Physiology of Mastication and Deglutition

Vocal Tract As we have discussed, the vocal tract consists of the nasal cavity, the oral cavity, the buccal cavity, and the pharyngeal cavity, which, because of is position, interfaces with the nose, the mouth, and the larynx. These regions of the pharynx are termed the nasopharynx, the oropharynx, and the laryngopharynx.

Vocal Tract In its nonbiological role, the vocal tract shapes sound waves for speech production which are then modified and resonated by the pharynx, oral, and/or nasal cavities. However, in addition to its nonbiological role, the biological function of this tract is to serve as a common pathway for breathing and swallowing. In its biological role, this tract is better described as the upper aerodigestive tract. In order to serve these functions efficiently and safely, the upper aerodigestive tract must be able to switch rapidly between an open airway for respiration and a closed airway for swallowing.

Upper Aerodigestive Tract It must also shift the position of the structures within the oral cavity, pharynx, and the larynx to operate differently during respiration, swallowing, or phonation. In study the anatomy of the articulatory system, you have, in essence, studied the anatomy of the swallowing mechanism. Although the structures are the same, the lips, cheeks, tongue, velum, and so forth, the functions are substantially different.

Stages of Deglutition Normal swallow consists of a set of behaviors resulting in food movement from the mouth to the stomach. The swallow is often described as containing four stages: the oral preparation stages; the oral stage; the pharyngeal stage; and the esophageal stage. In the oral preparatory phase, food is taken into the mouth, manipulated into a cohesive bolus, and held there momentarily as it is prepared for transport. Preparation may include chewing, incorporating saliva into the bolus to from a smooth and cohesive mass, breaking the bolus into small parcels, and positioning it for transport through the oral cavity.

Stages of Deglutition In the oral stage, the bolus is propelled posteriorly toward the oropharynx. As the bolus reaches the oropharynx, a pharyngeal swallow response is initiated, setting into motion a series of airway-protective and bolus-propulsive events associated with the pharyngeal stage. The esophageal phase is initiated by relaxation of the upper esophageal sphincter at the top of the esophagus that allows the bolus to being its descent toward the stomach.

Cranial Nerve Role in Swallowing Efficient swallowing demands cooperation and coordination of the cranial nerves that are also involved in speech production. The trigeminal (V) cranial nerve plays an important part in controlling the muscles of mastication as well as with innervating the mucous membranes of the upper and lower gums and teeth. The facial (VII) cranial nerve plays an important role in saliva production and in taste sensation of the bolus on the anterior two-thirds of the tongue. It also supplies the buccinator muscles and the lower portion of the orbicularis oris.

Cranial Nerve Role in Swallowing The glossopharyngeal (IX) cranial nerve contains both motor and sensory fibers supplying the tongue and pharynx. Specifically, its sensory branches supply the mucous membrane of the pharynx, palatine tonsils, the fauces, the soft palate, and the taste receptors on the posterior one-third of the tongue. Together with the motor fibers from the vagus (X) cranial nerve, it innervates the upper pharyngeal constrictor muscles.

Cranial Nerve Role in Swallowing The motor and sensory fibers of the vagus cranial nerve supply the mucous membranes of the pharynx and soft palate, base of the tongue and the larynx, as well as the muscles of the pharynx, soft palate (except the tensor veli palatini), and the larynx. The hypoglossal (XII) cranial nerve is best known for its innervation of all of the intrinsic muscles of the tongue, as well as some of the extrinsic muscles, such as the styloglossus, hyoglossus, and genioglossus.

Saliva Production A smooth, moist mucosal surface covers the entire oral cavity and the aerodigestive tract. In the oral cavity, glandular tissue located beneath the mucosal surface excretes saliva through tiny openings called salivary ducts. Saliva consists of a watery serum-like component, a thicker mucous component, and an enzyme responsible for breaking down starch. Saliva has booth antibacterial and antacid properties which help to prevent tooth decay and infections of the oral mucosa.

Saliva Production It is a critical component in swallowing for two reasons. First, incorporating saliva into dry or sticky foods through chewing results in the formation of a manageable bolus. Second saliva keeps the oral mucosa moist, which reduces friction along the path of the bolus and thus aids in deglutition. There are three major sets of salivary glands in the oral cavity: the parotid, the sublingual, and the submandibular glands.

Saliva Production Saliva is produced at a rate of 0.1 ml/min during rest and up to 4 ml/min during active secretion. The submandibular glands are responsible for up to 70% of oral secretions while the parotid glands produce another 25%, and the sublingual glands the last 5%. Not only can stimuli such as the presence of food in the mouth promote salivation, so too can the sight and smell of food Even the thought of food can influence the amount and rate of salivary flow.

The Oral Preparatory Phase The oral preparation stage is a voluntary phase characterized by chewing and the breakdown of food to a consistency appropriate for swallowing. This phase is of variable duration depending upon the ease of mastication, oral motor efficiency, and whether one wishes to savor the taste. During this stage, lip closure is maintained to prevent food from falling out of the mouth.

