1- introduction 2012 (mod)

7/27/2019 1- Introduction 2012 (Mod)

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Introduction


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Digestive System

consists of:


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Objectives

After studying this chapter,

The student should be ableto:


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Explain how food is swallowed &transported to the stomach.

Describe digestion in thestomach.

How gastric secretion isregulated.

i.e. stimulated & inhibited.

List the factors that govern

emptying of the stomach.


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List major components ofpancreatic secretion.

Give their functions.

Explain how secretion iscontrolled.


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Give the functions of theliver.

Explain the role ofbile in thedigestive process & itscontrol.

Explain functions ofgallbladder& control.


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Intestinal secretion &motility.

Regulation of intestinalwall.

Factors affectingintestinal absorption.


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List the functions of thecolon.

List the various types &functions of motility.

Explain howdefecation occurs.


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Digestive system

Gastro intestinal tract Digestive glands

1.salivary glands

2.liver

3.Pancreas


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General Structure ofthe Digestive

System


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Layers of Alimentary Canal Organs


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Cross section of the gut.


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Functions of GIT


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Functions of digestive System:

It is the normal site for:Food & fluids enterance to body:

so it supplys & provides' body with :

Nutritive subs.Vitamins.

Minerals.

Fluids


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1.digestion of food: break down of

complex: Protein, fat & carbohydrate

2.Secretion of digestive juice: variousenzymes & hormones.

3. Motility: Propulsion + mixing' contents

(mostly anal wards)4.Absorption: mainly in small intestine:

absorption of products of digest &

vitamins & minerals: all pass from m.m toblood or lymphatics.

5.Excretion: of undigested & unabsorbed

substances.

6.it has immune function.


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2) Absorption

Transfer of the

contents of the gutfrom lumen into the

mucosa to blood.


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:3) SecretionTransport of fluid,

electrolytes, peptides, .etc

from blood to GIT lumen.( about 7500 ml/day)

Usually isotonic with

plasma (except saliva ishypotonic.)


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4) RES (Immune System)

Humoral antibodies &cellularimmune system

(lymphocytes in Peyerspatches in SI ).

Protect the body against

micro-organisms in lumen ofgut.


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5) Endocrine function

Release of

somatostatin, intestinalglucagon, gastrin,

secretin, CCK etc


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Regulation of GIT functions

(1)Nervous:

neural

Regulation

(2) Hormonal

regulation


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Musculature of the digestive tract is

composed ofsmooth muscle fibers.

Except for the striated muscles of:1. Mouth.

2. The upper part of the oesophagus.

3. The external anal sphincter.


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All smooth muscles are involuntary,controlled by:

Autonomic nerves:

a) Extrinsic: (symp & parasymp)b) Intrinsic: (enteric nerve plexus)

1) The myenteric (Aurbach's plexus)

2) The submucosal (Meissner'splexus)

a) Intr insic innervat ion o f the gu t


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a)Intr insicinnervat ion o f the gu t

The enteric nerve plexus comprises:

1) The myenteric (Aurbach's plexus):

lies in the musculosa between thecircular & longitudinal muscle layers.

It is concerned with controlling the motoractivity of the alimentary canal.

It is mainly excitatory to the tonic &

rhythmic contractions, but it also contains some inhibitory

neurons.


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2) The submucosal

(Meissner's plexus):

lies in the submucous layer.

It is concerned with controlling the local

secretory function of the GIT by

stimulating the local exocrine &

endocrine secretory cells.

It contains the neurons of the sensory

afferent nerves which arise from themucosal layer.


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Innervation of the gut.

b) Extr insic innervat ion o f the


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b)Extr insicinnervat ion o f the

gu tThe intrinsic neural plexuses of

the ENS are:

connected together & modulated by extrinsic

autonomic nerves (sympathetic &parasympathetic).


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The parasympathetic supply is

generally excitatory to the enteric

nervous system:

It stimulates the motility and secretion.

However, it contains also some

inhibitory vagal fibers to the stomachand the gastrointestinal sphincters.


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The sympathetic nerve

supply generally inhibits

secretion and motility.However, it contains also

some excitatory fibers.


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Sensory fibers arise from

the luminal epithelium and

wall of the gut to the entericplexuses and from there to

the prevertebral ganglia,spinal cord and brain stem.


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Gastro intest inal Reflexes1) Local enteric reflexes:integrated in the ENS.

e.g. reflex gastrin secretion & myenteric

(peristaltic) reflex & gastrocolicreflex.

2) Ganglionic reflexes:

integrated at the prevertebral sympganglia (celiac, mesenteric &hypogastric).

e.g. enterogastric reflex.


