Gastrointestinal physiology

Transport & mixing of food in the GIT:

Dr.M.A.M.Shaikhani.

Food ingestion: stimulated by hunger & Appetite.Mastication swallowing.

Mastication(chewing):

By incisors & molar teeth controlled by:Chewing reflex: stimulated by bolus presence in the mouth causing

reflex inhibition of mastication muscles , lower jaw drops & initiating a stretch reflex of jaw mastication muscles leading to their rebound contraction & repeating again the same series of events until the bolus in fully masticated for easy swallowing.

Importance of chewing:

1.Exposing the total surface area of foods specially the raw vegetables to GIT secretions & enzymes.

2.Prevents excoriation of GIT mucosal surfaces.3.Increasing the easiness of food emptying from stomach to the

s.intestine.

Swallowing process (deglutition)

The Swallowing Process

Swallowing(deglutition). Of 2 stages: 1.Volantary stage :queezing the food & rolling it posteriorly into the pharynx by tongue pressure upward & backward against the palate. 2.Automatic(involuntary) stage :it is automatic & can not be stopped voluntarily. of 2 stages: 1.Pharyngeal stage . 2.esophageal stage.

1.Pharyngeal stage:

The food enters the pharynx stimulates the swallowing receptors around the pharyngeal opening & impulses pass to the brain stem to initiate a series of automatic pharyngeal contractions as follows by 4 stages: A. Soft palate is pulled upward to close the posterior nares to prevent food regurgitation into the nasal (as it occurs in palatal palsy due to 9th & 10th cranial nerve palsy. ) B. Palatopharyngeal folds on either sides are pulled medially to approximate each other forming a narrow sagital slit through which the food must pass into the posterior pharynx allowing the passage of only well masticated bolus ( lasts < 1 second.)

 1.Pharyngeal stage:

C. Strong approximation of laryngeal vocal cords with the backward swinging of epiglottis(due to the presence of a ligament which prevent its upward movement ),blocks the opening of larynx preventing passage of food bolus into the trachea & so preventing suffocation .The vocal cords & larynx are more important than epiglottis as laryngeal or vocal paralysis causes strangulation while removal of epiglottis usually dose not cause serious disability in swallowing. D. Upper 3-4 cms. Of esophagus forms the pharyngeo-esophageal sphincter(remains tonicly contracted between swallowings to prevents air swallowing into esophagus & stomach during respiration), become relaxed during this stage by the upward movement of larynx& also the entire pharynx contracts allowing easy movement of food from posterior pharynx into the upper esophagus.

 Whole pharyngeal stage lasts for 1-2 seconds ,during this short period the respiration is stopped & is hardly noticeable

during this the swallowing center inhibits the respiratory center of medulla preventing respiration& allowing swallowing to proceed without serious affection of respiration.

Neuronal control of pharyngeal stage of swallowing:

It is a reflex act & not controlled by higher brain centers.Sensory divisions of trigeminal & glosopharyngeal cranial nerves

transmit impulses from the most sensitive areas of the pharynx (ring around the pharyngeal opening & the tonsillar pillars)

to a center in the medulla oblongata called tractus solitarius The successive stages of swallowing are automatically controlled by

the motor division; a neuronal area throughout the reticular substance of medulla& lower pons ( swallowing or deglutition center).

The motor impulses from this center pass to the pharynx& upper esophagus transmitted through 5th,9th,10th & 12th cranial nerves & by few of the superior cervical nerves .

2. esophageal stage of swallowing:

Esophagus conducts food from pharynx to the stomach .Normally it has 2 types of peristaltic movements(PMs):1.primary PMs: is continuation of the PMs that begins in the

pharynx & spreads to the eso. During eso. Stage of swallowing & pushes the food from pharynx to the stomach in 5-8 seconds helped by gravity.

2.Secondary PMs : starts when the primary PMs fails to move all the food from the eso. Causing distention of the eso.initiating secondary PMs.

