Download doc GIT

PHGY 210 – Digestion
Lecture02
Friday, March 24, 2006
Ann Wechsler
Lecture 02 – March 24th 2006
In the GIT, there are 3 identifiable forms of activity:
1- Motility
2- Secretion
3- Absorption
Motility: What brings about propulsion & the physical breakdown of food



The key elements involved in the motility are the 2 layers of muscles: longitudinal &
circular fibers of the muscularis externa.
The uppermost regions (mouth, pharynx and upper 1/3 of the esophagus) as well as
the external anal sphincter are striated muscles. They have specific properties.
The remaining regions are composed of smooth muscles. Their regulation will be
diff.
Propulsion (Flow in the GIT)



Propulsion in the GIT depends on gradients of pressure (High to Low).
Also depends inversely on resistance. The more resistance there is downstream, the
more difficult it is to propel some substances downstream.
There are 2 types of contractile activity within the GIT:
o The length of the GIT periodically contracts at various points which
gives rise to sausage-like segments. These are followed by periods of
relaxation. → Very effective mixing mvt. They can also be involved in
the propulsion of the meal away from the mouth.
o Propagated wave of contraction (PERISTALSIS): moves contents in one
direction → plays major role in most regions of the GIT in mediating
propulsion
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
page 1
PHGY 210 – Digestion
Lecture02




Friday, March 24, 2006
Ann Wechsler
Resistance: at normal circumstances, the meal encounters few. It used to be thought
that the various sphincters which separate the organs of the GIT played a major
role regulating the transport of the meal.
We now know that, normally, these sphincters do NOT offer resistance. They
actually open reflexely, anticipating the arrival of the meal contents in the lumen of
the GIT. They allow the meal to go thru and close to prevent regurgitation of the
contents from 1 organ to the organ above it.
Pathologically, there are situations that prevent the sphincters to relax or more
commonly, there are situations that prevent the sphincters to close completely &
allow for reflux to happen. However, NORMALLY, there is little or no resistance.
Normally, the flow is slow (several hours for the meal to go the GIT), aboral (away
from the mouth towards the anus) and meets little/no resistance.
First propulsive force : DEGLUTITION (Vander’s pp. 590-593)



Immediately after digestion
Deglutition is a process which
involves highly coordinated
muscular activities.
Result: transport of the food thru
the oral cavity, across the pharynx,
into the esophagus & all the way
into the stomach.
We usually discuss the mechanical of deglutition in terms of 3 phases:
 Oral: transport thru the oral cavity
 Pharyngeal: transport across the pharynx
 Esophageal: transport into the stomach
All these phases proceed smoothly and sequentially
Deglutition (swallowing): is accomplished thru a complex series of highly coordinated muscular
mvts aimed at building up pressure, temporarily sealing off of compartments to prevent
dissipation of pressure and decreasing resistance (by relaxation of the sphincters)
Oral phase
 Transport from anterior portion
of the mouth into the pharynx
 voluntary control
 You take a morsel of food, you
chew it & it becomes coated w/
saliva.
 Thru the combined actions of
muscles in cheeks, lips tongue,
the food is brought on the upper
surface part of the tongue.
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
page 2
PHGY 210 – Digestion
Lecture02




Friday, March 24, 2006
Ann Wechsler
Once it is in that position, thru the elevation of the upper tip of the tongue and retraction of
the lower tip, the food is rolled b/w the tongue and palate, it takes a rounded shape and
becomes known as the bolus.
It rolls to the back of the mouth and the bolus is projected into the pharynx
The oral phase of deglutition is under voluntary control. You can start/stop to swallow at will.
From evolutionary standpoint: transport across the rest of the GIT is involuntary (except
defecation, which can be initiated voluntarily). The initiation is voluntary, but the act itself is
carried out by highly coordinated reflexes (by definition, involuntary)
Oral phase:
 transport of bolus (masticated, ensalivated mass of food) from anterior to posterior
portion of mouth
 This involves a series of reflexes coordinated in deglutition centre in medulla oblongata
Cortical vs Medullary centers



There is a region in the
precentral cortex of the gyrus
that, when stimulated, will
initiate swallowing.
 Also involves reflexes that are
initated by presence of food in
the mouth that are coordinated
in the medulla oblongata
which is the deglutition
center. → can carry out the
sequence of mvt involved in
the oral phase of deglutition
 When
you
voluntarily
swallow, the impulses from
the cortical center facilitate
the reflexes which are
coordinated in the deglutition centre (this also applies to the defecation)
Voluntary – in cortex
Deglutition centre – “involuntary” – in the medulla
Oral Phase
1. ability to initiate: voluntary (cortex)
2. coordinated movements: reflex, involuntary (medulla)
Pharyngeal Phase




Pharynx: where the respiratory and digestive pathways cross
Pathway from the nose, across the pharynx into the trachea & pathway across the mouth, thru
the pharynx and into the esophagus
It is obvious that for swallowing to occur safely and w/ right pressure for the bolus to enter
esophagus, all the openings leading from the phrarynx, have to be closed (protect the resp
passages).
The pharangeal phase is strictly involuntarily (automatic). Once the bolus has entered the
pharynx, we can no longer control anything.
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
page 3
PHGY 210 – Digestion
Lecture02
Friday, March 24, 2006





