L09-GI function & control

Question 1

Gastrointestinal System components

Answer 1

A) GASTROINTESTIINAL TRACT

1) Alimentary canal

• oral cavity
• pharynx
• oesophagus
• stomach
• small intestine (duodenum, jejunum, ileum)
• large intestine (caecum, colon, rectum)
• anus

2) Sphincters

• upper esophageal sphincter
• lower esophageal sphincter
• pyloric sphincter
• ileocaecal sphincter
• anal sphincter

B) ACESSORY ORGANS

3) Salivary glands
4) Liver & gall bladder
5) Pancreas

Question 2

GIT innervation overview

Answer 2

Controlled involuntarily via the autonomic nervous system:

1) Parasympathetic

• Vagal nuclei
• Sacral spinal cord

2) Sympathetic
- paravertebral and prevertebral ganglia

The above two leads to the local enteric nervous system:

3) Submucosal plexus (Meissner’s plexus)
4) Myenteric plexus (Auerbach’s plexus)

Question 3

GIT sympathetic innervation

Answer 3

Originates from preganglionic nerve fibres from the central nervous system. Via the thoracolumbar region of spinal cord, they enter:

1) Paravertebral sympathetic ganglion
- Superior cervical ganglion
2) Prevertebral sympathetic ganglion
- Coeliac ganlion
- Superior mesenteric ganglion
- Inferior mesenteric ganglion
3) Hypogastric plexus

Function: Usually inhibit motility & secretion; excitatory to sphincters

Nature: Adrenergic

Question 4

GIT parasympathetic innervation

Answer 4

Originates from the vagal nuclei in the medulla oblongata and the neurons in the sacral spinal cord. Connected to the GIT via the vagal nerve and pelvic spinal nerves respectively.

Extrinsic innervation terminates on neurons of enteric nervous system, influences motor & secretory activity via the enteric nervous system

Function: Usually stimulates motility & secretion

Nature: Cholinergic

Question 5

Enteric Nervous System

Answer 5

Consists of:

1) Myenteric plexus
- between circular and longitudinal muscles of muscularis externae)
- control smooth muscle activity (muscularis externae)
2) Submucosal plexus
- in submucosal layer
- control submucosal gland secretion and smooth muscle activity of muscularis mucosae (secretomotor)

• Submucosal & myenteric plexuses contain motor, sensory & interneurons;
• Form reflex arcs that are located wholly within the GIT
• Can coordinate GI activity in the absence of extrinsic innervation; also influenced by extrinsic autonomic innervation
• Releases mediators like acetylcholine, nitric oxide, vasoactive intestinal peptides (VIP), enkephalins, serotonin, substance P & other neuropeptides

Question 6

Processing of Food in GI Tract

Answer 6

1) Digestion

• Chemical breakdown of food into molecules able to cross mucosa and absorbed into the blood
• Availability of digestive enzymes depends on regulation of secretion; enzyme functions at optimal pH

2) Absorption

• Absorption of nutrients, vitamins, minerals, water & electrolytes
• Large absorptive capacity
• Almost all digestive products absorbed completely in early part of small intestine
• About 90% ingested and secreted fluid absorbed in small intestine, remainder absorbed in colon, about 100 ml excreted in feces

Question 7

Exocrine Secretions of GI System

Answer 7

Salivary glands

• amylase (digest starch)
• electrolyte & water (moisten food)
• mucus (lubrication)

Esophagus

• mucus (lubrication)

Stomach

• mucus (lubrication)
• HCl (kill microbes, activate enzymes)
• intrinsic factor (absorption of vitamin B₁₂ in small intestine)
• enzyme (protease)

Pancreas

• enzymes (protease, lipase, amylase)
• Bicarbonate (neutralize HCl from stomach, activate enzymes)

Liver

• Bile salt (emulsification of fat)
• bicarbonate (neutralize HCl from stomach, activate enzymes)
• organic waste products (to be eliminated in feces)

Small intestine

• mucus (lubrication)
• enzyme (disaccharidease, peptidase)
• electrolyte & water (maintain fluidity of lumenal content)

Large intestine

• mucus (lubrication)

Question 8

Functions of GI Organs

Answer 8

Mouth

• intake of food
• mastication
• site of carbohydrate digestion

Pharynx

• Swallowing

Esophagus

• Transit of food to stomach

Stomach

• Store, mix and dissolve food
• controlled emptying
• initiation of protein digestion

Duodenum

• Carbohydrate digestion & absorption
• Protein digestion & absorption
• Fat digestion & absorption

Jejunum

• Electrolyte & water transport

Ileum

• Bile salt & Vitamin B₁₂ transport

Colon

• Storage, concentration of undigested material
• Electrolyte & water transport
• Propulsion of content

