the largest and most diversified endocrine system in the body. There are a
variety of substances that regulate it;
DES = diffuse endocrine system (scattered in mucosa)
There are 3 types of glandular cells:
 Autocrine  regulates itself
 Paracrine regulates nearby cell
 Endocrine a glandular cell releases its product into the circulation (capillary)
modulates activity of cell at a distance
Gut regulatory peptides
No steroid hormones released in the gut
1. released from mucosa into portal blood  liver systemic circulation
2. have multiple targets (released by gut and affects nearly any muscular or glandular cell):
excitatory & inhibitory
3. interact w/ one another and w/ neurotransmitters
a) synergistically (enhance each other’s activity)
b) antagonistically (decrease activity)
* A number of peptide agents are released from endocrine cells in the mucosa of the stomach and
mechanical stimulation, coincident w/ intake of food
Released into the portal circulation, the gut peptides pass thru the liver to the heart, and back to
the digestive system to regulate its mvts and secretion
Summary of GI tract regulation
There are 3 diff. component participating in the regulation of the GI tract activity:
1. Short enteric (intramural) reflexes (key regulation)
2. Long extrinsic (ANS) reflexes
Regulate the activity
3. Hormonal factors
of the short reflex
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
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