Phy 1: Neurohormonal control of GIT

What does the digestive canal consist of

Mouth, esophagus , stomach , small intestine , large intestine , rectum & anal canal

What do the digestive glands include

Include salivary glands, liver & pancreas.

How is the GIT functions regulated

This is done by:-
I. Nervous regulation. II. Hormonal regulation.

What is nervous regulation

It is done by the autonomic nervous system which has an extrinsic component and an intrinsic component. - The extrinsic component is the sympathetic and parasympathetic. - The intrinsic component is called the enteric nervous system.

What are the components of intrinsic innervation/ Enteric nervous system

Components:

- It is formed of 2 plexuses:

i) Myenteric (Auerbach ‘s plexus) and

ii) Submucosal plexuse (Meissner ‘s plexus).

- The intrinsic or enteric nervous system can direct all functions of the GIT even in absence of extrinsic innervation.

- The number of their neurons is equal to the number of the neurons in the entire spinal cord.

- Extends from the lower oesphagus till end of GIT

What are the connections of the enteric nervous system

-The ganglia receive input from the parasympathetic and sympathetic nervous system which modulate their activity.

- The ganglia also receive sensory information directly from mechano- receptors and chemoreceptors in the mucosa and send motor information directly to smooth muscle , secretory and endocrine cells.

- The ganglia of the two plexuses are connected by interneurons

What are the functions of the plexuses?

Functions:

i- Submucosal plexuse controls secretory activity and blood flow

ii- Myentric plexus controls motility

What are the types of extrinsic innervation?

1) Parasympathetic innervation

2) Sympathetic innervation

What are the divisions of parasym. innervation, and their functions?

1) Cranial division:

Origin:

Vagus nerve, vagal fibers supply esophagus, stomach, duodenum, small intestine and the proximal half of large intestine.

2) Sacral division

Origin:

It arises from 2 sacral segments of the spinal cord, passing through the pelvic nerve and supplies distal half of large intestine.

What is the function of parasym. innervation?

• Stimulates all functions of the gut.

• Produce vaso-dilatation.

What is the origin of sym. innervation?

Origin:

It arises from T5-L2 segments of the spinal cord, relaying in collateral ganglia.

What is the function of sym. innervation?

• Inhibit all functions of the gut.

• Produce vaso-constriction.

What are the types of GIT reflexes?

a) Short reflexes

b) Long reflexes

What are the properties of short reflexes and their function?

Properties:

- Occur entirely in ENS.

- CNS is not involved.

Function:

- Responsible for self regulation of GIT functions e.g GIT secretions.

What are the properties of long reflexes and their function?

Properties:

- Involves CNS & mediated by autonomic nerves\

Functions:

They regulate GIT functions in response to:

• Impulses from outside the gut e.g olfactory & visual stimuli.

• Pain & emotions.

What does the hormonal regulation system of GIT functions consist pf?

The digestive system has its own endocrinal system.

Consisting of:

Special cells in GIT mucosa called APUD cells = amine precursor uptake & decarboxylation, they secrete special hormones called GIT hormones.

What systems control GIT hormone secretion?

1- Nervous factors:

• Enteric nervous system.

• Autonomic nervous system. 2- Chemical factors, either - Luminal e.g products of digestion. - Blood born material e.g hormones

What is the classification of GIT according to structural similarity?

1) Gastrin family: Gastrin cholecystokinin

2) Secretin family: Secretin. Vaso-active intestinal peptide (VIP). Glucose dependent insulinotropic peptide (GIP).

What is the gastrin hormones structure, source, and actions?

Structure:

• A polypeptide hormone present in several forms : G14, G17 , G34.

• All forms have the same C terminal 5 amino acids.

Source:

• G cells: in the antral mucosa of stomach.

• TG cells: widely distributed in stomach & small intestine

Actions:

1- Stimulate HCL & pepsinogens secretion.

2- Stimulate gastric motility.

3- Stimulates GIT mucosal growth.

What factors stimulate gastrin hormone?

1) Nervous factors:

Vagal stimulation → increases gastrin.

2) Luminal factors:

• Distension of the stomach by food.

• Decreased acidity.

• Increased amino acids

What factors inhibit gastrin hormone?

a) Luminal factors:

Increased acidity

b) Blood borne factors:

• Secretin

• GIP&VIP.

• Somatostatin

What is the secretin hormones structure, source, and actions?

Structure:

• A polypeptide formed of 27 amino acid.

Source:

• S cells in the upper small intestine.

Actions:

• It stimulates the exocrine pancreatic secretion rich in HCO3 by

1- Neutralization of duodenal acidity.

2- Providing the alkaline medium needed for the action of pancreatic enzymes.

N.B smokers complain of peptic ulcer and hyperacidity because nicotine destroys S cells.

What factors stimulate secretin hormone?

• Acidity of the fluid passing by the duodenum.

There is +ve feedback between secretion of secretin and acidity, increased acidity → increased secretin which by its turn stimulates pancreatic exocrine secretion rich in bicarbonate.

What factors inhibit secretin hormone?

• Decreased acidity.

• Somatostatin.

What is the cholecystokinin hormones structure, source, and actions?

Source:

• I cells in the upper small intestine.

Actions:

• Stimulates contraction of the gall bladder → increased bile flow.

• Stimulates pancreatic secretion rich in enzymes.

• Secretes bicarbonate from pancreas

What factors stimulate cholecystokinin hormone?

• Fatty acids and monoglycerides in the duodenum.

• Products of protein digestion.

What factors inhibit cholecystokinin hormone?

• Somatostatin

What is the GIP (Glucose Dependent Insulinotropic peptide) hormones structure, source, and actions?

Structure:

• AA polypeptide consist of 42 AA

Source:

• K cells of upper small intestine

Actions:

• Gastric acid secretion

• Insulin secretion, some types of diabetes mellitus are due to deficiency of GIP.

N.B: the old name of GIP is Gastric Inhibitory Peptide

What is the VIP (Vasoactive Intestinal Peptide) hormones source and actions?

Source:

• Nerves of GIT

• Brain

• Blood

Actions:

• Decreased gastric acid secretion

• Increased intestinal secretions

What is the somatostatin hormones source and actions?

Source:

• D cells in pancreas & intestine

Actions:

• Decreases gastric acid secretion

• Deceases pancreatic secretion.

What is the Motilin hormones source and actions?

Source:

• Special cells in the duodenum during fasting states

Actions:

• Regulates interdigestive motility