Rectum & Anus

• defecation

Question 9

Functions of GI motility

Answer 9

• Chewing & swallowing
• Propulsive movement from mouth to anus
• Mixing of luminal content
• Reduce size of food particles
• Storage function
• Sphincter function
• Optimize time needed for digestion & absorption
• Defaecation

Question 10

Phases of GI Control

Answer 10

Three Phases:

1) Cephalic phase

• Stimuli from higher centers (thought, sight, smell of food) or mouth (taste, chewing of food)

2) Gastric Phase

• Stimuli from food in the stomach (gastric distension, chyme acidity, osmolarity)

3) Intestinal Phase

• Stimulus arising from chyme in intestine

Question 11

GI Control Mechanisms

Answer 11

Regulated by GI reflexes which involve:

1) Neural regulation (Neurocrine)

• Short neural reflex
• Long neural reflex

2) Chemical regulation

• Endocrine
• Paracrine

Question 12

GI neurocrine Control

Answer 12

Higher centers in CNS can initiate control from CNS to enteric nerve plexuses and effector cells via ANS

1) Short reflexes (Enteric reflexes)
- from GI receptors through enteric nerve plexuses to effector cells
2) Long reflexes
- from GI receptors to CNS by afferent nerves & back to enteric nerve plexuses and effector cells by way of ANS
- e.g. vagovagal reflex

Parasympathetic: excitatory

Sympathetic: inhibitory

Question 13

GI Chemical regulation

Answer 13

Regulation of GI functions like:

• Rate & composition of GI secretion
• Motility of gut & sphincters, gallbladder
• Release of other GI peptides & hormones
• Trophic effects on exocrine glands & mucosa

Mucosal endocrine cells are not organized into discrete isolated glands, but as single cells scattered over a wide area of the mucosa

Hormones:

• Gastrin
• cholecystokinin (CCK)
• secretin
• Gastric inhibitory peptide (GIP)
• Motilin

(-) Vasoactive intestinal peptide (VIP)

(-) Bombesin or Gastrin releasing peptide (GRP)

(-) Enkephalin

Paracrine:

• somatostatin
• histamine

Question 14

Gastrin features

Answer 14

Action:

• Stimulates gastric acid secretion
• Trophic effect of gastric oxyntic gland mucosa

Site of release: Antrum (& duodenum)

Triggers:

• Peptides
• Amino acids
• Gastric distension
• Vagal stimulation

Question 15

cholecystokinin features

Answer 15

Action:

• gall bladder contraction
• pancreatic enzyme secretion
• pancreatic bicarbonate secretion
• growth of exocrine pancreas
• inhibits gastric emptying

Site of release: duodenum & jejunum

Triggers:

• Peptides
• Amino acids
• Fatty acids > 8C

(- Acids ?)

Question 16

Secretin features

Answer 16

Action:

• pancreatic bicarbonate secretion
• Biliary bicarbonate secretion
• growth of exocrine pancreas
• Pepsin secretion
• inhibits gastric acid secretion
• inhibits trophic effect of gastrin

Site of release: duodenum

Triggers:

• Acid

(- fat ?)

Question 17

Gastric inhibitory peptide features

Answer 17

Action:

• stimulates insulin releas
• inhibits gastric acid secretion

Site of release: duodenum & jejunum

Triggers:

• glucose
• amino acids
• Fatty acids

Question 18

Motilin features

Answer 18

Action:

• stimulates gastric motility
• stimulates intestinal motility

Site of release: duodenum & Jejunum

Triggers:

• Nerves
• Fat
• Acid

Question 19

Vasoactive Intestinal peptide features

Answer 19

Action:

• Relaxes sphincters
• Relaxes gut circular muscles
• Stimulates intestinal secretion
• Stimulates pancreatic secretion

Site of release: mucosa & smooth muscle of GIT

Question 20

Bombesin/ GRP features

Answer 20

Action: Stimulates gastrin release

Site of release: gastric mucosa

Question 21

Enkephalins

Answer 21

Action:

• Stimulates smooth muscle contraction
• Inhibits intestinal secretion

Site of release: Mucosa and smooth muscle of GIT

Question 22

Somatostatin features

Answer 22

Action:

• inhibits gastrin release
• inhibits peptide hormone release
• inhibits gastric acid secretion

Site of release: GI Mucosa and pancreatic islets

Triggers:

• Acid
• Vagus inhibits release

Question 23

Histamine features

Answer 23

Action: Stimulates gastric acid secretion

Site of release: parietal cells (oxyntic gland mucosa) and ECL cells

Triggers: Gastrin