git-revision for gastrointestinal physiology

7/31/2019 Git-revision for Gastrointestinal Physiology

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SMALL INTESTINE

M- mixing ad propulsion mainly

Electrical- small bowel slow waves, migrating motor complexes

Mechanical- peristalsis - segmentation contractiontonic contraction-movement of villimovement of ileocaecal valve - peristaltic rush

S- enzymes, HCO3 and salt & water, mucus (goblet cells)

- enzymes (pancreas)

- bile, HCO3 (liver)

- secretin (duodenal S cells)

- CCK (lining cells of duodenum)

- GIP (duo. & jejunum mucosal cells)

- motilin (duo. & jejunum mucosal M cells)

- VIP (jejunum)

- substance P (endothelial cells of GI)- gastrin release peptide (GRP)wide spread distribution

D- carbohydrates, fat, polypeptides, nucleic acids

A- peptides by active transport

- amino acids, glucose & fructose by secondary actve transport

- fats by simple diffusion

- water by osmosis

- ions, minerals & vitamins by active transport

OF- maintains fluidity of intestinal content

- lubrication, neutralize HCl entering small intestine

- hydrolytic function- aqueous component of succus entericus provide water & thus helpin all hydrolytic of enzyme reactions of digestion of various food particles.

LARGE INTESTINE

M- segmental mixing, mass movement for propulsion

S- mucus (goblet cells)

D- none (except by bacteria)

A- ions, water, minerals, vitamins, small organic molecules produced by bacteria, salts,glucose alcohol, drugs (anaesthetic agents, sedatives, steroids)

OF- storage& concentration of ingested matter.

- lubrication,- synthesis & absorb folic acid, vitamin B12, vitamin K by bacteria flora- excretion of heavy metal (mercury), lead, bismuth, arsenic.- defecation


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1. INNERVATIONS OF GITPurpose- motility

- secretion

i. Autonomic nervous system extrinsic NS (extrinsic regulation). Afferent- carry sensory from chemoreceptors, mechenoreceptors in GIT

spinal cord, brain stem.

Efferent- carry information from spinal cord, brain stem GIT Parasympathetic

o GI motility & secretiono through vagus & pelvic nerveso preganglionic fibers enteric nervous system

Sympathetico GI motility & secretiono postganglionic nerves GIT

ii. Enteric nervous system (ENS) intrinsic NS known as little brain. regulation is by negative feedback control by release of a variety of GI

hormones.

Myenteric (Auerbachs) plexus Submucous (meissners) plexus Along entire length of gut. Located in submucosa between lies between longi & circular layers of ms circular muscle & muscularis

mucosa

provides motor innervations to 2 ms layersBest developed in submucosa Present in striated ms portion of oesophagus-Innervates glandular

epithelium,muscularis mucosa,

submucosal blood vessels,

intestinal endocrine cells.


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2. SPHINCTERS Some are tonically contracted to close off the GIT from outside world & to

keep materials from passing freely from one section of the tract to another.

They are or selective retention of solutes / chime in specific sites to promoteoptimal digestion & absorption.

i. Upper oesophageal sphincter

o True sphinctero Formed by cricopharyngeal muscle (striated)o Prevent entry of air into oesophagus during normal respiration.

ii. Lower oesophageal sphincter

o also known as cardiac sphincter.o situated at distal 2 cm of oesophaguso is a functional/ physiological sphincter.o principle function is to prevents regurgitation of gastric contents into

oesophagus when the intragastric pressure is markedly raised (after a

heavy meal or ingestion of carbonated drinks) , the resistance of LES is

overcome and air escapes into the mouth (belching).

o gastrin increases the tone of LES.iii. Pyloric sphincter

o separates storage component of GIT (stomach) from the digestive andabsorptive components (small & large intestine) duodenum.

o regulated gastric emptying and prevents duodenal-gastric reflux.o secretin, CCK causes contraction delay gastric emptying.

iv.