Ann Wechsler
This
involves
coordinated
reflexes, activated by pharyngeal
receptors that send afferent
impulses to the deglutition center
in medulla.
The efferent output gives rise to a
# of events.
There are a series of protective
rxns that separate the digestive
tract from the respritory tract.
To further control this, there is
temporary apnea (for fractions of
seconds).
Respiration
is
temporarily arrested.
Then, the upper esophageal
sphincter relaxes (to ↓ resistance
& release pressure) and the
pharynx muscles contract to push
the bolus down (generated by
pressure gradient).
The passages into the nose:







Soft palate is raised for the food not to go into the nose.
Base of tongue raised: prevent food from going back into the mouth.
Trachea (entire larynx) is raised upwards and moved forward to fit under the base of tongue.
Upward mvt of the larynx is the major protective mechanism of the resp syst (glottis is
closed, the vocal cords are pulled together). → apnea
Then, relaxation of the UES and the pharynx muscles push bolus down.
The bolus also presses down on the epiglottis. Epiglottis also flips over and offers further,
secondary protection.
There are about 25 diff. muscles that must contract to assure safe swallowing.
Pharyngeal Phase – Involuntary
1. passages into nose, mouth and trachea
are blocked
2. apnea
3. UES relaxes
4. Pharynx muscles contract
In elderly: aspiration pneumonia  inappropriate
deglutition, so food goes into lungs and brings
bacteria in
Pharyngeal Phase
under involuntary control – consists of
a) a series of protective reflexes, initiated by
stimulation of afferent fibres in the
pharynx, organized in deglutition center,
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
page 4
PHGY 210 – Digestion
Lecture02
Friday, March 24, 2006
Ann Wechsler
closing off nasal, oral and larygeal cavities, preventing misdirection of the bolus
simultaneously…
b) transfer to esophagus, as pharyngeal muscles contract and upper esophageal sphincter
relaxes
Deglutition Reflexes
The pharyngeal reflex consists of the following steps:
1- There are pharyngeal receptors that send information, thru afferent nerves, to the
deglutition center in the medulla.
2- From here, there is efferent info sent to the diff. parts of the pharynx in a coordinated
fashion in order to control the whole INVOLUNTARY phase of pharyngeal
deglutition.
Upper esophageal sphincter



Corresponds anatomically to the cricopharyngeus muscle.
This muscle is usually kept contracted → always closed (normally)
Train of impulses that are mediated to this muscle along the vagus nerve keeps it closed.
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
page 5
PHGY 210 – Digestion
Lecture02





Friday, March 24, 2006
Ann Wechsler
This is striated muscle, so there is direct innervation by the vagus nerve.
There is release of ACh that acts on nicotinic receptors (block with curare) causing
contraction of the cricopharyngeal muscle.
During swallowing, the muscle is relaxed. Relaxation is mediated by the arrest of these
impulses.
From the deglutition center, there is inhibition of these excitatory impulses that arrive to the
muscle.
Vagus nerve innervates the cricopharyngeus thru ACh (Nicotinic receptors)
 closure – impulses originate in CNS, mediated by vagus, releasing ACh,
causing muscle contraction
 relaxation – mediated by cessation/arrest of impulses, results in muscle
relaxation
Pharyngeal phase
1. Involuntary
2. Rapid – takes 1/5th of a second (we are not aware that we stopped breathing and talking)
3. “stereotyped” -- Automatic. The sequence proceeds w/o modification in normal
circumstances
4. temporospatial coordination – muscles contract in appropriate order & w/ particular
strength (sequencial and appropriate matter for this stereotyped response)
Esophageal phase








Simplest portion of the digestive.
No digestion or absorption.
The main fctn are muscular to allow transport of the bolus from the pharynx all the way along
the esophagus into the stomach.
It is a very muscular organ. Both the circular and longitudinal layer are well-developed.
Upper regions of the
esophagus are striated, and
the lower regions becomes
smooth
Body of esophagus lies
within thoracic cavity –
therefore, it is subjected to
the intrathoracic pressure (it
is negative, -5 to -10
mmHg; so the pressure in
the esophagus is also
negative).
The consequence of this is
that you have a (-) pressure
in the organ, and above it,
in the pharynx it is higher
(atmospheric) and below it, in the stomach, it is positive (+5-10mmHg).
So we can appreciate the role of the UES and LES  w/o contracted sphincters you would
always be aspirating air and saliva into the esophagus (UES) and refluxing the acidic gastric
contents into the esophagus (LES).
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
page 6
PHGY 210 – Digestion
Lecture02
Friday, March 24, 2006
Ann Wechsler



The innervation of
these upper and lower
esophagus regions is
diff.  both by vagus
nerve.
In striated region, the
innervation is direct
(somatic vagal fibers
release ACh which acts
on nicotinic receptor).
The smooth muscle
 indirect innervation.
It synapses w/ enteric
neurons, which will
then act on the
muscular cells.
There are 2 forces that play a role in pushing down the bolus:
1. gravity (minor role → facilitates flow of non-viscous material down the esophagus)
2. peristalsis (major mech to carry bolus down the esophagus) – propagated wave of
contraction that narrows the lumen and sets up a gradient of pressure which favors aboral
mvt







