Gastrointestinal physiology Transport & mixing of food (Cont).

Dr.M.A.M.Shaikhani.

Colonic movements(Ms):2 types:1.Mixing or segmentation movements(SMs); same as SI . Large circular constriction contractions scattered along colon. Caused by the combined contraction of circular muscles (about

2.5 cms) & longitudinal muscle fibers( which are arranged into 3 longitudinal strips called (Tenae coli)cause the colonic wall to bulge outside into baglike sacs causing the appearance of haustrations .

Help in mixing colonic contents & absorbing water & electrolytes from the wall so concentrating it to semifluid, mush, semimush, solid & finally hard food residues called stool.

2.propulsive or mass Ms: forward Ms. Help to push stool towards the rectum & initiating

the defecation reflex. more abundant in the transverse & sigmoid colon, stimulated by

distention or irritation of colon.

HAUSTRAL CONTRACTIONS

Foodresidue

Haustra

MASS MOVEMENTS

Food residue

Rectum

Defecation reflex:

The rectum is usually empty.

when the stool enters the rectum

the sensation of its presence is transmitted to the myenteric plexus & through parasympathetic pelvic nerves to the spinal cord to initiate colonic contractions .

External sphincter relaxation occur through skeletal motor nerves starting defecation when the situation is proper as there is higher centers control from conscious cortex over the whole defecation reflex.

Figure 24.25

The Defecation Reflex

Figure 24.25

Stretchreceptors

Sensoryneurone

Analsphincters

Parasympatheticneurone

Somaticneurone

DEFECATIONREFLEX

Rectum

Reflexes in the colon and rectum

Mass movements

+

+

Food in stomach

Food in

duodenum

Faeces

Defaecation reflex

Colonrectum

anus

Ach

Sp

inal co

rd

IAS

EAS

FAECES

+Distention

-

VIPATP

+

-Pudendal

nerve

The Defecation Reflex: Summary

Removes undigested faeces from the body.

Stretch receptors in GIT wall detect distension of rectum.

Parasympathetic reflex causes contractions of the sigmoid colon & rectum + relaxation of internal anal sphincter.External anal sphincter (under voluntary control) consciously relaxed if appropriate.

Secretary functions of GIT:Of 2 types:1.Enzymes helping digestion.2.Mucous for lubrication & protection of GIT mucosal surfaces from excoriation. Anatomical types of secretary glands:1.Mucous glands: 2 types:a.single cell (goblet cells).b.complex cell mucous glands.2.Crypts of Liberkhan: deeper & contains specialized secretary cells. 3.Deep tubular glands in stomach & upper duodenum secreting acid & pepsinogen.4.Complex glands like salivary ,pancreatic & hepatic glands.

3Mechanisms of stimulation of GIT glands:

1.Local contact of food with GIT mucosal surfaces activating enteric nervous system through:

A.chemical irritation.

B. tactile irritation.

C.distension.

2.Autonomic stimulation:

specially through the parasympathetics (vagi & other cranial parasymp. Nerves) stimulating eso.,stomach,pancreas, Brunner glands in duodenum & glands of distal colon,while secretion in the remainder of SI & 1st 2/3 of large intestine(LI) is stimulated by local myenteric nerves & hormones locally in each segment.

The sympathetics also slightly increase the glandular secretion but through vascular constriction reduces secretion as an overall effect.

Secretion of saliva:Daily secretion is 1 liter in a rate of 0.5 ml/min. during day & very

little during sleep. It contain 2 major types of secretions:1.serous secretion containing ptyalin ,an alpha-amylase for digesting

starch.2.mucous secretion for lubrication. salivary glands consist of acini & ducts ,in the acini there is

primary secretion of ptyalin,mucous & extracellular fluid while in the ducts there is K+ & HCO3- secretion & active Na+ reabsorbtion & passive Cl- reabsorbtion,so as a result there is high K+ & HCO3- & low Na+ & Cl- in the saliva.

Functions of saliva:1.Digesting starch by ptyalin.2.Maintaining oral hygiene through:A.washing away pathogenic bacteria & food particles.B.it has bactericidal activity through its thiocyanate , proteolytic

enzymes as lysozyme & protein antibodies contents.

Nervous regulation of salivary secretion: Stemulation through parasympathetic NS(PNS) from salivary

nuclei located at the brain ponto-medulary junction excited by taste(specially sour) & tactile stimuli(as presence of smooth objects in mouth),

inhibited or stimulated by higher centers specially the appetite center located close to PNS center in the anterior hypothalamus which function in response to signals from taste & smell areas of the cerebral cortex or amygdala.

salivation also occurs in response to reflexes in the stomach& upper intestine by the presence of irritating food or nausea since saliva has diluting & acid neutralizing effect.

