GIT

Mouth , Pharynx And EsophagusLec. 3

Ingestion of food
The amount of food that a person ingests is determined principally by intrinsic desire for food called hunger . Type of food that a person preferentially seeks is determined by appetite Two mechanics of ingestion especially : 1- mastication ( chewing ) 2- swallowing ( deglutition )

MOUTH

The time food spends in oral cavity is 10—20 seconds.The mouth chews and lubricates food, starts its digestion , and maneuvers food to the pharynx for swallowing. In the mouth , food is mixed with saliva and propelled into the esophagus . peristaltic waves in the esophagus move the food into the stomach .Mastication( Chewing), the anterior teeth (incisors) providing a strong cutting action and the posterior teeth (molars), a grinding action.

Most of the muscles of chewing are innerved by the somatic motor branch of the fifth cranial nerve to the skeletal muscles of the mouth and jaw and chewing process is controlled by nuclei in the brain stem . rhythmic chewing motions are reflexly activated by the pressure of the food against the gums hard palate at the roof of the mouth and tongue Chewing is important for digestion of all foods especially fruits and raw vegetables because these have indigestible cellulose membrane around their nutrient portions that must be broken before the food can be digested .. Large food particles can be digested but they are cause strong and painful contraction of the esophageal muscles .

Edentulous patients are generally restricted to a soft diet and have difficulty eating dry food The number of chews that is optimal depends on the food, but usually ranges from 20 to 25.

Salivary Glands and Saliva

A number of salivary glands scattered throughout the oral cavity . the principal glands of salivation are : 1. parotid glands : rich in amylase . its innervated by glossopharyngeal cranial nerve ( IX ). 2. Submandibular glands under the lower jaw . its innervated by Facial cranial nerve ( VII) . 3. Sublingual glands :They are found under the tongue . its innervated by Facial cranial nerve ( VII) supply .

Composition of Saliva :

1. daily secretion ranges between 800 – 1500 ml .( average value 1000ml ) . 2. two types of protein secretion :a. serous secretion that contain ptyalin an (α - amylase ) which is an enzyme for digesting starches .b. Mucus secretion that contains mucin for lubricating3. PH between 6 – 7 a favorable range for the digestive action of ptyalin .4. Ionic composition of saliva :1) potassium and HCO3- greater than plasma . 2) Na+ and chloride are several times less than in plasma .


 Aldosterone increases the K+ and reduces the Na+ concentration ,a high salivary Na+ / K+ ratio is seen when aldosterone is deficient in Addison 's disease

Function of Saliva . Under basal awake conditions, about 0.5 milliliter of saliva, almost entirely of the mucous type, is secreted each minute; but during sleep, secretion becomes very little. T his secretion plays an exceedingly important role for maintaining healthy oral tissues. The mouth is loaded with pathogenic bacteria that can easily destroy tissues and cause dental caries. Saliva helps prevent the deteriorative processes in several ways. 1.The flow of saliva itself helps wash away pathogenic bacteria as well as food particles that provide their metabolic support.


2. saliva contains several factors that destroy bacteria. One of these is thiocyanate ions and another is several proteolytic enzymes—most important, lysozyme—that (a) attack the bacteria, (b) aid the thiocyanate ions in entering the bacteria where these ions in turn become bactericidal, and (c) digest food particles,thus helping further to remove the bacterial metabolic support.3. saliva often contains significant amounts of protein antibodies that can destroy oral bacteria, including some that cause dental caries. In the absence of salivation, oral tissues often become ulcerated and otherwise infected, and caries of the teeth can become rampant.

OTHER FUNCTIONS1. keep the mouth moist. 2. facilitate swallowing .3. serves as a solvent for molecules that stimulates the taste buds . 4. Aid speech by facilitating movements of the lips and tongue 5. the PH of saliva ranges 6 – 7 which help to neutralize gastric acid , it relieve heartburn when gastric juice ( acidic ) regurgitated into the esophagus .

Salivary secretion saliva is controlled mainly by parasympathetic nervous signals from the superior and inferior salivary nuclei in the brain stem . Salivary nuclei are located approximately at the juncture of medulla and pons.

Stimulation of the parasympathetic nerve supply causes profuse secretion of watery saliva and vasodilatation in the glands which is due to local release of VIP which is a co-transmitter with ACH in some of postganglionic parasympathetic nervous . Atropine and other cholinergic blocking agents reduce salivary secretion.

Stimuli that increase secretion of saliva :

1.taste and tactile stimuli from tongue and other areas of mouth and pharynx . also certain tactile stimuli such as the presence of smooth objects in the mouth ( e.g. pebble ) cause marked salivation , whereas rough objects cause less salivation . 2. stimulation of vagal afferent fibers at the gastric end of the esophagus. 3. salivary secretion is easily conditioned as shown in Pavlov's original experiments .

4. when a person smell or eats favorite foods salivation is greater than when disliked food is smelled or eaten . 5. in humans , the sight and even thought of food causes salivary secretion ( makes the mouth water ) . 6. salivation also occurs in response to some GI abnormality . 7.Sympathetic stimulation can also increases salivation to less extent than does parasympathetic stimulation .