Ileocaecal sphinctero separates ileum from caecum.o regulates ileal flow into the colon.o distension of ileum relaxation of sphinctero distension of colon contraction of sphinctero under control of vagus, sympathetic, ENS.

v. Sphincter of Oddio regulates the movements of contents of the common bile duct into the

duodenum.

vi. Internal & external anal sphincterso Internal is formed by both the longitudinal & circular smooth muscle .o under involuntary control.o External sphincter (contain striated muscle), under both voluntary &

involuntary control.


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MOTILITY

1. Motility moves food from mouth to anus and mechanically mixes food.2. GI smooth muscles are composed of single unit smooth muscle (with the exception of

pharynx, upper 1/3 oesophagus, external anal sphincter- all of which are striated

muscle)and electrically connected by gap junction.3. Different regions exhibit different types of contraction. Some segments are tonically

contracted but others exhibit phasic contraction.

4. Phasic contractions occur in oesophagus, gastric antrum, small intestine whichcontract & relax periodically.

5. Tonic contraction occurs in sphincters.6. Intestinal muscles exhibit spontaneous slow wave potentials. When slow wave

reaches threshold, it fires action potential & contract.

7. Slow waves originate in the interstitial cells of Cajal.8. Between meals, migrationg motor complex (MMC) moves food remnants from upper

GIT to lower regions (house-keeping).

9. Peristaltic contractions are progressive waves of contraction. Peristalsis is mediatedby the ENS and altered by hormones, paracrine signals, meuropeptides.

10.Segmental contractions are primarily mixing contractions.11.Depolarisation of circular muscle leads to contraction of ring of smooth muscle

diameter of that segment.

12.Depolarisation of longitudinal muscle leads to contraction in longitudinal direction length of that segment.

PERISTALTIC CONTRACTIONS

create forward movement stimuluswhen the wall is stretched (the response is called myogenic reflex. are most important in oesophagus in normal digestion periods they are limited to short distances a deep contraction (peristaltic wave) from behind the point of stimulation and passes

along the intestine towards the rectum at rates varying 2-25cm/sec with varying

intensities

when there is obstructionperistaltic rushes (very intense peristaltic waves) occur.SEGMENTATION CONTRACTION

ring-like contractions that appear at fairly regular intervals along the gut, thendisappears and are replaced by another set of ring contractions in the segments

between the previous contractions

in the contracting segment, circular muscles contract while longitudinal muscles relax. these contractions may occur randomly along the intestine or at regular intervals. Alternating segmental contractions chum the intestinal content, mixing them with

various secretions of intestine & keeping them in contact with absorptive epithelium.

no net forward movement.


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TONIC CONTRACTIONS

relatively prolonged contractions that in effect isolate one segment of intestine fromanother.

slow transit permit longer contact of the chime with the enterocytes & promotes absorption.

PHYSIOLOGY OF SWALLOWING

Food Stimulus

Swallowing receptor areas of pharynx Receptors(especially on tonsillar pillars)

Sensory portions of trigeminal and Afferentsglossopharyngeal nerves

Deglutition or swallowing center Centerin the medulla and lower pons

Respiratory center Motor impulses

Deglutition apnoea V,IX, XI, XII cranial nerves Efferents(respiration inhibited) +

Few superior cranial nerves

Series of automatic pharyngeal Effects

muscle contractions &Responses

Sequence of events in swallowing(Entire process takes 1-2 seconds)


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1. Tongue sweeps hard palate moving food backward into oropharynx (Oral/buccalphase- voluntary).

2. Nasopharynx closes (superior pharyngeal sphincters)(pharyngeal phase-involuntary.)

3. Respiration inhibited.4. Laryngeal muscle contract to close glottis and elevate larynx.5. Peristalsis initiated by superior pharyngeal constriction and continued by middle and

inferior pharyngeals.

6. Upper oesopharyngeal sphincter (UES) relaxes(oesopharyngeal phase- involuntary)7. Peristaltic wave proceeds down oesophagus.8. Receptive relaxation occurs in the stomach.9. Lower oesophageal sphincter (LES) dilates just prior to contraction wave.

DISORDERS OF SWALLOWING

1. Abolition of deglutition reflexIt causes regurgitation of food into the nose or aspiration into the larynx and trachea.

