Physiology of the Digestive System Note PDF

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15.PHYSIOLOGY AND THE DIGESTIVE SYSTEM1. THE GASTROINTESTINAL TRACTA. Continuous hollow tube - about 8 meters long i. Mechanically processes and moves food through the tract ii. Chemically processes and digests food iii. Absorbs nutrients and water B. The accessory organs - control secretions and br...

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15.02.2019

PHYSIOLOGY AND THE DIGESTIVE SYSTEM 1. THE GASTROINTESTINAL TRACT A. Continuous hollow tube - about 8 meters long i.

Mechanically processes and moves food through the tract

ii.

Chemically processes and digests food

iii.

Absorbs nutrients and water

B.

The accessory organs - control secretions and breakdown food i.

Teeth

ii.

Tongue

iii.

Salivary Glands

iv.

Liver

v.

Gallbladder

vi.

Pancreas

2. THE DIGESTIVE PROCESS

3. NEURAL AND HORMONAL CONTROL MECHANISMS •

Peristalsis & motility

•

Secretion & absorption

A. Autonomic nervous system i.

Parasympathetic

ii.

Sympathetic

iii.

Enteric nervous system

B.

Gut peptides i.

Paracrine

ii.

Hormonal

4. AUTONOMIC NERVOUS SYSTEM (ANS) A. Extrinsic nerves - long reflexes – external stimuli B. Involves the CNS C. Causes changes in motility and secretion

5. ENTERIC NERVOUS SYSTEM (ENS) A. Contains all the elements of a nervous system B. Intrinsic control – short reflexes - internal stimuli C. Communicates with the parasympathetic and sympathetic but D. Autonomous E. Two well-organized neural plexuses: i.

2

myenteric plexus: between longitudinal and circular layers of muscle and is involved in control of digestive tract motility.

ii.

submucosal plexus: is located between the circular muscle and the luminal mucosa senses the environment of the lumen and regulates gastrointestinal blood flow and epithelial cell function.

6. HORMONAL CONTROL A. In excess of 22 hormones and paracrine hormones B. Secreted by Enteroendocrine cells in the mucosa i.

Single cells scattered through GI tract

ii.

Densely packed secretory vesicles

iii.

Sense luminal contents: chemical, osmotic and pH

iv.

Release hormones and paracrine

7. MOTILITY A. Phasic contractions: short lasting contractions - movement of material in the small intestine i.

Peristalsis (waves of contractions-20cm)

ii.

Segmented contractions (10cm)

B.

Tonic contractions: long lasting contractions - closing of a sphincter i.

Controlled movement of material though the tract

ii.

Maintains ordered sequence of events

iii.

Compartmentalisation ensures processes are complete before passing to next area

8. PERISTALSIS A. Propulsion of material- peristalsis B. Mediated by neurones in myenteric plexus C. Interstitial cells of Cajal i.

ICC are pacemakers of the gut (like in the heart)

ii.

Found in myenteric plexus

iii.

Electrical activity spreads via gap junctions from ICCs to muscle

iv.

ICCs produce slow waves

v.

differs in different regions of the GI tract

D. The frequency of ICC slow waves differs in different regions of the GI tract:

3

i.

3 per minute in the stomach

ii.

11-12 per minute in the duodenum

iii.

8-9 per minute in the ileum

iv.

3-4 per minute in the colon

4

9. SLOW WAVES A. Slow waves- slow undulating changes in resting membrane potential - not action potentials (different from SA node in heart) B. Don’t cause contraction until the threshold (-40mV) is reached C. Activated by: i.

Distention - bolus of food (chyme)- stimulates stretch receptors (local reflex): oral contraction and aboral relaxation

ii.

Parasympathetic nerves (ACh)

10.

SEGMENTED CONTRACTIONS & MASS MOVEMENTS A. Colon: 99% of the time i.

Retain material (e.g. water reabsorption and fermentation)

ii.

Mixing contents

B.

Mass movement of material into aboral end of colon i.

Gastro-colic response

ii.

2-3 times per day

11.

HIRSCHPRUNGS DISEASE – TOXIC MEGA COLON A. Congenital disorder presents shortly after birth B. All or part of colon has no innervation C. Aganglionic segment is strictured, proximal to the strictured segment, and the colon is dilated. D. 1:5000 children affected E. Surgical removal of the colon F.

Key points i.

GI tract is made up of alimentary canal and accessory organs, including oesophagus/stomach/small intestine and large intestine

ii.

Regional specializations to optimize digestion and absorption.

iii.

Function: breakdown food and absorb nutrients

iv.

Mechanical and chemical digestion

v.

Absorption and secretion

vi.

The lining of the GI tract is subdivided into layers (mucosal, submucosal, and muscle layers).

vii.

There are three major control mechanisms: hormonal, paracrine, and neural.

5

viii.

12.

Control can be both excitatory and inhibitory (speed up or slow down).

THE MOUTH – START OF THE JOURNEY A. Mastication: breakdown food B. Taste: inform brain about edibility/duration C. Saliva production: lubrication, protection and digestion D. Swallowing: movement of digested macromolecules to stomach

13.

THE MOUTH - TASE Papillae: ‘taste organs’ A. Raised protrusions on the tongue B. Visible to the naked eye C. Located on the tongue, soft and hard pallet, pharynx, epiglottis and larynx. D. Contain the taste buds i.

Each taste bud contains 3 kinds of cells:

ii.

Epithelial cells,

iii.

Support cells,

iv.

