12 Adult Swallow Physiology

An adult or mature swallow is a rapid, well coordinated, pressure-driven event. Although a swallow happens in under a second, for ease in understanding this complex activity, academicians and clinicians break the process up into phases, thereby giving the swallow process an artificial serial simplification. This can be done by looking at the direction of bolus flow, in which case the stages would be the horizontal and vertical stages of bolus flow (Kennedy & Kent, 1985), or by phases related to anatomic cavities. In this later scenario, these phases include the oral, pharyngeal, and esophageal phases. The oral phase may be further subdivided into oral preparation and oral transportation. An anticipatory phase that begins before the food or liquid enter the mouth, may also be included.

The use of simplified swallow phases to discuss this complex parallel activity supports early understanding of swallow physiology (Box 1.43). However, it is important to note, that once the bolus is prepared and transported to the pharynx, the swallow occurs in under a second. Thus, there is considerable overlap in the various swallow events. Further, there are idiosyncratic patterns that are seen in some, but not all, individuals. Although used to support understanding, discussing swallow physiology in a sequential order is simplified and artificial. Box 1.44 shows a full swallow across 6 images.

Anticipation of Oral Intake

The anticipatory phase, sometimes referred to as the pre-oral stage, includes all the sensory input and reactions that occur before the food or liquid enter the mouth. Cognitive processes, such as a feeling of hunger, or sensory stimulation at the sight of food, may all be considered pre-oral components of the swallow. The sight and/or smell of the food may aid in desire and increase the experience of hunger. Of course, these sensory stimuli can also reduce desire. Once the food or liquid is delivered into the oral cavity, the oral phase begins.

Oral Swallow Physiology

Important actions in oral swallow physiology include oral preparation and mastication of the oral intake followed by oral transportation of the prepared bolus from the anterior to posterior oral cavity.

Acceptance and Mastication of Oral Intake

Oral preparation deals with breaking down the food (if needed). In other words, food is reduced into smaller particles, and mixed with an adequate amount of saliva so that the bolus maintains a level of cohesion during transport. Oral preparation may be divided into the initial transport and reduction components. The initial transport refers to the accurate positioning of food in the oral cavity so that it can be easily broken down. For example, you would likely position a piece of steak onto the lateral tooth surface for chewing. The reduction component refers to the actual mastication or reducing of the food and mixing it with saliva to begin the bolus development. When mastication is required, the chewing pattern generator is employed. Chewing or mastication is controlled through activation of the brain stem and involves the motor nucleus of CN V (trigeminal motor nucleus). The triggering stimulus for the chewing pattern generator results in rhythmic activation of jaw opening and jaw closing muscles. Note that not all material placed in the oral cavity will require preparation or mastication (e.g., liquids).

In healthy individuals, food reduction is performed while the material is contained  (anteriorly, posteriorly, and laterally) in the oral cavity. Typically the lips are approximated to provide an anterior barrier and support anterior containment of the bolus. When this does not occur, anterior oral containment may be breached and observed as drooling or food loss from the mouth. During bolus preparation, posterior containment can be obtained by tongue base to soft palate, which is motivated primarily by the palatoglossus muscle. Without adequate oral control, poor posterior containment may result in premature spillage into the pharynx. Reduced posterior oral containment can only be directly observed with an instrumental exam; i.e.,it is not directly observed in the clinical exam. When posterior containment is breached, the pharynx is not ready for the incoming bolus and the airway is unprotected. Lateral containment is achieved through cheek tension in combination with buccal teeth approximation.

Once the food has been prepared into a bolus, it is transported from the anterior to the posterior oral cavity to prepare it for passage to the pharynx. The swallow is a pressure-driven event and the generation of intrabolus pressure begins with oral compression on the bolus. The transportation of food or liquid from the anterior to the posterior oral cavity is facilitated by contraction of the lips and buccal muscles, elevation of the velum, and, for some, anterior tongue tip contact to the hard palate. The upward tongue tip provides some of the initial motivation on the bolus to move it in the posterior direction. A central groove is noted in the mid-tongue and serves as a funnel for the oral bolus. As the bolus moves down the angled tongue body, the tongue tip continues to compress against the hard palate invigorating the bolus in a posterior direction. This continued and progressive compression is essential to generate adequate bolus propulsion. Concurrently the velum elevates, and the posterior tongue depresses. The bolus is now positioned in the posterior aspect of the oral cavity and ready to be propelled into the pharynx. This oral transit usually occurs in less than one second.