It may occur

when Ix or X nerve is paralysed in lesions of medulla when pharynx is anaesthetised with cocaine (deglutition reflex is abolished

temporarily)

2. Aerophagia It refers to unavoidable swallowing of air along with the swallowing of food

bolus and liquids.

It usually occurs in nervous individuals having low tone of the upperoesophageal sphincter (UES). Some of the gases present in the air swallowed are absorbed, partly the air is

regurgitated into the oral cavity and out in the atmosphere (belching), and

majority of air passes on the colon and is then expelled as flatus through the

anus.

3. Cardiac achalasia It is a neuromuscular disorder of the lower two-thirds of oesophagus. It is characterised by failure of the lower oesophageal sphincter (LES) to relax

during swallowing and food accumulates in the oesophagus.

4. Gastroesophageal reflux disease (GRD) There is incompetence of LES causing reflux of acidic gastric contents into

oesophagus. oesophageal pain (heart burn) irritation of oesophagus or

bronchioles (due to aspiration).

Conditions associated with GRD Conditions in which emptying of the stomach is not normal (e.g.

pyloric sphincter disease)

When the gastric contents are pushed up against the oesophagus (e.gWhen the patient is lying down).

when the LES is forced through the diaphragm into the thoracic cavity(e.g. hiatal hernia)


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during pregnancy, the growing foetus may push the top of the stomachinto the thorax. The low intra-thoracic pressure (compare to the higher

intra-abdominal pressure) causes the LES to expand, allowing reflux to

occur.

conditions in which LES fails to contract between swallows.

5. Dysphagia It is a term used to denote difficulty in swallowing due to any cause.

GASTRIC MOTILITY

1. Receptive relaxation (storage function, accormodation) is a vaso-vagal reflex

is initiated by distension of stomach abolished by vagotomy. The oral portion of stomach relaxes to accormodate the ingested food. The inhibitory neurotransmitter is either VIP or nitric oxide

2. Mixing and digestion The caudal region (distal body & antral part) contracts to m.ix food with

gastric secretions and begin the process of digestion. The size of the particle

is reduced.

(a)Gastric Slow wavesGSW (Basal Electrical RhythmBER)o represents a wave of depolarisation of stomach cells initiated by

pacemaker cells from the circular muscles of fundus on the greatercurvature to pyloric sphincter.

o consist of upstroke and plateau phase (rate 3-4 waves/min)o electrophysiologically unknown.o assumed that upstroke & plateau are due to influx of sodium & calcium.o in stomach, Ach increases contractile activity (produces peristalsis)by

increasing the amplitude & duration of plateau phase. Thus frequency of

slow waves set the maximal frequency of contraction.

o other agents that initiate contraction of gastric smooth muscle aregastrin, nicotine, barium, potassium.

o agents that inhibit the activity are enterogastrone, E, NE, atropine &calcium.

(b)Migrating Motor Complexes (house-keeping)o are propulsive movements initiated during fasting.o begin in the stomach and move undigested material from the stomach

and small intestine into the colon.

o housekeeping function that sweeps food remnants and bacteria out of theupper GI into large intestine.

o repeats every 90-120 min during fasting.o correlates with high levels of motilin.


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3. Gastric Emptying (GE)The caudal region of the stomach contracts to propel.

The rate of GE is fastest when stomach contents are isotonic.If hypotonic or hypotonic GE slowed.

fats inhibit GE (delayed GE time) by stimulating release of CCK. H+ in duodenum inhibits GE via direct neural reflexes. (via interneurons in the

GI plexus).

GE results from a progressive wave of forceful contractions whichsequentially involves antrum, pylorus, proximal duodenumall 3 functions as

a unit.

Chyme is pumped in a small at a time into small bits- the peristaltic waveswhich provide this pumping action is known as pyloric pump.