Neuronal control of eso. Stage:

The pharyngeal & upper 1/3 esophageal muscles are striated responsible for PPMs controlled only by skeletal motor nerve in the glossopharyngeal & vagi ,while the lower 2/3 of esophagus is smooth muscles supplied by the myenteric aurbach plexus controlled by PS fibers of the vagi & can be excited even when vagi are sectioned (as in operations for peptic ulceration) & function independently of vagi supporting secondary PMs helping to push food to the stomach helped by gravity.

Secondary PMs are initiated partly by the intrinsic neuronal circuit in the esophageal myenteric plexus & partly by the vagi afferent & efferent fibers .

Swallowing Summary:

– Begins as a voluntary activity.– Completed as an involuntary activity– Involves Mouth, pharynx & esophagus.

• Oral phase is voluntary.• Pharyngeal & esophageal phases are involuntary.

– Larynx is raised.– Epiglottis covers the entrance to respiratory tract.

– Involuntary muscular contractions & relaxations are coordinated by the swallowing center in the medulla.

The antireflux mechanisms of lower esophagus:

2 important mechanisms in the lower eso allows smooth easy passage of food to the stomach & prevent reflux of acidic stomach contents into the lower eso. Causing reflux acid esophagitis :

1. Muscle of the lower 2-5 cms. Of eso. Above its junction with the stomach is anatomically not different from the rest of eso. But it is slightly thickened & function as lower eso, shpincter(LES).It remains tonicly contracted between swallowings to prevent reflux but in some pathological conditions it is lax causing reflux esophagitis & in other pathological states it fails to relax during eso. Stage of swallowing causing type of dysphagia called achalasia.

2. Short portion of eso. below the diaphragm before reaching the stomach has a valve like action as it become curved during any increased intra-abdominal pressure as coughing,straining or hard breathing so preventing reflux of acidic stomach contents into lower eso

The antireflux mechanisms of lower esophagus:

PregnancyLarge mealsOverweight

BendingTight clothing

Smoking (relaxessphincter)

EsophagusEsophagus FundusFundus

BodyBody

AntrumAntrum

PyloricPyloricSphincterSphincter

DuodenumDuodenum

Motor functions of stomach:The stomach consists of:1. body(corpus). 2.Antrum. 3.Fundus ( physiologically a part of body but

anatomically a separate part)The main motor functions of stomach are:1.storage of food: By receptive relaxation of stomach to accommodate with large

quantities of food entering the stomach up to about 25 liters .2.Mixing & propulsion of food : by mixing or constrictor waves which moves towards the antrum along the stomach

wall & occurs every 20 seconds( initiated by basic electrical rhythm BER consisting of electrical slow waves) causing mixing of food with secretions & provide weak propulsion to move food forwards towards the antrum.it starts near the mid point of stomach when it is full & higher up & stronger when it not full to push the last amount of stored food into the antrum& when completely empty the stomach become quiescent until new food enters.

 The real mixing occurs when the peristaltic waves of stomach passing along the antrum towards the pylorus ,is opposed by the impeding action of pyelorus which allows the passage of only few mls.of well digested food to pass to the duodenum.This two opposing forces allows the PMs to dig deeply into the antral contents causing mixing of stomach contents to form a paste like substance called chyme.

3.Emptying of the stomach: facilitated by the pyeloric pump which consists of the PMs of the

stomach & opening of the pyelorus:Regulation of stomach emptying & pyeloric pump:By stomach & duodenal factors:Stomach factors or signals:

1. distension of stomach by food stimulates gastric emptying.

2.Gastrin hormone :released by antral mucosal in response to presence of certain type of food leading to increase PMs & inhibits pyloric tone facilitating gastric emptying.

