5.Swallowing Models: The Four-Stage Model and Process Model

Explanation

In this module, 2 major systemic models are used to describe the physiology of normal swallowing.

Four-stage model¹:This swallowing model is used to describe how water or other liquids are swallowed. The 4 stages are defined by where the liquid is, from the anticipatory stage to the esophageal stage, and the stages proceed continuously without overlapping.
Process model²:This swallowing model is used to describe how food is chewed and swallowed. It describes how food can be present in both the oral cavity and pharynx during mastication.

Explanation

The four-stage model is based on the concept of the command swallow, which we hold the liquid bolus in the oral cavity for a very short time before swallowing it when we are given a signal (a command). Many studies based on the four-stage model have properly described the movements of the anatomical structures and bolus during command swallowing of liquid. However, it is difficult to generalize the swallowing pattern described by this model to daily eating and drinking behaviors.
Below is a slow-motion videofluoroscopic (VF) image of command swallowing of liquid barium (click the image to play). The barium is initially held in the oral cavity (preparatory stage). During this stage, the fauces is closed by the tongue and soft palate to separate the oral cavity from the pharynx. This prevents the food bolus from entering the pharynx before the onset of swallowing in healthy individuals. Once swallowing begins, the barium is transported through the pharynx into the esophagus.

Explanation

The four-stage model consists of 5 stages when we include the anticipatory stage before liquid enters the mouth.
Anticipatory stage :The stage before liquid is taken into the mouth.
Preparatory stage :The stage during which a food bolus is formed in the oral cavity. The fauces is closed by the tongue and soft palate to shut off the oral cavity from the pharynx. The bolus is held by the tongue(see ₍₁₎ in the figure below).
Oral stage :The stage during which liquid is transported from the oral cavity to the pharynx. Once the bolus is ready to be swallowed, the tongue, while holding the bolus, begins touching the anterior part of the palate while the posterior part of the tongue begins descending. At the same time, the soft palate is lifted backward to open the oropharyngeal isthmus, allowing the bolus to be squeezed by the tongue and palate and transported into the pharynx(see ₍₂₎ below).
Pharyngeal stage :The stage during which the bolus passes through the pharynx as a result of continuous activities of pharyngeal nerves and muscles. The bolus is transported from the pharynx into the esophagus through the upper esophageal sphincter (UES) (see ₍₃₎ and ₍₄₎ below).The detailed process is described in the following slide.
Esophageal stage :The bolus that has entered the esophagus is transported downward by peristalsis and gravity through the inferior esophageal sphincter until finally reaching the stomach (see ₍₅₎ below).

Explanation

Pharyngeal swallowing is a sequence of movements involving more than 30 nerves and muscles and occurs within 1 second.
Veloopharyngeal closure is achieved by lifting of the soft palate and contraction of the upper pharynx.
The tongue base contracts, moves backward, and, in conjunction with oropharyngeal contraction, pushes the bolus downward.
The hyoid bone and larynx are lifted anterosuperiorly. The lifting of the larynx and the posterior contraction of the tongue base fold the epiglottis backward.
With the folding of the epiglottis, the glottal closure and the arytenoid adduction completely shut off the lower airway from the pharyngeal cavity.
The larynx elevates and the cricopharyngeus muscle relaxes to open the UES.

Explanation

When eating food, chewed food is transported to the oropharynx in a manner different from in command swallowing of liquid. It is difficult to describe this flow of food movement by the four-stage model, so the process model was proposed to describe how food is chewed and then swallowed.

The features of the process model are as follows:

• When eating food, chewed food is transported through the fauces to the oropharynx and valleculae and it accumulates there before the onset of the pharyngeal stage of swallowing.
• After a part of chewed food has been transported to the oropharynx, the remaining part of food in the oral cavity continues to be chewed.
• This means that food is present in both the oral cavity and pharynx before swallowing.

Explanation

The process model consists of the following 4 stages defined by movement of the organs involved in swallowing₍₃₎. Because stage II transport occurs during mastication, it is possible that 2 stages occur at the same time, unlike in the four-stage model.

Stage I transport: The stage when ingested food is transported to the molar region. The whole tongue moves backward to carry the food placed on the tongue to the molar region(ie, tongue pull-back) and then turns to the side to place the food on the mandibular occlusal plane.

Processing: The stage when food is chewed and crushed into small pieces, mixed with saliva, and transformed into aswallow bolus. In conjunction with cyclic masticatory movements of the jaw, the tongue, cheeks, soft palate, and hyoid bone also move cyclically in a coordinated manner.

Stage II transport: The stage when chewed food is transported to the oropharynx. Once in a swallowable form, a part of the chewed food is placed in the center of the tongue and is transported by the squeezing motion of the tongue to the oropharynx(ie, squeeze-back).

