Illustration of three physiological models of swallowing : two-stage, four-stage, and process models
A brief summary of the current understanding of these swallow stages and the differences among them is clearly outlined in the following text.
Swallowing is traditionally divided into four stages: the oral preparatory, oral propulsive, pharyngeal, and esophageal stages . The four-stage swallowing model is used to explain the command swallow (swallowing under order during an experiment) and the discrete swallow (one spontaneous swallow) of liquids and the concept that bolus propulsion to the pharynx normally does not occur until the initiation of swallow onset. However, this model does not adequately represent the mechanism of eating and swallowing solid food. One of the most important fundamental differences between the processes of drinking liquids and eating solids, other than the consistency of the food, is the chewing process before swallowing as well as the coordination between chewing and intraoral food transport with swallowing. This means that the major difference is in the oral stage, whereas the pharyngeal and esophageal stages have minor differences (Fig. 3.1).
3.1 Four-Stage Model
The four-stage model comprises the oral preparatory stage , oral propulsive stage , pharyngeal stage , and esophageal stage .
3.1.1 Oral Preparatory Stage
A liquid bolus is collected in the anterior part of the oral cavity and placed in a swallow-ready position. Two regions in the oral cavity may hold a liquid bolus prior to initiating the swallow: the floor of the mouth in the anterior part of the oral cavity (dipper type) and the tongue surface against the hard palate (tipper type) . The lips are sealed, and the tongue tip elevates to contact the anterior alveolar ridge and thus prevent drooling . The tongue creates a cuplike shape to entrap the bolus on its dorsum. The posterior part of the oral cavity is closed by tongue–palate contact behind the bolus, which is mainly controlled by the palatoglossus muscle , to prevent premature leakage into the oropharynx before the swallow.
3.1.2 Oral Propulsive Stage
The tip and sides of the tongue are anchored to the alveolar ridge. The posterior region of the tongue is depressed to open the posterior portion of the oral cavity. The bolus is subsequently propelled from the oral cavity to the pharynx through the fauces by a stripping action of the tongue along the palate from the anterior to posterior part using a squeezing mechanism . The bolus is pushed to the back of the mouth and toward the pharynx (Fig. 3.2).
Diagram of normal swallowing of liquid bolus. The bolus is readily held between the anterior surface of the tongue and hard palate. The posterior tongue–palate contact separates the oral cavity and pharynx . The posterior tongue drops down and the soft palate rises with swallowing onset , allowing the bolus to flow into the pharynx (Reproduced from  with permission)
3.1.3 Pharyngeal Stage
The pharyngeal stage usually occurs subsequent to the oral transport stage. The pharyngeal swallow is voluntarily initiated by propelling the bolus from the pharynx into the esophagus. This stage is the most critical stage of the swallow because airway protection occurs, preventing the bolus from entering the respiratory system.
The physiological activities during the pharyngeal stage occur step by step as follows:
The soft palate is raised and contacts the lateral and posterior pharyngeal walls primarily by the levator and tensor veli palatini muscles , palatopharyngeus muscle , and superior pharyngeal constrictor muscle , completely closing the velopharyngeal part to isolate the nasopharynx from the oropharynx and prevent food material from entering the nasal cavity (nasal regurgitation ).
The larynx and hyoid move superiorly and anteriorly by contraction of the suprahyoid and thyrohyoid muscles . This anterosuperior movement of the larynx and hyoid bone is important for several reasons [1, 5, 6]. First, this movement brings the larynx to a position under the tongue base and out of the descending bolus pathway and may facilitate the epiglottis to tilt backward. Second, the elevation of the pharynx and larynx shortens and widens the pharynx , creating a vacuum of negative pressure (suction force) in the hypopharynx to pull the bolus into the esophagus. Third, this action creates a pulling force to open the cricopharyngeal muscle and UES by tracking the cricoid cartilage up and forward away from the posterior pharyngeal wall .
The base of the tongue is retracted toward the posterior pharyngeal wall , preventing the food from reentering the mouth and delivering the bolus to the pharynx by the tongue’s driving force.
The contractions of the pharyngeal constrictor muscles from the upper to lower part help to squeeze the bolus downward, reducing the pharyngeal volume and increasing the pharyngeal pressure .
Closure of the larynx occurs. Laryngeal closure involves three main aspects: the TVC , laryngeal vestibule (false vocal folds and aryepiglottic folds ), and epiglottic inversio n [7–9].
TVC : TVC closure has been described as the first event of laryngeal closure [7, 8]. Recent CT study , however, revealed that timing of TVC closure was adjusted by the bolus viscosity , suggesting TVC as adaptable component of swallow. TVC closure may occur before swallowing or at any time during swallowing responding to bolus flow.
At the level of the laryngeal vestibule , the false vocal folds contract, and the arytenoids tilt forward and inward to contact the epiglottic base.
The epiglottis is retroverted. It folds backward over the top of the larynx , and the base of the epiglottis contacts the adducted arytenoids , tilting forward. This mechanism protects the airway, diverting the bolus into the pyriform sinus by sliding down on the epiglottis .
The pharyngeal stage ends when the UES relaxes and permits the bolus to enter the esophagus entirely. The opening of the UES is dependent upon the following three conditions : relaxation of the tonically contracted UES; traction forces through the anterior hyolaryngeal excursion ; the distensibility property of muscles, which allows the UES to stretch and accommodate the bolus passage; and the increase in intrabolus pressure with the expanding effect of the approaching bolus.
This stage starts once the bolus passe s through the UES. The UES is under tension in the resting state and relaxes while food passes into the esophagus. After the bolus has successfully entered the esophagus, the cricopharyngeal muscle returns to its contracted state to prevent the bolus from flowing back into the hypopharynx in a retrograde manner (pharyngeal regurgitation ), which can possibly lead to aspiration.
Primary esophageal peristalsis is activated in response to arrival of the bolus; this stretches the esophageal lumen when it occurs in connection with the pharyngeal swallow, propelling the bolus toward the lower esophageal sphincter (LES ) . The wavelike process of secondary esophageal peristalsis then occurs by local distension to squeeze the bolus into the stomach under control by the autonomic nervous system. Likewise, gravity partly assists bolus transfer in the upright position. The esophageal transit time should normally range from approximately 8 to 20 s . The LES relaxes so that the bolus can enter the stomach. The LES is also contracted at rest, as is the UES, after the bolus passes into the stomach to prevent gastroesophageal regurgitation . The swallowing process is complete at this point, and digestion begins after the bolus enters the stomach through the LES .