kuliah 1. fungsi mekanik sistem pencernaan

7/25/2019 Kuliah 1. Fungsi Mekanik Sistem Pencernaan

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FUNGSI MEKANIK

SISTEM PENCERNAAN

Rahmatina B. HermanBagian Fisiologi

Fakultas Kedokteran Universitas Andalas


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Introduction

The primary function of the digestive system is totransfer nutrients, water and electrolytesfrom the

ingested food into the bodys internal environment

The ingested food is essential as:

- an energy source from which the cells can

generate ATP to carry out their particular

energy-dependent activities

- a source of building supplies for the renewal

and addition of body tissues


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The digestive system performs:

Introduction..

1. Motility along gastrointestinal tract

2. Secretionof digestive juices3. Digestion of food

4. Absorption the small absorbable units

Excretionof the waste materials

Regulationof digestive function through neuralreflexes and hormonal pathways

Protectionagainst any damages

Four basic digestive processes:


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General Principle of Gastrointestinal Motility

Characteristics of gastrointestinal wall:Layers of the wall (inward):

- Serosa: continues onto mesentery

- Muscularis: smooth muscles> longitudinal layer: outer

> circular layer: inner

- Submucous: smooth muscles

> muscularis mucosae, usually longitudinal- Mucosa

Each layer functions as syncytium


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General Principle of Gastrointestinal Motility..


7/29Organization of the intestine wall into functional layers


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Basic Electrical Activity

Slow Waves:- undulating changes in the resting membrane

potential (-50 - 60 mVolts)

- intensity: 515 mVolts

- frequency: 3-12/min (3 in the body of the stomach,

12 in the duodenum, 8-9 in the terminal ileum)

- might be caused by a slow undulation of the

sodium-potassium pumping activities- rarely cause muscle contraction

- Basic Electrical Rhythm (BER) control the

appearance of intermittent peristaltic


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Basic Electrical Activity..

Spike Potential:- is true action potential

- generated at the peaks of the slow waves, when

the peaks rise above - 40 mVolts

- the depolarizing portion of each spike potential is

due to Ca++influx through Ca-Na channel and

repolarizing portion is due to K+ efflux

- stimulation by stretch, acetylcholine,parasympathetics, and specific GI hormones

- rate: 1-10/minute

- duration: 10-20 msecond


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Basic Electrical Activity..


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Contraction of GI Smooth Muscles

Rhythmic:- during the spike potentials generated at the

peaks of the slow waves, the large quantities of

Ca++

enter the fibers through Ca-Na channel- Ca++acting through a calmodulin control

mechanism, activate the myosin filaments

attractive forces between the myosin and theactin filaments muscle contraction


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Contraction of GI Smooth Muscles..

Tonic:- is continuous contraction, often lasting several

minutes or even several hours

- not associated with the BER

- often decreases or increases in intensity, but continues

- might be caused by:

> repetitive spike potential

> hormones or other factors that bring about

continuous depolarization of the smooth muscle

membrane, without causing action potentials

> continuous Ca++influx, bought about in ways not

associated with changes in the membrane potential


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Basic electrical rhythm (BER) relation to muscle contraction


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Gastrointestinal Motility

Propulsivemovement- The basic propulsive movement is peristalsis

- Reflex response through myenteric plexus that isinitiated by distention peristaltic reflexor

myenteric reflex- Constriction at the upper of the stretch and

relaxation (receptive relaxation) at the lower of thestretch

- The wave contraction moves in an oral-to-analdirection

- The peristaltic reflex plus the anal-ward direction ofthe peristalsis movement is called the law of gut


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Peristalsis


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Gastrointestinal Motility..

