Gastrointestinal tract is a continuous tube that consists of the mouth, pharynx, esophagus,

stomach, small intestine, large intestine, and anus.The lumen of this tube is continuous with the external environment. The accessory organs are the salivary glands, exocrine glands, and biliary system (liver and gallbladder).

pharynx

Upper esophygeal

sphincter

Lower esophygeal sphincter

stomach

Descending colon

duodenum

cecum

anus

rectum

liver

Salivary gland

larynx

esophagus

Functions of the digestive system

• This system has four functions:1. Motility is the muscular contractions that mix and

move the contents forward of the digestive tract. - propulsive movements - mixing movements2. Secretion is the transfer of digestive juices by

exocrine glands into the digestive tract.3. Digestion is the chemical change (hydrolysis) of

large molecules (e.g., carbohydrates, proteins, and fats) into their smaller subunits (e.g., starch into glucose, proteins into amino acids, etc).

4. Absorption is the passage of the products of digestion (e.g., glucose), along with water, vitamins, and electrolytes, from gastrointestinal lumen into the blood and lymph.

Fig. 15-1, p. 466

GlucoseGlucose

Our diets contain mainly three types of food; Carbohydrates, fats, and proteins. These large molecules must be broken down into smaller absorbable units by process of digestion.

Carbohydrates monosaccharides

Proteins aminoacids

Fats fatty acids and monoglycerides

*** The process of digestion is done by digestive enzymes which add water molecules to large molecules to break down them into smaller ones (Hydrolysis)

An example of hydrolysis. In this example, the disaccharide maltose (the intermediate breakdown product of polysaccharides) is broken down into two glucose molecules by the addition of H2O at the bond site.

Maltose

The wall of the digestive tract consists of four layers.

• The mucosa lines the luminal surface. Its inner epithelial layer has exocrine and endocrine cells. The lamina propria is a middle layer of connective tissue. The muscularis mucosa is a sparse layer of smooth muscle.

• The submucosa is under the mucosa This connective tissue has large blood and lymph vessels. It contains a submucous plexus.

• The muscularis externa is the main smooth layer of the digestive tube. It is between the submucosa and outer serosa. The muscularis externa has an inner circular layer and an outer longitudinal layer. Their contractions produce the propulsive and mixing movements.

• A myenteric plexus is between the two smooth muscle layers.

Autonomous smooth muscle function

• Smooth muscle in GIT shows rhythmic spontaneous variation in it’s resting membrane potential (slow waves). It is also called basic electric rhythm (BER). They do not cause contraction by themselves, but they bring smooth muscle to their thershold and action potentials occur and at that time contraction developed.

*** Smooth muscle cells is connected to the adjacent smooth muscle cells by cap junction and electrical activity in one muscle can pass through these cap junction so the whole muscle sheet acts as functional syncytium.

• This slow cyclic electrical activity is originated from pacemaker cell called Cajal cells which found between circular and longitudinal layers.

BER

Muscle contraction

Digestive motility and secretion are regulated by four factors.

1. Smooth muscle cells display rhythmic, spontaneous variations in membrane potentials. This is autonomous smooth muscle function. The inherent rate of several digestive processes (e.g., peristalsis and segmentation) depend on pacesetter cells in the tract with this activity.

2. The enteric nervous system consists of myenteric plexus and submucous plexus. Some neurons promote contraction of the smooth muscle of the digestive tract or affect secretion of digestive juice, other neurons stimulate secretion of GIT hormones

3. Extrinsic nerves of the autonomic nervous system innervate digestive structures from the outside. They affect digestive tract motility and secretion and modify the activity of enteric nervous system.

– Stimulation of parasympathetic system increases GIT motility and secretion

– Stimulation of sympathetic system decreases GIT motility and secretion

4. Endocrine glands within the mucosa release hormones that have either excitatory or inhibitory influences on smooth muscle and glands secretion

GIT receptors affect digestive activities through neural and hormonal pathways

**Conditions or changes inside git are sensed by receptors inside the tract.

***There are three types of such receptors; chemoreceptors, mechanoreceptors, and osmoreceptors

*These receptors respond to changes in the digestive tract. *Their activation produces short and long reflexes that

affect: - smooth muscle (motility) - exocrine glands (secretion of digestive juice) - endocrine glands (secretion of local hormones)

