MASTICATION

MASTICATION

Mastication or chewing is the first mechanical process in the gastrointestinal (GI) tract, by which the food substances are torn or cut into small particles and crushed or ground into a soft bolus.

Significances of mastication

1. Breakdown of foodstuffs into smaller particles

2. Mixing of saliva with food substances thoroughly

3. Lubrication and moistening of dry food by saliva, so that the bolus can be easily swallowed

4. Appreciation of taste of the food.

MUSCLES AND THE MOVEMENTS OF MASTICATION

Muscles of Mastication

1. Masseter muscle

2. Temporal muscle

3. Pterygoid muscles

4. Buccinator muscle.

Movements of Mastication

1. Opening and closure of mouth

2. Rotational movements of jaw

3. Protraction and retraction of jaw.

CONTROL OF MASTICATION

Action of mastication is mostly a reflex process. It is carried out voluntarily also. The center for mastication is situated in medulla and cerebral cortex. Muscles of mastication are supplied by mandibular division of 5th cranial (trigeminal) nerve.

DEGLUTITION

Definition

Deglutition or swallowing is the process by which food moves from mouth into stomach.

Stages of Deglutition

Deglutition occurs in three stages:

I. Oral stage, when food moves from mouth to pharynx

II. Pharyngeal stage, when food moves from pharynx to esophagus

III. Esophageal stage, when food moves from esophagus to stomach.

ORAL STAGE OR FIRST STAGE

Oral stage of deglutition is a voluntary stage. In this stage, the bolus from mouth passes into pharynx by

means of series of actions.

Sequence of Events during Oral Stage

1. Bolus is placed over postero-dorsal surface of the tongue. It is called the preparatory position

2. Anterior part of tongue is retracted and depressed.

3. Posterior part of tongue is elevated and retracted against the hard palate. This pushes the bolus

backwards into the pharynx

4. Forceful contraction of tongue against the palate produces a positive pressure in the posterior part of

oral cavity. This also pushes the food into pharynx.

PHARYNGEAL STAGE OR SECOND STAGE

Pharyngeal stage is an involuntary stage. In this stage, the bolus is pushed from pharynx into the esophagus. Pharynx is a common passage for food and air. It divides into larynx and esophagus. Larynx lies anteriorly and continues as respiratory passage. Esophagus lies behind the larynx and continues as GI tract. Since pharynx communicates with mouth, nose, larynx and esophagus, during this stage of deglutition, bolus from the pharynx can enter into four paths:

1. Back into mouth

2. Upward into nasopharynx

3. Forward into larynx

4. Downward into esophagus.

However, due to various coordinated movements, bolus is made to enter only the esophagus. Entrance of

bolus through other paths is prevented as follows:

1. Back into Mouth

Return of bolus back into the mouth is prevented by:

i. Position of tongue against the soft palate (roof of the mouth)

ii. High intraoral pressure, developed by the movement of tongue.

2. Upward into Nasopharynx

Movement of bolus into the nasopharynx from pharynx is prevented by elevation of soft palate along with its extension called uvula.

3. Forward into Larynx

Movement of bolus into the larynx is prevented by the following actions:

i. Approximation of the vocal cords

ii. Forward and upward movement of larynx

iii. Backward movement of epiglottis to seal the opening of the larynx (glottis)

After origin, the peristaltic contractions pass down through the rest of the esophagus, propelling the bolus

towards stomach. Pressure developed during the primary peristaltic contractions is important to propel the bolus. Initially, the pressure becomes negative in the upper part of esophagus. This is due to the stretching of the closed esophagus by the elevation of larynx. But immediately, the pressure becomes positive and increases up to 10 to 15 cm of H2O.

2. Secondary Peristaltic Contractions

If the primary peristaltic contractions are unable to propelthe bolus into the stomach, the secondary peristaltic contractions appear and push the bolus into stomach. Secondary peristaltic contractions are induced by the distention of upper esophagus by the bolus. After origin, these contractions pass down like the primary contractions, producing a positive pressure.