Swallow Trigger

As the bolus is transported toward the pharynx, we transition from the oral or horizontal phase of the swallow to the pharyngeal or vertical phase of the swallow. In a healthy young adult, the pharyngeal phase is initiated or triggered when the bolus interacts with the posterior oral cavity. When swallowing on command, for a young healthy adult, the swallow is typically triggered when the bolus is around the area of the faucial pillars. Spontaneous swallows may be triggered lower down in the pharynx, such as in the vallecula. Also, with advanced age, the trigger may occur when the bolus is lower in the pharynx. Similar to chewing, the swallow trigger is mediated by a central pattern generator located in the medulla of the brainstem. The initiation of the triggered pattern occurs as the result of a combination of specific inputs to mechanoreceptors whose afferents feed into the nucleus tractus solitarius via the solitary tract) located in the brainstem swallow center. (Learn more about the central pattern generator and the swallow center in the section on neurophysiology.) The biomechanical event most typically noted at the initiation of the swallow trigger is the onset of hyolaryngeal elevation.

Pharyngeal Swallow Physiology

The goal of the pharyngeal phase of the swallow is to transport the bolus through the pharyngeal conduit toward the esophagus without breaching the nasal cavity or airway entrance, and without leaving material in the pharynx upon completion. This is accomplished through multiple events — closing the nasal cavity, closing and, thereby protecting, the airway entrance, propelling the bolus through the pharyngeal tube, and opening the entrance to the esophagus to allow passage of the bolus. These events must be accomplished with adequate pressure and be sufficiently timed to produce an efficient swallow. Although these actions are interdependent, for ease in understanding, they will be presented as separate events.

The pharyngeal swallow takes about 800 ms. However, at times the bolus will enter into the pharynx and the pharyngeal swallow will not trigger immediately. This delay is not typically seen in young healthy adults. When it is observed, this delay time may be calculated and used to estimate the amount of risk to the patient. Consider that while the bolus is in the pharynx, if the swallow is not triggered, then the airway is not protected. Therefore the risk of penetration and/or aspiration is increased with a delayed trigger response.

Pharyngeal Pressure

Generation of intrabolus pressure begins in the oral cavity with compression on the bolus. The tongue tip makes contact with the palate and the increasing palatal contact reduces the size of the oral cavity, squeezing down around the tail of the bolus. If tongue-to-palate compression in the oral cavity is weak, the imparted pressure on the bolus will be reduced. Further, at the interface between the oral and pharyngeal cavity (from horizontal to vertical), the tongue base compresses the tail of the bolus toward the posterior pharyngeal wall, imparting a positive pressure on the bolus as it is propelled into the pharynx. Sufficient pressure must be maintained to propel the bolus through the vertical system, while various swallow-related biomechanical events co-occur. If pressure is inadequate, then bolus propulsion may be insufficient. Incomplete or inadequate bolus transfer through the pharynx results in bolus residue in the pharynx after the swallow, putting the individual at risk for penetration and/or aspiration after the swallow. Adequate horizontal propulsion of the bolus, supported by the continued pressure gradient from pharyngeal constriction and gravity, will yield efficient and timely propulsion of the bolus through the vertical system to the esophagus with minimal to no bolus residue in the pharynx upon completion of the pharyngal swallow. (Watch Box 1.45 for a schematic of pressure patterns during the swallow.)

Maintenance of intrabolus pressure during the pharyngeal phase of the swallow is dependent on the biomechanical behavior of the pharynx. Openings into the pharynx (velopharyngeal port, oral entrance, and laryngeal entrance) need to be securely closed in a well-timed fashion. If the timing is off, or if the musculature is inadequate to achieve optimal valve closure when needed, pressure will leak from the system. This results in a drop in pressure and, therefore, inefficient bolus propulsion through the pharynx. Further, these leaks, if present, may result in altered bolus pathway for some (or all) of the bolus. Leakage in the velopharyngeal valve may result in bolus penetrance into the nasal cavity; poor airway protection may occur if there is a leak in the airway protection mechanisms. Overall, a loss of pressure will reduce bolus propulsion and may result in residue after the swallow. However, if valve closure is secure and well timed, the pharynx will provide continuous compression on the tail of the bolus supporting efficient bolus transfer.

