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Chapter 31

Gastrointestinal Regulation and Motility

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We require:

carbohydrates (mainly glucose)

proteins (essential amino acids)

fats (but Western diet fats too high)

vitamins

minerals

Nutrition

Carbohydrate 50%

Fat 35%

Protein 15%

Intake (normally 3000-6000kcal per day & depends on

Geography

Occupation

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Gastrointestinal system

consists of

Gastrointestinal (GI) tract

Accessory glandular organs

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Anatomy and functions of the GI tract

GI tract mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus

Accessory Glandular Organs

salivary glands, liver, gallbladder, pancreas

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Histology/organization of the Gut Wall

From esophagus to anus, GI tract has the same basic arrangement of tissues.

There are 4 layers that can be distinguished

• Mucosa

• Submucosa

• Muscularis

• Serosa

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Layers of Alimentary Canal

Serosa

Submucosa

Mucosa

Circular muscle layerLongitudinal muscle layer

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MucosaSerosa

SubmucosaLongitudinalmuscle

Circular muscle

Layers of Alimentary CanalMyentericplexus

Submucosalplexus

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1.Digestion of food and absorption of nutrients are accomplished in a long tube connected to the external world at both ends

2.Secretion and motility of “the tube” are major themes in understanding the gut.

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1. Ingestion : process of eating

2. Propulsion : passing of food down the GI tract

• swallowing : voluntary

• peristalsis : reflex, involuntary, involves alternating contractions of muscles in body walls of

GI organs

3. Mechanical digestion

• prepares food for chemical digestion

• includes chewing, mixing with saliva by tongue action, churning in stomach...

Processing of food by the DS

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4. Chemical digestion

• catabolic steps in which food is broken down to

basic building blocks

• accomplished by enzymes in digestive juices

5. Absorption : passage of food particles into the blood-

lymph

6. Defecation: elimination of indigestible food

substances

Processing of food by the DS

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Gastrointestinal Motility: Movements that mix and circulate the gastrointestinal contents and propel them along the length of the tract

Secretion: Process by which the glands associated with the gastrointestinal tract release water and substance into the tract

Digestion: Process by which food and large molecules are chemically degraded to produce smaller molecules that can be absorbed across the wall of the gastrointestinal tract.

Absorption: Process by which nutrient molecules are absorbed by cell that lin the gastrointestinal tract and enter the bloodstream

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I REGULATION OF GASTROINTESTINAL TRACT FUNCTIONS

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Cell Hormone Site

G Gastrin (G) autrumn, duodenum

I Cholecystokinin (CCK) duodenum 、 jejunum

S Secretin duodenum 、 jejunum

D Somatostatin (SS) Stamoch, duodenum, pancreas, colon

L Enteroglucagon Small intestine, colon

PP Pancreatic polrpeptide (PP) pancreas

EC1 Substance P (SP) Stamoch, intestine

D1 VIP Stamoch, intestine, pancreas

P bombesin Antrum, duodenum

N neurotensin ileum

B insulin pancreas

A glucagon pancreas

K Gastric inhibitory polypeptide(GIP)

duodenum 、 jejunum

Mo motilin Stamoch, intestine

Endocrine Cell and gut hormoneEndocrine Cell and gut hormone

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Types of secretionTypes of secretion

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Function of GI hormones

1. Regulate the secretion and motility of GI tract

Gastrin HCl secretion, gastric empty

2. Trophic action

Gastrin stomach and duodenum mucosa

3. Regulate the release of other hormones

GIP insulin

SS GH, gastrin

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Innervation of the GI tract1. Central nervous system

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CNS to gut connections

SPINAL CORDdorsal root

ganglion

Spinal afferent

Vagalafferent

Vagal efferent

nodose ganglion

BRAIN STEM

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The autonomic nervous system Sympathetic system:

Noradrenaline

Gut secretions (+)

Sp

inal co

rd

Pons/medulla

Midbrain

Gut sphincters (-)

Pancreas (+)

ACTION

Rectum (+)defaecation

ACTION

Gut wall (+)

adrenaline

EFFECTSEFFECTS

Salivary glands (+)

X IX

VII

Cranial nerves

Parasympathetic system: Acetylcholine (Ach)

(+) salivary glands

(+) gut blood 2 (-) vessels

1/2 (-) gut wall, (+) sphincters

(+) secretion

Adrenal medulla

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Innervation of the GI tract

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Innervation of the GI tract2. Intrinsic (enteric) nerve plexuses

Located

in the submucosa (submucosal or Meissner’s plexus) and between circular and longitudinal muscle layers (myenteric or Auerbach’s plexus)

Control

Motility - Myenteric plexus

Secretion - Submucosal plexus

through release of neurotransmitters

Excitatory - Acetylcholine, Substance P

Inhibitory - VIP, nitric oxide

Excitatory - Acetylcholine

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The enteric nervous system

Deep muscular plexus

Submucosal

artery

Muscularis mucosa

Submucosalplexus

MUCOSA

Myenteric plexus

Longitudinal muscle

Circular muscle

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The enteric nervous system coordinates

digestion,

secretion

motility

to optimize nutrient absorption.