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3) Central nervous reflexes:

integrated in the CNS (the brain

and spinal cord).

e.g. peristaltic reflex in the

upper half of the oesophagus (vagovagal reflex) & the spinal

defecation reflexes. R t i


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Receptors in

'wall: (short

reflexes)

Mechanoreceptors

Stim. By stretch

Chemoreceptor

Stimulated by:Products of food digestion &

acids & Osmotic changes

Send impulses to

Local n. plexus

Or through afferent

autonomic n. as

vagus nerve

Receptors out side the gut


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Receptors out side the gutstimulation of:

visual, alfactory or taste receptors can lead to GIT

responses through :- long reflexes

They affect the gut function through efferent

autonomic nerves.

Or leads to:

- Hormonal release:

Release of local hormones: act only on GIT,

affect secretion and motility.


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Release of GIT hormones is regulated by:

1- Nerve impulses :- Local

- Autonomic

2- Chemical factors:

- food digestion products.

- pH changes.

-osmolarity ..

-all can stimulate ' mucosal cells

to produce : hormones..


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Smooth m of GIT functions as a

syncytium

*The longitudinal m.fs are arranged in bundles &extends down the Intestinal tract.

*m.fs. are electrically connected through hugenumber ofgap junctions with low resistance forion movement, so electrical signals travel fromone cell to the next very rapidly along the bundles.

*The bundles fuse with one another at many pointsso they function as a syncytium that allow spreadof action potential in all directions in the muscles.*N.B.

(there are few connections between longitudinal &circularm layers.)

Electrical activity of GIT smooth


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Electrical activity of GIT smoothmuscles: it is of2 types :

1-slow wave (BER):

it is not action potential, just

small changes in RMP.2- Spike potential:

they are true action potentials ,they occur on peaks of slow

waves


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1) Basic electrical rhythm (BER)

The smooth muscle of the GIT

has spontaneous rhythmic

fluctuations in membranepotential between about65 &

45 mV.

Except in the oesophagus & the

proximal portion of the stomach.


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BER

Initiated by the interstitial cells of Cajal,

stellate mesenchymal pacemaker cells

with smooth muscle-like features that send

long branched processes into the intestinalsmooth muscle.

The BER itselfrarely causes muscle

contraction, but spike potentialssuperimposed on the most depolarizing

portions of the BER waves do increase

muscle tension.

BER f GIT th l & it l ti


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BER of GIT smooth muscle & its relation

to muscle contraction.


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The rate of the BER is

about 4/min in the stomach.

It is about 12/min in the duodenum

about 8/min in the distal ileum.

about 2/min at the cecum.

about 6/min at the sigmoid.

The function : to coordinate peristaltic and other

motor activity.Contractions occur only during the depolarizing

part of the waves.


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2) The Spike

The depolarizing portion(Ca2+ influx). The repolarizing portion (K+ efflux).

Many polypeptides &

neurotransmitters affect the BER: e.g.Ach increases the number of

spikes & the tension of the smooth

muscle epinephrine decreases the number of

spikes and the tension


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Stimulatory effect of acetylcholine andinhibitory effect of epinephrine.

Th P i t lti M t


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The Peristaltic Movement

(Peristalsis)

Propels the food or chyme along the

GIT & It has a mixing function.

Peristalsis is a wave ofconstriction -usually preceded by relaxation- of the

gut which proceeds analwards .

It is a local enteric reflextriggeredby distension of the gut.

(called the myenteric orperistaltic

reflex .


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Peristaltic reflex

Stimulus: distension of the gut

(e.g. by a bolus of food)

Receptors: receptors in the gutmucosa.

send impulses to neurons of themyenteric plexus.


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Effect

Ring constriction proximal to the

bolus:

(contraction of the circular m fibers &

relaxation of the longitudinal m fibers).

Receptive relaxation distal to the

bolus:

(relaxation of the circular m fibers &

contraction of the longitudinal m fibers).

This pulls the gut wall over the bolus

hel in its ro ulsion analwards.


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Peristalsis


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The Law o f the Gut:

A distension of the gut

produces a peristaltic wave

that starts at the site ofdistension & proceeds

analwards.


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Electrical activity of GIT smooth


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ymuscles: it is of2 types :

1-slow wave (BER): it is not action potential,just smallchanges in RMP due to

cyclic changes of electrcal pace maker cells whice rspecialised cells,present among smooth muscles, itcauses opening of ion channels with inward current

that generate the slow wave activity.2- Spike potential: they r true action potentials ,theyoccur on peaks of slow waves, they r due to muchCa++& little Na+ influx.

Factors that make the membrane more excitable r:

strech , acetylcholine,parasympathetic nerves &several GIT hormones. The sympathetic,noradrenaline & certain hormones r inhibitory factors.

1- introduction 2012 (mod)

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