The Oral Preparatory Phase If chewing is required, the jaw will move in a rotary grinding motion, and the tongue will move laterally to place food on the teeth and then to gather it up while mixing with saliva. Also, the facial muscles are held close to the dental arches to prevent food from slipping off the teeth into the buccal sulcus. At the same time, the soft palate is pulled downward and forward to help keep food from prematurely falling off the back of the tongue into the throat. At the end of the oral prep stage, the food has been formed into a ball or bolus that can be swallowed easily.

The Oral Preparatory Phase The tongue then holds the bolus of food to be swallowed either on the surface of the tongue with the tip sealed against the roof of the mouth--the tipper position--or on the floor of the mouth in front of the tongue--called the dipper position. If liquid or food does not require chewing, the soft palate will remain pulled down and forward against the back of the tongue to hold the liquid or other foods in the oral cavity until the swallow is initiated.

Mastication During Oral Preparation Jaw movement for chewing has been found to be a highly stereotyped behavior—much less variable than for speech production. The movements associated with chewing can be categorized as those that involve translation, rotation, and a combination of the two. Translational movements occur when the jaw is displaced in a gliding action in a single direction around the TMJ, as in mandibular protrusion, retraction, and lateralization. Translational movements are accomplished primarily by the medial and lateral pterygoid muscles and the posterior portion of the temporalis muscle.

Mastication During Oral Preparation Rotation refers to the swinging action around the TMJ, as would be evident on mandibular elevation and depression. Rotational movements are accomplished by the masseter, temporalis, and medial pterygoid for jaw elevation, and by the lateral pterygoid for jaw depression. During chewing, the combined actions of translation and rotation characterize mandibular movement. As the jaw opens and closes, there is a concomitant protraction, retraction, and lateralization about the TMJ.

Mastication During Oral Preparation The overlap in muscles and functions suggests that many combinations of muscle activation are used during any episode of chewing. This occurs in complementary function with the tongue such that the bolus is manipulated and positioned for grinding and reducing between the occlusal surfaces of the teeth.

Oral Phase The oral stage begins when the lips seal and compress against the incisors. If the bolus is held in the dipper position, the tongue must then pick it up and place it in the tipper position. The midline of the tongue applies pressure to the bolus from the front to the back of the hard palate propelling the bolus backward to the region where the soft palate approximates the back of the tongue. The movement spreads avalanche-like over the dorsum of the tongue along a central groove that acts as a ramp or chute for the food.

Oral Phase When the bolus reaches the tongue dorsum, the back of the tongue elevates postero-superiorly, the palatoglossal arches narrow, and the bolus is poised at the oropharyngeal isthmus awaiting entry into the oropharynx. The sensory information from the tongue motion moving the bolus of food as well as the arrival of the bolus at the posterior oral cavity sends nerve impulses to the region in the brainstem to trigger the swallow response. This stage is a voluntary phase, even though we do it unconsciously, that typically takes less than 1 second to complete.

Pharyngeal Phase The pharyngeal phase begins with the triggering of the swallow response at the oropharyngeal isthmus. Sensory information from the tongue and the oropharyngeal isthmus is received at a special sensory site in the brainstem. This site then sends nerve impulses to another motor site in the brainstem to coordinate the timing and almost simultaneous occurrence of the following actions: closure of the velopharyngeal port to prevent food from entering the nose;

Pharyngeal Phase elevation and forward movement of the larynx out of the pathway of the bolus; closure of the vocal folds, the false folds, and the epiglottis to protect the airway; opening of the upper esophageal sphincter; propulsion of the bolus by the base of the tongue into the pharynx; contraction of the lateral and posterior pharyngeal walls to meet the bolus;

Pharyngeal Phase movement of the bolus by peristaltic action of the pharyngeal constrictor muscles down the pharynx and through the upper esophageal sphincter. All of these behaviors take no longer than 1 sec in duration and respiration ceases for approximately .30 to .70 of a second. If the swallow is not triggered by the bolus, none of these activities take place. The bolus may be pushed into the pharynx by the tongue and come to rest in either the valleculae or pyriform sinuses, recesses in the pharynx, or it may fall into the open airway.

Esophageal Phase Once the bolus is squeezed down the length of the pharynx and passes through the upper esophageal sphincter, the larynx descends, the airway and velopharyngeal port open, respiration resumes, and the upper esophageal sphincter closes to prevent food from being redirected back into the pharynx and possibly the now-opened airway. The peristaltic action in the esophagus takes over bolus propulsion through the esophagus and through the lower esophageal sphincter into the stomach.

Esophageal Phase As the food approaches the lower esophageal sphincter, the muscle fibers comprising the valve relax and food passes into the stomach. Once through the lower esophageal sphincter, the valve again tenses to prevent food and stomach acid from back flowing out of the stomach and into the esophagus—an occurrence known as reflux. Normal esophageal transit time is 8 to 20 second.