Pharyngeal swallow: The stage when the food bolus is transported through the pharynx into the esophagus through the UES. The movements of the pharynx and larynx during the processing and swallowing of solid food are almost the same as those during liquid swallowing.

Explanation

This is a lateral videofluoroscopic image of a young healthy adult volunteer eating a 6-g of barium-coated banana (click the image to play). Ingested food into the mouth is immediately transported by the tongue to the molar region (stage I transport), followed by mastication. During mastication, the tongue sequentially transports processed food pieces to the middle pharynx (stage II transport). Food boluses accumulated in the vallecula and those transported from the oral cavity are combined and are swallowed.

Explanation

During mastication, cyclic masticatory movements of the jaw are coordinated with cyclic movements of the soft palate, tongue, and cheeks⁴.
The soft palate is lifted when the mouth opens and descends when the mouth closes⁵ (click the image on the left to play).
The tongue and cheeks move in conjunction with the masticatory movements of the mandible to place food on the mandibular occlusal plane⁶.
The oropharyngeal isthmus is kept open during mastication due to periodic movements of the soft palate and tongue. Closure of the oropharyngeal isthmus to hold food in the oral cavity, as observed during liquid swallowing, does not occur.

Explanation

Once in a swallowable form, a part of the chewed food is gathered in the center of the tongue and transported through the fauces to the oropharynx. This transport is called as stage II transport.
Stage II transport occurs while the mouth is closed. First, the anterior part of the tongue touches the hard palate behind the maxillary frontal teeth. The tongue-palate contact area gradually expands backward while squeezing the chewed food bolus into the middle pharynx (ie, squeeze-back).
Stage II transport is mostly due to squeeze-back of tongue and does not require gravity.
Stage II transport occurs intermittently during mastication. During the subsequent mastication events, the transported bolus is accumulated around the dorsum of tongue and vallecula in the oropharynx. Food pieces remaining in the oral cavity continue to be chewed, undergo stage 2 transport, and accumulated in the oropharynx.

Explanation

Slow-motion movie1
A slow-motion movie of stage II transport (click the image to play). After the mouth is closed, the tongue places a food bolus on its dorsal surface and squeezes it against the palate while transporting it to the oropharynx.
Slow-motion movie2
Stage II transport occurs intermittently during mastication. The lower movie clip on the left (click the image to play) shows that after stage II transport, mastication occurs 3 times, followed by another stage II transport process and then by swallowing.

Explanation

Simultaneous intake of solid and liquid food is followed by a different mode of food movement into the pharynx⁷⁾.
When solid and liquid food is taken in simultaneously, a food bolus is likely to be transported to the lower pharynx before the onset of swallowing.

Upper and lower movies on the left
Because the fauces is open during mastication, the liquid component of the food flows by gravity into the lower pharynx while the solid component is being chewed in the oral cavity (upper movie on the left; click the image to play). When the examinee takes in food while on hands and knees on the floor in order to eliminate the effect of gravity, food is transported up to the oropharynx, but not to the hypopharynx (lower movie on the left; click the image to play).
These findings demonstrate that solid and liquid food taken simultaneously is transported by both the tongue's active movements and gravitational flow. The gravitational force plays a particularly important role in allowing liquid food to flow into the lower pharynx.

References

1. Leopold NA, Kagel MC. Dysphagia--ingestion or deglutition?: a proposed paradigm. Dysphagia 12 (4): 202-6, 1997.
2. Palmer JB, Rudin NJ, Lara G, Crompton AW. Coordination of mastication and swallowing. Dysphagia 7 (4): 187-200, 1992.
3. Matsuo K, Palmer JB. Anatomy and physiology of feeding and swallowing: normal and abnormal. Phys Med Rehabil Clin N Am 19 (4): 691-707, 2008.
4. Matsuo K, Palmer JB. Process Model. In: Eiichi S, Mukai Y, eds. Dysphagia Rehabilitation. 2nd ed. Tokyo: Ishiyaku publisher, pp. 62-77, 2007.
5. Matsuo k, Metani H, Mays KA, Palmer JB. Tempospatial Linkage of soft palate and jaw movements in feeding. The Japanese Journal of Dysphagia Rehabilitation 12 (1): 20-30, 2008.
6. Mioche L, Hiiemae KM, Palmer JB. A postero-anterior videofluorographic study of the intra-oral management of food in man. Arch Oral Biol 47 (4): 267-80, 2002.
7. Saitoh E, Shibata S, Matsuo K, Baba M, Fujii W, Palmer JB. Chewing and food consistency: effects on bolus transport and swallow initiation. Dysphagia 22 (2): 100-7, 2007.