The following points highlight the three important stages of swallowing. The stages are: 1. Buccal or Oral Stage 2. Pharyngeal Stage 3. Esophageal Stage.
1. Buccal or Oral Stage:
By the action of tongue, and muscles contracting against the palate, the food is converted into bolus which is lubricated by saliva. The bolus is then pushed by the tongue to the posterodorsal aspect of the tongue. This is called the preparatory position. In this process, the tongue moves upwards and backwards pressing against the hard palate.
From the preparatory position, the rest of the first stage of swallowing is by a reflex action, wherein the following muscles of the tongue are involved.
i. Myelohyoid
ii. Styloglossus
iii. Hypoglossus
The above muscles contract, and push the food into the pharynx. The first stage thus gets completed. The oral phase of deglutition is affected in the following diseases affecting tongue, lips, palate, etc.
i. Inflammatory
ii. Neoplastic
iii. Congenital anomalies
iv. Paralytic
2. Pharyngeal Stage [Very Important] (Fig. 5.8):
During this stage, food has to pass from pharynx into esophagus avoiding the respiratory tract in the process.
The objectives of this stage are:
(a) Protective,
(b) Propulsive.
Food in the pharynx can take any of the four directions, namely:
a. Back into the mouth
b. Up into the nose
c. Forwards and downwards into the larynx and respiratory tract
d. Down into the esophagus
Except for movement down into the esophagus, normally entry of bolus into all the three other regions will be prevented.
1. Movement back into the mouth is blocked by the tongue compressing against the hard palate and blocking the passage.
2. The soft palate moves upwards and backwards closing the posterior nasal aperture.
The muscles in the soft palate which contract are:
i. Tensor veli palatini
ii. Levator veli palatini
3. Due to inhibition of respiration and approximation of the vocal cords which close the larynx and also the epiglottis forming a hood over the larynx, food is prevented from entering into larynx. During the second stage of deglutition, respiration is reflexly inhibited and speech is interrupted.
4. Since all the other three routes are blocked, food can easily go into the 4th outlet namely esophagus. During deglutition, the cricopharyngeal muscle (upper esophageal sphincter) relaxes and facilitates the entry of bolus from pharynx into the esophagus.
Propulsive aspect:
Once the food is in the pharyngeal area the muscles of the pharynx act on it. The important pharyngeal muscles brought into action are: Superior, middle, inferior—constrictors of the pharynx.
Superior and middle constrictors go into powerful contraction known as pharyngeal peristalsis. Inferior constrictor is also known as cricopharyngeus. This is located at the pharyngoesophageal junction forming the upper esophageal sphincter or cricopharyngeal sphincter.
Once the pharyngeal peristalsis starts, cricopharyngeal sphincter relaxes as a part of the swallowing reflex and lets the food pass into the esophagus.
The upper esophageal sphincter has a length of about 4 cm and when there is no deglutition, it is in a state of powerful contraction. Second stage of swallowing is further helped by upward and forward movements of larynx, the epiglottis helping to divert the food to the sides of larynx.
Disturbances of second stage:
Conditions affecting larynx, pharynx, soft palate are:
i. Inflammatory
ii. Neoplastic
iii. Congenital anomalies
iv. Paralytic
The two stages are well made out especially when solids are swallowed. Very often while swallowing liquids, 1st and 2nd stages are merged into one called the buccopharyngeal stage, i.e. the pharyngeal stage hardly takes a second.
The temporary cessation of breathing which occurs during swallowing is called deglutition apnea.
3. Esophageal Stage:
Normally, when deglutition is not taking place, the upper and lower ends of esophagus remain in a contracted state. Within the esophagus, at rest, there is a small negative pressure and the two walls of esophagus are in contact with each other.
Once food passes through the upper esophageal sphincter into the esophagus, the pharyngeal peristalsis continues in the esophagus as primary esophageal peristalsis.
Peristalsis is a coordinated phenomenon in which a wave of contraction will be preceded by a wave of receptive relaxation and travels down a hollow organ (or viscera) and as a rule, this wave travels from oral towards the aboral direction.
If a primary wave is unable to propel the bolus into the stomach, then secondary peristaltic wave starts in the esophagus and completes the act of swallowing. This is brought about by distension of wall of esophagus due to the presence of food.
Tertiary esophageal contractions are irregular contractions usually found in the lower part of esophagus. The cause and nature of these contractions are not well understood. These are not normally seen in human beings.
Factors influencing third stage of deglutition are:
i. Consistency of food
ii. Vagal stimulation
iii. Gravity
Helped by these peristaltic contractions and in the erect posture facilitated by gravity also to a certain extent, the food reaches the esophagogastric junction. This junction is guarded by a physiologic sphincter known as cardiac sphincter.
Cardiac sphincter extends above and below the diaphragm by about 1-2 cm. Normally this area remains contracted but during the third stage of swallowing the sphincter relaxes to allow the food to enter stomach. The relaxation thus produced is known as receptive relaxation.
The cardiac sphincter is relaxed by vagal stimulation, action of acetylcholine, secretin and VIP (vasoactive intestinal polypeptide). It is powerfully contracted by action of gastrin.
Difficulty in swallowing in any stage is known as dysphagia.
Achalasia cardia:
In this condition, the food is held up in the lower region of esophagus which is in a state of spasm. It is believed to be due to degeneration of the myenteric plexus or Auerbach plexus in that region. In this condition, very little food enters into the stomach. Esophagus above the spasm is abnormally dilated. Treatment is usually by surgery.
Third stage can also be disturbed due to stricture, neoplasms and inflammation of esophagus.
Sometime, food may regurgitate from the stomach into the esophagus because the cardiac sphincter is often in a relaxed state. This is detrimental because the acid contents of stomach can damage the esophageal wall which is normally not exposed to acid.
This condition is called reflux esophagitis. This causes typical burning pain behind the sternum. The pain of reflux esophagitis is called heart burn as the pain is felt behind the sternum. In hiatus hernia also, there can be heart burn.
Stricture of esophagus due to consumption of strong alkali or acid is associated with dysphagia. Dysphagia also occurs in esophageal carcinoma.
Belching:
It is the involuntary expulsion of swallowed air. Air is normally swallowed during food intake.
Some hysterical people swallow too much of air “Aerophagia”. Once air collects in the upper part of the stomach, the intragastric pressure may increase so much that the cardiac sphincter relaxes resulting in the expulsion of air through esophagus and mouth.