.Esophageal secretions:

no enzymes , only mucous secreted by simple & complex mucous glands.

CONTROL OF SALIVARY SECRETION

cerebral cortex

salivary centrein medulla

autonomic nerves

salivary glands

salivary secretion

pressure receptorsand chemoreceptors

in the mouth

other inputs:smell& taste centers

conditionedreflex

simplereflex

Gastric secretions: 3 types of secretary glands

1.Oxyntic(parietal) glands :in the body & fundus of S ,80% of Stomach glands ,secret Hcl,pepsinogen,intrinsic factor & small amount of mucous .It contains 3 types of cells:

a.mMucous neck cells secreting mucous.

b.Peptic or chief cells secreting pepsinogen.

c.Parietal or oxyntic cells secreting Hcl& intrinsic factor essential for vitamin B12 absorption.

2.Pyeloric glands : in antrum ,secret mainly mucous & very important hormone called Gastrin & small amounts of pepsinogen .

3.Numerous mucous glands:between the above 2 main glands

,secret large amounts of mucous which covers & protects the S wall by a protective layer from digestion by acid-pepsin.

less important enzymes include gastric lipase,amylase & gelatinase.

HCl

Gastrin

Histamine

Pepsinogen

Activation of pepsinogen to pepsin:

Done by Hcl as it is inactive in an alkaline medium.

Regulation of gastric acid secretion:

stimulated by 3 hormones:ACH(PNS),gastrin & Histamine:

1.PNS (vagus) stimulation :secreting ACH which can be blocked by anticholinergic drugs pirenzepen used for peptic ulcer(PU) therapy.

2.Gastrin release from antral glands which can be blocked by proglumide.

3.Histamine release stimulating H2 receptors which can be blocked by H2 blockers as cimetidin(Tagamet).

The 3 above hormones secret Hcl through activation of the proton pump(H+-K+ ATPase)which is the final common pathway in acid secretion which can be blocked by omperazole ,an effective therapy for peptic ulceration & hyperacidity.

Figure 24.14

The Secretions of Hydrochloric Acid

GASTRIN

histamine

Parietal cell

ECL cell

Chief cell

D-cell

somatostatin -

+

noradrenaline,

CCK, VIP & CGRP

Ach

H+

-

+

+

Gland lumen

+

THE CONTROL OF ACID SECRETION

Acid inhibitory therapy

H2 receptor antagonists

Histamine

Parietal cell

H2 receptor

H+/K+ ATPase

(the proton pump)

H+K+

Proton pump inhibitors

Peptic ulcer

reflux oesophagitis “heart burn”

Tagamet, Zantac,

Pepcid AC

Omeprazole

(Losec/Nexium)

Gastrin:Proglumide

Vagus:pirenzepine

Inhibition of gastric acid secretion:By: 1.Acid feed back(FB) inhibition in the presence of excess acid

when the S (PH) becomes 3 or less. In patients with (PU) this (FB) inhibition is abnormal so Hcl

continue to be secreted in spite of very high acid& low PH in the stomach leading to PU.

2.Through the enterogastric reflex in the presence of excess acid,fat & protein breakdown products,hyperosmolar fluid ,distention or any irritating factor in the upper SI which cause the release of several inhibitory intestinal hormones as secretin,CCK,Gastrin inhibitory peptide & somatostatin.

3.Interdigestive period: in this period between meals the S secrets few mls. Of gastric juice containing little enzymes, more mucous & moderate amounts of HCO3 called non-oxyntic type of secretion .This interdigestive type of secretion may change with high enzyme-acid content in patients with PU & those with emotional upsets.

.

G. Phases of gastric secretion:Consists of 3 phases;1.cephalic phase : 1/5 of gastric secretion associated with eating a meal. occurs before or while the food is eaten.It is stimulated centrally.2.Gastric phase: 2/3 of total acid secretion associated with eating a mael. Occurs when the food enters the S stimulated by long vagovagal

reflexes,local enteric & gastrin mechanisms.3.Intestinal phase: acid secretion in response to presence of food in upper intestine

specially the duodenumdue to small amounts of gastrin released by duodenal mucosa in

response to distention & chemical irritation.

1. CEPHALIC PHASE

Sight, smell orthought of food

- Parasympathetic activationof gastric motility & gastric juice secretion

Vagusnerve

Food arrival causes muscular reflexes & gastrin secretion byG cells.

Gastrin stimulates secretion from both chief &parietal cells.

2. GASTRIC PHASE

Gastrin

GOGOFOODFOOD

Arrival of food in duodenum triggers release of hormonesthat inhibit gastric motility &secretions.

3. INTESTINAL PHASE

Circulation

Secretin &Cholecystokinin (CCK)