Gustatory receptor cells (taste cells)

v.

Innervated by gustatory afferent nerves

14.

THE MOUTH – A TASTE BUD A. 50-100 continuously maturing cells B. Bud shields the cells from the oral cavity C. Apical microvilli increase surface area D. Receptor proteins exposed through the taste pore (3-5µM) E. Ion channels/receptors located on villi

F.

i.

All flavours are encoded by 5 taste modalities:

ii.

Sweet, Salt, Sour, Bitter, Umami

iii.

One taste cell responds to one modality

iv.

Ionotropic receptor: Salt Na+, Sour H+ gated ion channels (K+/H+)

v.

Metabotropic receptors: Sweet, Bitter, Umami All flavours are encoded by 5 taste modalities:

i. 6

Sweet-High calorie foods

ii.

Salt - Electrolytes

iii.

Sour- Proteins

iv.

Bitter- Often poisons/dangerous chemicals

v.

Umami (Japanese for delicious- meat and cheese) -amino acids (MSG, glutamate and aspartate) to grow and repair tissue

G. Tongue has regional sensitivity to different taste modalities: i.

Sour is usually perceived at the sides

ii.

Bitter is perceived at the back

iii.

Salt and sweet are perceived at the front

H. Sensitivity to all tastes is distributed across the whole tongue I.

Differences in intensity of sensation from different regions (Olfactory system is sensitive to 1000s of modalities)

15.

THE MOUTH – SALIVA PRODUCTION A. Hypotonic solution containing more than 99% water and 1% dry matter such as proteins i.

Water

ii.

Bicarbonate ions- buffering capacity, protects from acid

iii.

Mucous

iv.

Enzymes

v.

α-amylase - released from the parotid gland which initiates carbohydrate digestion,

vi.

lipase - secreted within glands of the mucosa of the tongue (linguinal)

vii.

Proteins (100s)

viii.

Multifunctional

ix.

16.

Anti- bacterial, Anti- viral, Anti- fungal, Tissue Coating, Lubrication & Viscoelasticity, Mineralization, Buffering, Digestion

THE MOUTH - SWALLOWING A. Oral phase (voluntary) i.

Preparatory phase - chewing and biting

ii.

Transfer phase - respiration inhibited and tongue forces bolus into pharynx

B.

Pharyngeal phase (involuntary) i.

C.

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Movement of bolus from pharynx into oesophagus Oesophageal phase

i.

Upper Oesophageal sphincter relaxes, and bolus moves into oesophagus

ii.

Start primary peristaltic wave (vagal nerves)

iii.

Secondary peristaltic wave (enteric nervous system)

iv.

Delivers bolus to the stomach

v.

Oesophageal manometry measures oesophageal motility & problems swallowing - dysphagia

17.

THE STOMACH & DUODENUM Gastric functions A. Motility i.

Gastric accommodation- Temporary storage reservoir

ii.

Trituration – Dissolve, mix and grind food particles

iii.

Gastric emptying - Control delivery to small intestine

B.

Digestion i.

C.

Initiate digestive process (proteins) via gastric secretion Protection

i.

Foreign invasion (acid/proteases)

ii.

Mechanical abrasion (mucus)

iii.

Prevents auto-digestion (mucus)

D.

Absorption i.

18.

Alcohol and fat-soluble drugs (diffusion)

THE STOMACH – GASTRIC JUICE A. Water and ions B.

HCl i.

Provides low pH (as low as 1 or 2!)

ii.

Prevents bacterial growth

iii.

Catalyses cleavage of pepsinogens to pepsin

C.

Pepsinogens i.

Proenzyme of pepsin

ii.

Pepsin breaks down proteins into peptides

D.

Intrinsic factor i.

Glycoprotein

ii.

Binds to vitamin B12- allowing digestion in the ileum

E.

Mucus i.

F.

8

Protects gastric mucosa Gastrin

i.

From ‘G cells’

ii.

Regulates acid secretion

19.

THE STOMACH GASTRIC SECRETION

20.

THE STOMACH – HCL SECRETION A. CO2 within the parietal cells reacts with H2O to form bicarbonate, and then H+ catalysed by the enzyme carbonic anhydrase B. H+ is actively transported by ‘proton pumps’ into the gastric lumen, in exchange for K+ ions C. Both K+ and Cl- diffuse passively down a concentration gradient out of the cell D. The bicarbonate produced is transported into the capillaries in exchange for Cl- ions by ‘antiporters’

21.

THE STOMACH – CONTROL OF MOTILITY & SECRETION

9

22.

THE STOMACH – GASTRIC/PEPTIC ULCERS A. Mucus entraps alkali i. B.

Fluid-gastric mucosal barrier Break in mucosal barrier

i.

C.

Exposes underlying tissue to corrosive action (acid, proteases) by Alcohol, vinegar, bile, stress, NSAIDS & bacterial infection (Helicobacter pylori). Symptoms

i.

Abdominal pain

ii.

Bloating

iii.

Nausea/vomiting

iv.

Bleeding- haemorrhage and anaemia

23.

THE DUODENUM First loop of the small I (10-15 inches long) Vital role in digestion of the bolus-often forgotten. A. ‘Brunner’s glands’ i.

Secrete an alkaline fluid composed of mucinacid function by coating the duodenal epithelium

ii.

Protecting it from the acidic chyme of the stomach

B.

Receives secretions from: i.

Gall bladder (liver)

ii.

Pancreas

C. Site of iron absorption

24.

THE DUODENUM - PANCREAS

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anti-

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