Role of Lower Esophageal Sphincter

Distal 2 to 5 cm of esophagus acts like a sphincter and it is called lower esophageal sphincter. It is

constricted always. When bolus enters this part of the esophagus, this sphincter relaxes so that the contents enter the stomach. After the entry of bolus into the stomach, the sphincter constricts and closes the lower end of esophagus. The relaxation and constriction of sphincter occur in sequence with the arrival of peristaltic contractions of esophagus.

DEGLUTITION REFLEX

Though the beginning of swallowing is a voluntary act, later it becomes involuntary and is carried out by

a reflex action called deglutition reflex. It occurs during the pharyngeal and esophageal stages.

Stimulus

When the bolus enters the oropharyngeal region, the receptors present in this region are stimulated.

Afferent Fibers

Afferent impulses from the oropharyngeal receptors pass via the glossopharyngeal nerve fibers to the

deglutition center.

Center

Deglutition center is at the floor of the fourth ventricle in medulla oblongata of brain.

Efferent Fibers

Impulses from deglutition center travel through glossopharyngeal and vagus nerves (parasympathetic

motor fibers) and reach soft palate, pharynx and esophagus. The glossopharyngeal nerve is concerned

with pharyngeal stage of swallowing. The vagus nerve is concerned with esophageal stage.

Response

The reflex causes upward movement of soft palate, to close nasopharynx and upward movement of larynx,

iv. All these movements arrest respiration for a few seconds. It is called deglutition apnea.

Deglutition apnea

Apnea refers to temporary arrest of breathing. Deglutition apnea or swallowing apnea is the arrest of breathing during pharyngeal stage of deglutition.

4. Entrance of Bolus into Esophagus

As the other three paths are closed, the bolus has to pass only through the esophagus. This occurs by the combined effects of various factors:

i. Upward movement of larynx stretches the opening of esophagus

ii. Simultaneously, upper 3 to 4 cm of esophagus relaxes. This part of esophagus is formed by the

cricopharyngeal muscle and it is called upper esophageal sphincter or pharyngoesophageal

sphincter

iii. At the same time, peristaltic contractions start in the pharynx due to the contraction of pharyngeal

muscles

iv. Elevation of larynx also lifts the glottis away from the food passage.

All the factors mentioned above act together so that, bolus moves easily into the esophagus. The whole

process takes place within 1 to 2 seconds and this process is purely involuntary.

ESOPHAGEAL STAGE OR THIRD STAGE

Esophageal stage is also an involuntary stage. In this stage, food from esophagus enters the stomach.

Esophagus forms the passage for movement of bolus from pharynx to the stomach. Movements of esophagus are specifically organized for this function and the movements are called peristaltic waves. Peristalsis means a wave of contraction, followed by the wave of relaxation of muscle fibers of GI tract, which travel in aboral direction (away from mouth). By this type of movement, the contents are propelled down along the GI tract. When bolus reaches the esophagus, the peristaltic waves are initiated. Usually, two types of peristaltic contractions are produced in esophagus.

1. Primary peristaltic contractions

2. Secondary peristaltic contractions.

1. Primary Peristaltic Contractions

When bolus reaches the upper part of esophagus, the peristalsis starts. This is known as primary peristalsis. After origin, the peristaltic contractions pass down through the rest of the esophagus, propelling the bolus towards stomach. Pressure developed during the primary peristaltic contractions is important to propel the bolus. Initially, the pressure becomes negative in the upper part of

esophagus. This is due to the stretching of the closed esophagus by the elevation of larynx. But immediately, the pressure becomes positive and increases up to 10 to 15 cm of H2O.

2. Secondary Peristaltic Contractions

If the primary peristaltic contractions are unable to propel the bolus into the stomach, the secondary peristaltic contractions appear and push the bolus into stomach. Secondary peristaltic contractions are induced by the distention of upper esophagus by the bolus. After origin, these contractions pass down like the primary contractions, producing a positive pressure.

 

Post a Comment

0 Comments