Fig. 15-3, p. 471

Externalinfluence

Local changes indigestive tract

Receptors in digestive tract

Intrinsicnerve plexuses

Extrinsicautomaticnerves

Gastrointestinalhormones

Smooth muscle(contraction for motility)

Exocrine gland cells(secretion of digestive juices)

Endocrine gland cells(secretion of gastrointestinaland pancreatic hormones)

Self-excitable

= Short reflex

= Long reflex

= Hormonal pathway

Chewing

** is the first step in digestive process

** is can be voluntary or reflex (mostly) action.

** It’s functions

1. Grinds and breaks food up into smaller pieces to facilitates swallowing

2. Mixes food with saliva

3. Stimulates taste buds by exposing them to food

Salivary Glands:

The Major Salivary Glands:

*Parotid-produces serous secretion containing alpha amylase enzyme (ptyalin)

*Submandibular-produces serous and mucous secretion

*Sublingual-similar to submandibular secretion

*The Minor Salivary Glands-buccal glands secret only mucus

Salivary Flow:*1-2 L/day

**Salivary flow ranges between 0.5 ml/min during basal flow and 5ml/min during maximum flow

Basic saliva components

•Water 99.5% •Ions: Na+, K+, Ca2+, Cl–, HCO3

– •Proline-rich proteins for protection of teeth enamel :

• Enzymes: ptyalin (from salivary glands), **lingual lipase (secreted from glands on the tongue).

• Immunoglobolins: IgA• Mucin: glycoproteins for lubrication of food and

protection of oral mucosa •Lysozyme, lactoferrin, thiocyanate ions

• pH of saliva is about 7

Fig. 15-4, p. 473

Cerebral cortex Other inputs

Salivary centerin medulla

Conditionedreflex

Pressure receptorsand chemoreceptorsin mouth

Simple reflex Autonomic nerves

Salivary glands

Salivary secretions

Control of salivary secretion.

Innervation of salivary glands

• Salivary secretion is continuous and can be reflexly increased. A simple, unconditioned salivary reflex is coordinated by the salivary center in the medulla. An acquired, or conditioned, reflex occurs without oral stimulation.

• Both branches of the autonomic nervous system increase the rate of stimulation.

Excitation of parasympathetic nerve fibers (facial and glossopharyngeal nerves) causes:

- Increased watery secretion rich in enzymes

Excitation of sympathetic nerve fibers (superior cervical ganglion) causes:

-slight increase in viscid saliva (rich in mucus)

Function of Saliva 1. Moistens oral mucosa. In fact, the mucin layer on the oral mucosa is

thought to be the most important nonimmune defense mechanism in the oral cavity. It facilitates speaking and chewing.

2. Moistens dry food, lubricates food to facilitate swallowing and cools hot food.

3. Provides a medium for dissolved foods to stimulate the taste buds.4. Buffers oral cavity contents. Saliva has a high concentration of

bicarbonate ions.5. Digestion. Alpha-amylase, contained in saliva, breaks down

polysaccharides into disaccharides, while lingual lipase helps break down fats.

6. Neutralizes any gastric acid that refluxes from stomachinto the lower esophagus.7. Mineralization of new teeth and repair of precarious enamel lesions.

Saliva is high in calcium and phosphate. It helps to minimize tooth decay

Function of Saliva (cont.)

8. Protects the teeth by saliva protein which contains antibacterial compounds. Thus, problems with the salivary glands generally result in dental caries. 

9. Controls bacterial flora of the oral cavityLysozyme, Secretory IgA, and Salivary Peroxidase play important roles in

saliva’s antibacterial actions. ***Lysozyme agglutinates bacteria and activates autolysins.

***IgA interferes with the adherence of microorganisms to host tissue. It neutralizes viruses, bacterial, and enzyme toxins***Peroxidase breaks down salivary thiocyanate which, in turn, oxidizes the

enzymes involved in bacterial glycolysis. ##Also saliva contains lactoferrin which binds free iron in the saliva causing

bactericidal or bacteriostatic effects on various microorganisms requiring iron for their survival.

Some of intraoral complications of salivary hypofunction include1. Candidiasis 2. Recurrent aphthous ulcers 3. Dental caries.