Each time we swallow, a
primary (single) peristaltic
wave is generated.
There is a definite link b/w the
# of times we swallow and the #
of primary peristaltic wave. (If
swallow once a minute → one
peristaltic wave)
It takes 8-10 secs for the bolus
to move from the lower part of
the UES to be brought into the
stomach
The primary peristaltis waves
are part of the deglutition
reflexes.
When the pharyngeal receptors
are stimulated, they set into
motion reflexes that result in
primary peristalsis moving
along the esophagus.
The stimulation of the pharyngeal receptors sends afferent impulses to the deglutition centers,
from where the vagal somatic fibers to the striated muscle portion will be activated & these
impulses arrive sequentially.
It is a smoothly propagated wave (in the striated muscle region).
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
page 7
PHGY 210 – Digestion
Lecture02


Friday, March 24, 2006
Ann Wechsler
The autonomic vagal fibers to the distal region of the esophagus are activated. But the
excitation arrives synchronously in all the vagal fibers to this region.
There is delay in the activation of enteric neurons, so the muscle here is also activated
sequentially (not b/c the impulses arrive sequentially which delay to the muscle, but b/c of the
progressive delay in activation of the enteric neurons)
The significance of having 2 types of innervation:







In the esophagus, there is a peristaltic wave moving over the entire organ (moves down).
The innervation & the responses of the musculature are diff. (striated and smooth).
If you cut the vagus nerve high up in the neck so that the somatic and the autonomic fibers
are disrupted, you can no longer generate a primary peristaltic wave. You need the vagus
nerve to be intact in order to do that.
If you leave the somatic vagal innervation intact, but you cut the vagus at the level of the
atria, trans-thoracically.
As long as you leave the somatic fibers intact and a few autonomic vagal fibers innervating
some of the enteric neurons intact, the primary peristaltic wave will be carried and
uninterrupted: the distal esophagus, once you have activated only a few of the enteric
neurons, there is relay b/w the enteric neurons that is preprogrammed so that the wave is
propagated.
So the vagus is ESSENTIAL for initiating peristalsis in proximal esophagus. (striated muscle)
But, if you activate just a few of the enteric neurons in smooth muscle, there will be
propagation of the peristaltic wave, so the continuation & propagation in the distal part relies
mainly on the intactness of the enteric innervation.
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
page 8
PHGY 210 – Digestion
Lecture02
Friday, March 24, 2006
Ann Wechsler
When you have a large bolus
of food:
 Local distension within
the body of the esophagus
created by the presence of
a large bolus, will gives
rise
to
secondary
peristaltic wave (stimulus:
local distension).
 This may be mediated by
short intramural enteric
reflexes (stimulate at one
pt and give rise to
activation at a diff. pt,
mediated
by
enteric
nerves).
 There are also activation
of sensory afferent vagal
fibers up from the gut to
the CNS, resulting in
efferent output.
 So we have several
secondary
peristaltic
waves generated until the
bolus has been displaced.
Lower Esophageal Sphincter






Corresponds anatomically to the
terminal 4 cm of the esophagus.
Not an anatomically distinct
region
(hardly
recognizable
anatomically)
Functionnaly imp. b/c it allows a
pH gradient of 5 units (esophagus
= 7 and stomach = 2).
It is contracted tighly, preventing
reflux of gastric contents. The
closure of this region to act as
sphincter is thought to be myogenic (property of that muscle to remain tonically contracted at
rest even in the absence of innervation, either vagal or enteric)
Relaxation: neurogenic results from the local release of NANC inhibitory neurotransmitter
During swallowing, it is the vagus nerve that activated these inhibitory enteric neurons
allowing for the relaxation of the sphincter.
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
page 9
PHGY 210 – Digestion
Lecture02
Friday, March 24, 2006
Ann Wechsler
Pharyngeal receptors











Enteric activation also brings of the relaxation of LES.
The LES normally straddles the diaphragm. 2 cm are in the thorax, and 2 cm are in the
abdominal cavity.
The area that lies in the abdominal cavity gives additional protection to the prevention reflux.
Any increase in the intra-abdominal pressure increases pressure equally on the stomach & on
LES.
In individuals that have a hiatus hernia (all LES is in the thoracic cavity), this doesn’t happen.
They have a greater
tendency to reflux.
In pregnancy, in later
months, women have a
tendency to reflux. b/c
as the uterus grows, it
displaces the abdominal
organs upwards and a
hiatus hernia develops.
The sphincter – intrinsic
property of the muscle,
helped
by
intraabdominal segment
Reflux – pyrosis (heart
burn)
It used to be thought that gastrin tightened the LES. → false
Gastrin plays a role in the secretion of acid, doesn’t cause the LES to contract, the only
hormone which acts to relax the LES is progesterone. (2nd reason why women who are
pregnant or are on the pill have higher tendency to reflux)
Sarantis Abatzoglou, Natasha Cohen, Emilie Trinh
page 10