Duodenal factors or signals: inhibiting pyeloric pump & gastric emptying,include:

1. distention of duodenum 2. acidity of duodenal mucosal.3. chyme osmolarity. 4. irritation of duodenal mucosal.6.Certain chyme breakdown products specially protein & to lesser

extent fat.7.Duodenal hormones that inhibit gastric emptying include mainly

cholycystokinin (CCK) stimulated mainly by fats entering the duodenum ,less important include: secretin &gastric inhibitory peptides(GIP).

Mechanical Processes: Mixing and EmptyingMechanical Processes: Mixing and Emptying

PyloricPyloricSphincterSphincter

(open)(open)

DuodenumDuodenum

FundusFundus

BodyBody

AntrumAntrum 3. A small amount 3. A small amount of chyme isof chyme ispushed intopushed intothe duodenum.the duodenum.

2. Most mixing 2. Most mixing occurs in theoccurs in theantrum.antrum.

ChymeChyme

Food

Peristaltic Peristaltic WavesWaves

1. Peristaltic Waves 1. Peristaltic Waves begin in the begin in the fundus.fundus.

Mechanical Processes: Mixing and EmptyingMechanical Processes: Mixing and Emptying

DuodenumDuodenum

4. When the 4. When the peristaltic wave peristaltic wave reaches thereaches thepyloric sphincterpyloric sphincterit closes.it closes.

PyloricPyloricSphincterSphincter(closed)(closed)

ChymeChyme

Mechanical Processes: Mixing and EmptyingMechanical Processes: Mixing and Emptying

DuodenumDuodenum

4. When the 4. When the peristaltic wave peristaltic wave reaches thereaches thepyloric sphincterpyloric sphincterit closes.it closes.

PyloricPyloricSphincterSphincter(closed)(closed)

5. The remainder 5. The remainder of the chyme isof the chyme ispropelledpropelledbackwards.backwards.

ChymeChyme

6. The next wave6. The next waveof peristalsisof peristalsispushes the chyme pushes the chyme forward again.forward again.

DuodenumDuodenum

PyloricPyloricSphincterSphincter(closed)(closed)

ChymeChyme

Factors in the Factors in the DuodenumDuodenum that Influence that Influence the Rate of Gastric Emptyingthe Rate of Gastric Emptying

1. Fat1. Fat

3. Hypertonicity3. Hypertonicity

2. Acid2. Acid

4. Distention4. Distention

 Hunger contractions or pangs:Rhythmical PMs in the body of stomach, when empty for long time

& they become extremely strong & fuse together to cause tetanic contractions lasting for 2-3 mins. associated with pain. It is more intense in young persons with high degree of GIT tone & greatly increase during low blood sugar.

  

        Movements of the small intestine(SI):Of 2 types:1.Mixing or segmentation contractions: stimulated by stretching of

SI by chyme, are localized concentric contractions spaced at intervals , each travel only about 1cm. &when one set relaxes another set begins forming chain of sausages.They mix the chyme in a frequency of 8-12/min.backed by enteric NS specially the myenteric plexus.

2.Propulsive or peristaltic movements: move anal wards much faster proximally than distally. Of a frequency of ½-2 cms./seconds so it needs 3-5 hours for the passage of chyme from the pylorus to the ileo-cecal valve.

        Movements of the small intestine(SI):Control of SI PMs:

By nervous & hormonal factors .they increase after a meal through the myenteric plexus by a reflex called the gastroenteric reflex Hormonal factors that increase PMs include CCK& serotonin & those inhibiting PMs include : secretin & glucagons.

In the ileo-cecal (IC) valve the chyme is blocked for several hs until the person eats another time when a new GE or GI reflex intensifies the peristalsis & pushes the chyme to the cecum.

        Function of IC valve:

Prevents the backflow of cecal or colonic contentsinto the SI &resists a back pressure of 60 cms Water .Allows the emptying of ileal contents into the cecum after the gastroileal

reflex Backed by:Myenteric plexus Prevertebral sympathetic gangliaHormone gastrin which increase ileal contractions& relaxes the IC sphincter or valve About 1500 mls.of chyme empty into the cecum each day.