Deglutition or swallowing is probably a reflex phenomenon. Higher centres facilitate this reflex. Once aroused, the swallowing centre in the medulla evokes the complete act of swallowing by discharge through six nuclei and the motor neurones.
This reflex act occurs in three stages:
(1) First or oral,
(2) Second or pharyngeal, and
(3) Third or oesophageal (Fig. 9.46).
(1) First Stage:
The first stage consists of the passage of material through the oral cavity into the pharynx which is under voluntary control. Due to the contraction of the mylohyoid, styloglossus and hypoglossus muscles, upward and backward movements of the tongue occur and the bolus of food which remains on the upper surface of the tongue is thrown back through the pillars of the fauces into the pharynx. During this phase mastication ceases and respiration is inhibited reflexly.
(2) Second Stage:
It consists of passage of bolus from the pharynx into the oesophagus which is reflex process and known as swallowing reflex. The contact of food material with the pharyngeal and peripharyngeal structures initiates and completes reflexly the second as well as third stage of deglutition. These reflexes are inhibited and abolished by cocainisation.
The soft palate is elevated and the nasopharynx is closed off from the rest of the pharynx. There are elevation and forward movements of the larynx along with the elevation of the hyoid bone. The vocal cords are adducted and there is momentary stop-page of respiration and speech.
With the entrance of bolus in the larynx, contraction of superior pharyngeal constrictor occurs inducing rapid pharyngeal peristaltic wave (primary peristaltic wave) which moves down the pharynx, propelling the bolus in front of it.
The wall and structure of hypopharynx are elevated to engulf the oncoming bolus. The oesophagus, which was kept closed, until now by the contraction of cricopharyngeus muscle, relaxes as the bolus approaches the oesophagus and thus the bolus enters the oesophagus and the pharynx reopens.
Mechanism of Protection of Airway during the Passage of Food through the Pharyngeal Cross-Roads:
There are four possible outlets from the oral pharynx through which food may be expelled:
(a) Back into the mouth,
(b) Up into the nasopharynx,
(c) Forward into the larynx, and
(d) Downward into the oesophagus.
The swallowing reflex is so co-ordinated that food passes only in one of these possible paths, namely into the oesophagus. Return in the mouth is prevented by the high pressure (even 100 cm of H2O) developed in this area (posterior half of oral cavity) due to forceful contraction of the tongue against hard and soft palates.
Combined actions of tensor veli palatini and levator veli palatini muscles stiffen the soft palate which presses against posterior pharyngeal wall and passage into nasopharynx is prevented. Entrance of food into the respiratory tract is prevented by inhibition of respiration.
The mechanism of protection of the larynx during the passage of bolus has been the subject of controversy and much study. But it is agreed that the epiglottis is carried into horizontal position by the backward movement of tongue and forward movement of larynx and these movements result drawing up of larynx under the base of the tongue where it is completely out of way of oncoming bolus.
(3) Third Stage:
It consists of passage of bolus through the pharyngo-oesophagus junction (i.e., upper oesophageal sphincter), body of the oesophagus and oesophagogastric junction (i.e., lower oesophageal sphincter).
i. The terminal part of the second stage and the first part of third stage cannot be differentiated, because both are same and one. The upper oesophageal sphinteric mechanism is dealt here along with remaining portion of the third stage.
ii. Upper Oesophageal Sphincter:
Upper oesophageal sphincter is 4cm in length and usually is located between 15 and 20 cm from the incisor teeth in the region of cricopharyngeus muslce at the level of cricoid cartilage which is characterised by a short zone of high intraluminal pressure (basal-resting pressure 20 to 30 cm of H2O above the atmospheric pressure).
Immediately after the onset of deglutition there is brief increase in the high resting pressure (lasting only a few tenths of a second) followed by marked fall below the resting pressure and there is simultaneously rise of pressure in the pharynx due to arrival of peristaltic wave caused by contraction of the superior and middle pharyngeal constrictor.
Thus, considerable pressure gradiant forms from the pharynx across his sphincter. As soon as the bolus reaches the spincteric junction the previously relaxed fibres of the upper sphincter contract and propagate the peristaltic contraction and pressure gradient into the upper oesophagus.
The positive pressure wave (maximum 70 – 90 cm of H2O above the atmospheric pressure) represents a continuation of the pharyngeal peristalsis through the upper sphincter. The pressure within the junctional area then returns to its previous resting basal level within 3-4 seconds.
iii. Body of the Oesophagus:
The primary peristaltic contraction originating in the pharynx passes over the pharynoesophageal junction and continues into the oesophagus driving the bolus onward maintaining the pressure gradient. The pressure pattern of this contraction consists of an initial negative wave followed by three positive pressure components.
Secondary peristaltic contraction also appears in the oesophagus as a result of distention of its wall due to bolus (local) which progresses down like the primary peristaltic contraction and the pressure wave produced by this contraction is similar to final pressure wave of the swallowing pressure pattern.
iv. Tertiary Oesophageal Contraction:
This type of contraction has also been observed which occurs irregularly and locally and is observed in the lower oesophagus mainly.
v. Lower Oesophageal Sphincter:
Although there was confused idea regarding the terminal few centimeters of oesophagus but it is agreed today that distal 2-5 cm of oesophagus possess a characteristic motor function:
a. Deglutition pressure pattern observed in the oesophagus does not propagate into this area.
b. Body of oesophagus and its terminal part react reciprocally to cholinergic and anticholinergic stimulation.
c. High pressure zone is extended from 1 – 2 cm below the hiatus to 1 – 2 cm above the hiatus of diaphragm.
d. Mean resting maximum pressure in this region was greater than the pressure in the fundus of stomach is 10.7 cm of H2O at the end of expiration and 3.5 cm of H2O at the end of inspiration.
As the peristaltic contraction in the body of oesophagus progresses and approaches the lower oesophagus, the lower oesophageal sphincter relaxes like the cricopharyngeus muscle at upper oesophageal junction and the pressure gradient between the lower oesophagus and lower oesophageal sphincter is formed so that oesophageal content is emptied into the sphincteric area. Then the sphincteric area contracts slowly and the sphincteric content is emptied into the stomach.
Common Disturbances in the Swallowing:
i. In the First Stage:
Inflammation of any oral structure, paralysis of tongue and congenital defects of oral structure result difficulty of swallowing.
ii. In the Second Stage:
Acute pharyngitis, tonsillitis, poliomyelitis diphtheria result difficulty in deglutition.
iii. In the Third Stage:
Diffuse spasm in the oesophagus producing dysphagia and cardiac pain, achalasia (lower sphincter does not relax) scleroderma (lower part of oesophagus generally in spam), chalasia (lower sphincter remain relaxed condition, inducing gastro-oesophageal reflux).
Nervous Mechanism:
The afferent impulses reach the swallowing centre through the glossopharyngeal and trigeminal nerves and internal branch of the superior laryngeal, recurrent laryngeal and oesophageal branch of the vagus. The efferent nerves are trigeminal, facial, vagus and hypoglossal which regulate the actions of pharynx, larynx and oesophagus musculature.