Overview of digestion … introduction to the GI system

Prof. Barry Campbell

Cellular & Molecular Physiology

e-mail: bjcampbl@liv.ac.uk

http://pcwww.liv.ac.uk/~bjcampbl

Key concepts in Digestion….

“The progressive breakdown of food into a form

suitable for absorption and the associated transport

processes”

Digestion therefore also includes: -

• The processes of secretion

• The processes of absorption

• Movement of the gut contents

• growth & differentiation

• The mechanisms protecting the gut from damage or attack, and

• the mechanisms controlling and integrating all of the above

DIGESTION

SCHEMATIC REPRESENTATION OF

MAIN REGIONS OF THE GASTROINTESTINAL TRACT

Oesophagus

Stomach

duodenum

jejunum

ileum

Colon

rectum & anus

FUNCTION:

- fat, protein, carbohydrate

digestion & absorption, Ca2+/Fe2+

- water and electrolyte transport

- bile salt & vit B12 transport

REGION:

Small intestine

Large intestine

- Transit

- Storage

- water and electrolyte transport

- defaecation

- storage, H+/peptic digestion

& intrinsic factor

Salivary glands -

synthesis/secretion:

amylase, mucus.

Water, electrolytes

Exocrine pancreas -

synthesis/secretion:

proteases,lipase,amylase.

HCO3- , water

Liver -

bile salt synthesis,

bile secretion.

Gall bladder -

storage and concentration of bile

SECRETIONS OF THE GUT

• Endocrine - Gut hormones

• Paracrine - Local regulators

• Neural

Intrinsic - Myenteric & submucosal nerve plexuses

Extrinsic - Afferent & efferent nerves

vagal & splanchnic trunks (autonomic nervous system)

Physiological control systems in the gut

Major hormones of the gut

STOMACH

Gastrin

Somatostatin

UPPER SMALL INTESTINE

Cholecystokinin (CCK)

Secretin

Gastric inhibitory peptide

Motilin

ILEUM AND COLON

Glucagon-like peptide 1 (GLP-1)

Peptide YY (PYY)

Neurotensin

Overview of extrinsic innervation:

Voluntary (Conscious control)

Hypoglossal

nerve (XII)

Pudendal

nerve

Pelvic

splanchnic

nerve

Vagus

nerve (X) LOS

UOS

IAS EAS

Autonomic (non-perception)

Superior

cervical

ganglion

Paraspinal cord

The autonomic nervous system Sympathetic system:

Noradrenaline

Gut secretions (+) e.g acid, enzymes

& electrolytes

Sp

inal c

ord

Pons/ medulla

Mid brain

Gut sphincters (-) reflex relaxation

Pancreas (+) exocrine &

endocrine secretion

ACTION

Rectum (+) defaecation

ACTION

Gut wall (+) increased

motility and tone

adrenaline

EFFECTS EFFECTS

Salivary glands (+) secretion of saliva

X IX

VII

Cranial

nerves

Parasympathetic system:

Acetylcholine (Ach)

(+) secretion of saliva salivary

glands

(+) vasoconstriction gut blood

2 (-) vasodilation vessels

1/2 (-) decrease motility gut wall,

(+) contraction sphincters

(+) secretion

Adrenal

medulla

Organisation of the gut wall

epithelium

Lamina propria

Muscularis mucosa

SUBMUCOSA

Lumen

MUCOSA

Mesothelium (SEROSA)

Myenteric plexus

Longitudinal muscle

Circular muscle

MUSCULARIS

PROPRIA

Intrinsic and extrinsic nerves of the digestive tract

Visceral

afferents

Parasympathetic

efferents

e.g. Stretch & chemosensitive

neurons

ABORAL ORAL e.g. secretory cell

e.g. post-ganglionic non-

adrenergic or cholinergic fibres

INTRINSIC

NERVES

EXTRINSIC

NERVES

Submucosal plexus

Myenteric plexus

Action potentials

Mechanical recording 1.5g

25 mV

BER

Control of smooth muscle contraction –

Interstitial cells of Cajal are pacemakers of the gut

Basal electrical rhythm (BER) originates in ICC, but itself does not cause

contraction; when spike potentials occur at maximum depolarisation of BER due to

Ca2+entry, the result is contraction

ICC cell network

L type Ca2+ current & action potential mechanism

Slow wave conducted to smooth muscle

Slow wave mechanism

~130 min

antrum

duodenum

Distal small intestine

Colon

Motility in the interdigestive period -

The Migrating Myoelectic Complex (MMC)

III – motilin, ghrelin

& vagus nerve

III – somatostatin,

Serotonin (5-HT)

and xenin

Feeding

disrupts the

cycle

71%

29%

Phases of MMC

I

II

III

IV

I - quiescence; II - random contractions; III - burst of contractions (max.

amplitude & duration); IV - rapid decrease of contractions.

Alternate contraction and relaxation of adjacent

segments causes thorough mixing of contents

Intestinal movement - segmentation

Intestinal movement - peristaltic reflex

Descending relaxation Ascending contraction

Orad Caudad

Circular smooth muscle

VIP/NO

Inhibitory motor

neurons

Ach = acetylcholine NO = nitric oxide TK = tachykinin VIP = vasoactive intestinal polypeptide 5-HT = 5-hydroxytryptamine (serotonin)

ACh/TK

ACh ACh Intrinsic primary afferent neurons (IPAN)

interneurons interneurons

Excitatory motor

neurons

TK/ACh >

>

>

>

> >

Distension (5-HT)

Animation at www.westga.edu/~lkral/peristalsis/

Removing indigestible material –

reflexes in the colon and rectum

Mass movements +

+

Food in stomach

Food in

duodenum

Faeces

Defaecation reflex

Colon rectum

anus

Ach

Sp

inal c

ord

IAS

EAS

FAECES

+ Distention

-

VIP

ATP

+

- Pudendal

nerve

Function & Dysfunction in the GI tract

Physiology Pathology

• Growth/development

• secretion

• absorption

• motility & signalling to

CNS

• surveillance (immuno/metabolic)

• co-ordination (neurons/hormones)

cancer

peptic ulcer, cystic fibrosis

malabsorption

oesophagitis, gastroparesis,

non-ulcer dyspepsia and

irritable bowel

ulcerative colitis, Crohn’s

disease, Coeliac disease

aganglionic colon

(Hirschsprung’s)

GI system case-based learning (CBL)

1. A problem that’s difficult to swallow

2. Troubles with gastrin

3. No more fish and chips!

4. Gap year travels in India

Indigestion

Gallstone pancreatitis

Cholera

Achalasia