Mixing:- The reflex initiated by stretching of the gut wall

- Quite different in in different parts of the gut

- Segmentation contraction and relaxation in arhythmic pattern

- Local constriction occurs in every few centimeters

- The next constriction occurs at the next site, so that

the chyme is divided and push back and forth whichmixes the luminal content

- Mixing process is also performed by peristalsis and

sphincter activities


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Migrating Motor Complex

Modification of the pattern of electrical and motoractivity in GI smooth muscle during fasting orbetween periods of digestion

The cycles of motor activity migrate from corpus of

stomach to the distal ileumThey migrate slowly at a rate of 6-12 cm/ minute andoccur at intervals 1.5-2 hours

Each wave end within 6-10 minutes

Function: to clear the stomach and small intestines ofluminal contents in preparation for the next meal

Immediately stopped by ingestion of food and returnto peristalsis


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Mastication (Chewing)

ReflexRhythmic: by motor branch of the 5thcranial nerve

Important for digestion of all foods, especially:

- cellulose membrane of fruits and vegetables

- grinding the food to a very fine particulate consistency- increases the total surface area of food exposed to the

intestinal secretion

- increases the ease of enzymatic activities- prevent excoriation of the GIT

- increases the ease with which food is emptied from the

stomach into the small intestine and succeeding

segments


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Swallowing (Deglutition)

A. Voluntary stage:squeezed into pharynx by pressure of tongue

B. Pharyngeal stage (involuntary): reflex- soft palate is pulled upward to close posterior nares- form a palatopharyngeal slit by palatopharyngeal folds- prevent passage of food into trachea by epiglottis and

vocal cords- pharyngoesophageal sphincter relaxes- the food is propelled into esophagus

C. Esophageal stage (involuntary)peristaltic:

primary peristalsis: continuation from pharynx secondary peristalsis: due to distention of esophagus

The swallowing center inhibits the respiratory center


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Swallowing (Deglutition)..


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Gastroesophageal Sphincter

2-5 cm above the junction of esophagus-stomach

Normally remains tonically constricted

Prevention of stomach content reflux into esophagus

Additional prevention of reflux by valve-like closure of

the distal end of the esophagus- valve-like mechanism of the short portion of esophagus

that lies immediately beneath diaphragm beforereaching stomach

- increased intra-abdominal pressure caves the esophagusinward at this point at the same time that this pressureincrease the intra-gastric pressure

- the valve-like closure of the lower esophagus preventsfrom forcing stomach contents into esophagus (during

walking, coughing, or breathing hard)


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Motor Functions of The Stomach

A.Storage of large quantities of food- As food enters the stomach, it forms concentric circles- Vagovagal reflex: reduces the tone in the muscular wall of

the body of the stomach

B. Mixing the food with the gastric secretions until it forms

a semifluid mixture- peristaltic constrictor rings- propulsive movement toward pylorus- retropulsion

C. Emptying of the stomach- role of antrum: constriction movement- role of pylorus (pyloric sphincter): pyloric pump

Hunger contractions: hunger pang- usually do not begin until 12-24 hours after the last ingestion

- reach greatest intensity in 3-4 days and the gradually weaken


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Motor Functions of The Stomach..


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Regulation of Stomach Emptying

The weak gastric factors that promote emptying:- Effect of gastric food volume on rate of emptying

> vagovagal and myenteric reflexes

- Effect of hormone gastrin

The powerful duodenal factors that inhibit emptying:

- Enterogastric nervous reflexes from duodenum> degree of distention of duodenum

> especially sensitive to the presence of irritant and acids

> also breakdown products of protein digestion

> degree of osmolality

- Hormonal feedback from duodenumcholecystokinin, secretin, gastric inhibitory peptide


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Motor Function of The Small Intestine

A.Mixing contractions (segmentation contractions)- BER with background excitation by the ENS

B. Propulsive movements (peristalsis)- increased due to:

distention gastroenteric reflexes through the ENS hormone gastrin, cholecystokinin, insulin, serotonin irritation peristaltic rush diarrhea

- decreased due to: hormone secretin and glucagon peristalsis in the fasting: migrating motor complex

C. Movements caused by the muscularis mucosa and

muscle fibers of the villi


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Motor Function of The Small Intestine..

Segmentation movement of the small intestine


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Motor Function of The Colon

A. Mixing movements - haustrationsB. Propulsive movementsmass movements

- slow analward movement of haustral contractions- mass movement : modification of peristalsis initiated

by: gastrocolic and duodenocolic reflexes

irritation and distention

parasympathetic stimulation

C. Defecation :- intrinsic defecation reflex- parasympathetic defecation reflex- internal anal sphincter

- external anal sphincter (voluntary)


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Motor Function of The Colon..

Defecation reflex


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kuliah 1. fungsi mekanik sistem pencernaan

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