Swallowing (deglutition)

• Is the second stage of food ingestion

• It consists of three stages

1. Voluntary stage- initiation stage

2. Pharyngeal stage

3. Esophageal stageInvoluntary stages

Bolus of food

Tongue presses the hard palate

forces the bolus to oropharynx

Tonsillar pillar areas

Swallowing is initiated by closing the mouth and pushing voluntarily the bolus by the tongue posteriorly and upward against the palate

Epiglottis closes the larynx

Reflexly Nasopharynx closed by elevation soft palate

Bolus entering

esophagus and UES

relax

The second stage starts when the bolus reaches posterior part of the mouth and early parts of pharynx. It is involuntary action and reflex in nature. Presence of food in above places stimulates pressure receptors there specially on tonsillar pillars. Sensory impulses carried to swallowing center in brain stem.

Pharyngeal stage (cont.)• Impulses coming from swallowing center to

pharynx and esophagus to finish stage 2 and 3 of the swallowing act. In pharyngeal stage, the following events occur:

1. pushing the soft palate upward to prevent reflux of food to nasal cavity.

2. to prevent passage of food into trachea. This done by:

a. The vocal cords are tightly closedb. The larynx is elevatedc. Epiglottis swing back over the opening of

larynx. 3. Palatopharyngeal folds are pulled medially

forming slit through which the good masticated food can pass easily.

4. Relaxation of upper esophageal sphincter 5. Pharyngeal muscle contraction starts

(peristalsis) from upper parts and spreading down ward

Pharyngeal stage (cont.)

• At the beginning of stage 2, inhibitory impulses from swallowing center to respiratory center to stop respiration.

• Pharyngeal stage lasts about 1 second.• The upper esophageal sphincter (the

upper 3 cm of esophagus) is closed all the time except during swallowing. It relaxes during pharyngeal stage to allow the bolus to pass into esophagus

)3 (Esophageal stage

• Movement of bolus through esophagus is through peristalsists:

• Primary peristalsis is continuation of pharyngeal peristalsis which takes 5-9 second to travel along the esophagus. This peristalsis is capable to push the bolus down ward.

• Secondary peristalsis starts if primary peristalsis fails to push the bolus downward and will continue until the esophagus is empty. This peristalsis is initiated by distention of the esophagus by retained food. It is due to stimulation of the myentric plexus in the wall of esophagus.

Peristaltic contraction in esophagus

Esophageal stage (cont.)Lower esophageal sphincter

(LOS)

Is located 3 cm above the junction between esophagus and stomach . It remains contracted all the time except during esophageal stage of swallowing.

*** LOS is important to prevent reflux of stomach content into esophagus.

** failure of LOS contraction causes reflux of stomach content to esophagus

causing reflux esophagitis.When intra-abdominal pressure is increased during coughing, the lower portion of the esophagus is closed by valve like action of this part

Relaxed muscles

Relaxed muscles

Bolus of food

Stomach

LOS opens Due to wave of relaxation transmitted

through MyentericInhibitoryNeurons

Stomach

VOMITING (EMESIS) ACTThe sudden and forceful expulsion of gastric and upperintestinal contents* It is controlled by neurons in medulla (the ‘vomiting

centre’)*It is triggered by one or more of the following stimuli:- excessive gastric or duodenal distension- noxious substances in stomach- certain smells or sights- emotional factors- touch receptors at back of throat- reflexes involving semi-circular canals (‘motionsickness’)- stimulation of the ‘chemoreceptor trigger zone’ by

circulating ‘emetics’

VOMITING CENTER

**is found in reticular formation of medulla.

**receives sensory impulses from pharynx, esophagus, stomach and upper parts of small intestine.

CHEMORECEPTOR TRIGGER ZONE

** is located on the floor of fourth ventricle of the brain.

** is stimulated by certain drugs, circulating emetic substances.

** it is also stimulated by impulses coming from vestibular apparatus.

THE VOMITING REFLEX SEQUENCE OF EVENTS

• It starts by salivation and sensation of nausea• Deep inspiration• Closure of glottis (to prevent passage of vomit into airways)• Elevation of uvula (to prevent passage of vomit into nasal cavity). • Relaxation of lower esophageal sphincter• Contraction of diaphragm and abdominal muscles causes increased intra-

abdominal pressure.• Rapid rise in intra-gastric pressure causes reverse expulsion of gastric

and upper parts of small intestine contents***Vomiting act is accompanied by generalized autonomic effects e.g:- sweating- tachycardia- Salivation- Sensation of nausea*** Vomiting of gastric content alone for prolonged time leads to metabolic

alkalosis