The factors regulating GE1. Fluidity of the chime

Liquids empty faster than solids

2. Gastric factors Volume of food in the stomach

o Greater volume greater stretching distension long vagalmediated & short intrinsic neural plexus mediate reflexes

strong peristalsis rate of GE

Gastrino Presence of meat in stomach release gastrin activity of

pyloric pump GE

Type of food ingestedo Carbohydrate rich food rapid GEo Protein rich food slow GEo Fat rich food slowest GE

3. Duodenal factors Enterogastric reflex

o Neurally mediated reflexo Initiated by generation of receptors in duodenal mucosao Important stimuli are

distension of duodenum acidity of stomach content (pH


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MOTILITY OF SMALL INTESTINE

1. Electrical movements Small bowel slow waves Migrating motor complexes

2. Mechanical Peristalsis Segmentation contraction Tonic contraction Movement of villi Movement of ileocaecal valve Peristaltic rush

MOTILITY OF LARGE INTESTINE

1. Haustal shultting Similar to segmentation contraction.

2. Peristalsis3. Mass movement4. Defecation


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SECRETION

SALIVARY SECRETION

Almost entirely under the control of parasympathetic system (promotes secretion). Initial fluid formation in the acinus is via a chloride pump, & electrolyte composition

is similar to interstitial fluid.

NaCl is reabsorbed in the ducts making the secretions hypotonic.Composition

o Low sodium, chloride because of reabsorption.o High in potassium , bicarbonate because of secretion.o amylase secreted in the active form & begins the digestion of carbohydrates.o Low tonicity: because of reabsorption of NaCl and impermeablity of ducts to water.

GASTRIC SECRETIONS

Cell Type Substance Stimulus for release Function of secretion

secreted

Mucous mucous Tonic secretion with Physical barrier between

neck cell irritation of mucosa lumen & epithelium

Bicarbonate Secreted with mucus Buffers gastric acid to

prevent damage to

epithelium

Parietal cells Gastric acid Ach, gastrin, histamine Activate pepsin, kills bacteria

intrinsic factor complexes with vitamin B12

to permit absorption

Entero- Histamine Ach, gastrin Stimulate gastric acid

chromaffin secretion

like cells

(ECL)

Chief cells Pepsin (ogen) Ach, acid, secretin Digest proteinGastric lipase Digest fats

D cells Somatostatin Acid in the stomach Inhibit gastric acid secretion

G cells Gastrin Ach, peptides & Stimulates gastric acid

amino acid secretion


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MECHANISM OF GASTRIC ACID SECRETION BY PARIETAL (OXYNTIC) CELL

RECEPTORS & SIGNAL TRANSDUCTION PATHWAYS IN THE PARIETAL ECLL


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REGULATION OF GASTRIC- ACID SECRETION

Regulation of gastric-acid secretion. In the corpus of the stomach, the vagus nerve not only stimulates theparietal cell directly by releasing ACh, it also stimulates both ECL and D cells. Vagal stimulation of the ECL

cells enhances gastric-acid secretion via increased histamine release. Vagal stimulation of the D cells alsopromotes gastric-acid secretion by inhibiting the release of somatostatin, which would otherwise inhibit-by

paracrine mechanisms-the release of histamine from ECL cells and the secretion of acid by parietal cells. In theantrum of the stomach, the vagus stimulates both G cells and D cells. The vagus stimulates the G cells via GRP

(gastrin-releasing peptide), promoting gastrin release. This gastrin promotes gastric-acid secretion by two

endocrine mechanisms: directly via the parietal cell and indirectly via the ECL cell, which releases histamine.

The vagal stimulation of D cells via ACh inhibits the release of somatostatin, which would otherwise inhibit-by

paracrine mechanisms-the release of gastrin from G cells and-by an endocrine mechanism-acid secretion byparietal cells. Luminal H+ directly stimulates the D cells to release somatostatin, which inhibits gastrin release

from the G cells, thereby reducing gastric-acid secretion (negative feedback). In addition, products of protein

digestion (i.e., peptides and amino acids) directly stimulate the G cells to release gastrin, which stimulates

gastric-acid secretion (positive feedback). ACh, acetylcholine; CCKb, cholecystokinin B; ECL,

enterochromaffin-like; ENS, enteric nervous system.


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SUMMARY OF DIGESTION AND ABSORPTION OF NUTRIENTS

git-revision for gastrointestinal physiology

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