Its activity is modified by information

from the CNS

from local chemical and mechanical sensors.

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Myenteric plexus

Submucosal plexus

Enteric nervous system

CNS

Sympathetic ganglia

Vagal nuclei

Sacral spinal cord

Preganglionic fibres

Postganglionic fibres

Preganglionic fibres

Parasympathetic n.s Sympathetic n.s.

Smooth muscle

Secretory cells

Blood vessels

Endocrine cells

Innervation of the GI tract

Postganglionic fibres

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Gastrointestinal reflex

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II GASTROINTESTINAL SMOOTH

MUSCLE

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Musculature of the GI tract

All smooth muscle except:

Upper third oesophagus – striated

Middle third of oesophagus – mixed

External anal sphincter – striated

Areas of striated muscle are areas that are under conscious control

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Form hollow tubes not contracting against skeleton

Form a syncitium - electrically coupled, joined by gap

junctions contractions synchronous

Actin:myosin ratio 15:1 (skeletal muscle 2:1)

Contractile elements not arranged in sarcomeres

not striated

Anatomical Properties of GI smooth muscle

5-10m

200m

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General Functional characteristicsGeneral Functional characteristics

1. Lower excitability, slower contraction and relaxation

2. Higher extensibility

3. Tonic contraction

4. Autorhythmicity

5. More sensitive to stretch, chemicals, cold and warm stimulation but not to electric stimulation

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Electrical activity of GI smooth muscleElectrical activity of GI smooth muscle

1. Resting potential

2. Slow wave or basic electric rhythmThe smooth muscle membrane slowly depolarizes and repolarizes in a cyclic fashion

3. Action potential

4. Relationship to contraction

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0

-60Mem

bra

ne

po

ten

tial

(m

V)

Tension

0

-60

Tension

Mem

bra

ne

po

ten

tial

(m

V)

Acetylcholine

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Slow waves are changes in resting membrane potential 3-12 cycles per minute depending on area of GI tract -

3/min in stomach, 12/min small intestine Always present but do not always cause contractions Frequency of contractions dictated by frequency of

slow waves Slow wave frequency and height modulated by –

• body temp & metabolic activity, • intrinsic & extrinsic nerves• circulating hormones

Slow waves in GI smooth muscle

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Relationship to contraction

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gNa,Cavoltage

delayed gK

+ gK(Ca)resting pot:

gNa + gK + Na pump

-60 mV

Membrane potential

Contractile force

Thresholdfor contraction

2 sec

Plateau:Na + Ca influx =K efflux

In interstitialpacemaker cells:slow closing

of delayed gK

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.

electricalthreshold

action potentials

slow waves

Force of contraction determined by additional neural andhormonal input.

Frequency of contractions-determined by basic electricalrhythm of interstitial pacemaker cells.

con

trac

tio

ns

elec

tric

al a

ctiv

ity

Slow Waves & Visceral Smooth Muscle Contractions

Excitatory(contracts smooth muscle)

Inhibitory(relaxes smooth muscle)

AChSubstance P

VIPNO

Contractionthreshold

AP threshold

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Contraction of GI smooth muscle

Out

In Ca2+

(Ca2+)4. calmodulin

Inactive myosin light chain kinase

Active myosin light chain kinase

MUSCLE CONTRACTION

Myosin -(PO4)2

Calcium activates contraction

Depends on influx of calcium from extracellular space through calcium channels

Calcium- calmodulin complex activates myosin light chain kinase

Contraction explained by sliding filament theory

Out

In Ca2+

(Ca2+)4. calmodulin

Inactive myosin light chain kinase

Active myosin light chain kinase

MUSCLE CONTRACTION

Myosin -(PO4)2

Out

In Ca2+

(Ca2+)4. calmodulin

Inactive myosin light chain kinase

Active myosin light chain kinase

MUSCLE CONTRACTION

Myosin -(PO4)2

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AP in GI smooth muscle

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4. Noradrenaline release

5. Sympathetic stimulation

more hyperpolarised smooth muscle, Less excitable and fewer contractions

Stimulus Effect on muscle

Electrical activity and muscle contraction

1. Stretch of GI tract wall

2. Acetylcholine release

3. Parasympathetic stimulation

more depolarised smooth muscle, more excitable.

Leads to action potential generation and smooth muscle contraction

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GI motility

There are many types of contractions in different areas of the GI tract.

Some muscles contract and relax in seconds – Phasic Contractions

-Peristalsis and Segmentation

Some maintain contractions over minutes or hours – Tonic Contractions

-Sphincter