Gastrointestinal tract movements

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 of Mastication

  • Masseter muscles
  • Temporal muscles
  • Pterygoid muscles
  • 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

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-
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. 
Due to various coordinated movements, bolus is made to enter only the esophagus.

 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).
  • 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.

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.

 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.

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