In addition to securing valves to eliminate pressure leaks in the pharynx, efficient bolus propulsion is maintained by reducing the size of the pharynx and squeezing the bolus. As noted earlier, compression begins in the oral cavity with the tongue against the palate. The tongue continues to press against the tail of the bolus until the bolus is in the pharyngeal cavity. At this point the tongue base will make contact with the posterior pharyngeal wall (a.k.a. tongue base retraction). Meanwhile, the pharyngeal constrictors are producing sequential collapse of the pharynx on the tail of the bolus from top to bottom of the pharynx (Pauloski, Rademaker, Kern, Shaker, & Logemann, 2009). In addition to this circumferential and downward compression, the longitudinal muscles in the pharynx work to shorten the length of the pharyngeal tube, reducing the required transfer length for the bolus to reach the esophagus. The result is a reduction (constriction) of the pharyngeal space on all sides.

Meanwhile, the hyolaryngeal trajectory provides some impetus for epiglottic inversion, as well as a mechanical pull on the anterior aspect of the UES region. As the bolus propels through the pharynx, the confluence of events results in UES opening. This opening generates a negative pressure bias that essentially sucks the bolus through the UES and into the esophagus.

In summary, adequate bolus propulsion through the pharynx is dependent on a continuous driving pressure. The bolus must enter the pharynx with adequate intrabolus pressure and driving force from the tongue base. If the bolus enters the pharynx with adequate pressure, then maintenance of pressure is supported by controlling valves in the pharyngeal tube, as well as shrinking of the pharyngeal compartment.

Velopharyngeal Closure

As the bolus enters the pharynx, the role of the velum changes from posterior oral containment to nasal cavity seal. The velum lifts and opposes the posterior aspect of the nasopharynx (at the level of Passavant’s pad) to close off the velopharyngeal port, thereby eliminating the risk of nasal penetration. Box 1.46 shows the movement of the velum during bolus passage through both the oral and pharyngeal cavities.

Bolus Flow through the Pharynx

The bolus travels over the base of the tongue and makes contact with the vallecula, at which point it splits over the larynx and passes through the channels along either side of the larynx into the pyriform sinuses. The two bolus pathways (one on each side) are then rejoined at the top of the upper esophageal sphincter region.

Pharyngeal Constriction & Shortening

As the bolus passes through the pharynx, the pharyngeal space is obliterated by the constriction imposed on the bolus by the pharyngeal constrictor muscles. This constriction goes from superior to inferior. In addition to the pharyngeal compression, the pharyngeal tube is shortened by the contraction of the stylopharyngeus muscle.

Hyo-laryngeal Trajectory

The hyo-laryngeal complex is attached to the skeletal framework via muscles and ligaments. Thus, the desired upward movement of the hyoid, and therefore the larynx, is dependent on stabilization of the mandible. Contraction of the submental muscles serve to initiate the ascent of the hyo-laryngeal complex as the bolus enters the pharynx. As the hyo-laryngeal complex nears the apex of its superior displacement (just superior to the ramus of the mandible), it moves anteriorly. (See Box 1.47 for a sample of the hyo-laryngeal trajectory during swallow.)

Upward and forward trajectory of the hyo-laryngeal complex during the swallow is essential for airway protection. It moves the airway opening out of the line of bolus flow. Further, this upward and forward movement initiates epiglottic inversion, which covers over the top of the airway entrance. Muscles largely responsible for this action include the ABD, MH and GH. Consider that all of these muscles attach to the mandible and hyoid. Due to this muscular anchoring between the mandible and the hyoid, when the mandible is stabilized, contraction  of these muscles will draw the hyoid toward the genium of the mandible in a superior (upward) and anterior (forward) direction. The hyo-laryngeal complex may become more compact during anterior excursion by engaging the thyrohyoid muscle, which will result in closer approximation of the hyoid bone and larynx.

Since muscles involved in hyo-laryngeal excursion receive bilateral cortical inputs, cortical lesions typically do not result in an asymmetrical trajectory pattern. However, brainstem or peripheral damage to any of the attached muscles may alter the symmetry of the hyo-laryngeal trajectory. That said, it is a difficult asymmetry to assess. A more likely clinical scenario is the observation of unilateral weakness during assessment of floor of mouth muscles.

Adequate hyo-laryngeal excursion is needed to fully achieve emptying of the vallecula space and inversion of the epiglottis. This muscle action paired with the pressure gradients from the moving bolus aid in inverting the epiglottis and emptying the vallecula space. When combined with relaxation of the CP muscle, hyo-laryngeal trajectory is also essential in achieving adequate UES opening.

Incomplete hyo-laryngeal excursion may be reduced in the superior and/or anterior trajectory, or delayed in timing. Reduced trajectory can result in inadequate airway protection during the swallow in two ways: inadequate epiglottic down-folding and incomplete removal of the airway entrance from the path of the bolus flow. When inadequate epiglottic down-folding is observed, it is more likely that vallecula residue will be noted after the swallow. Limits in the hyo-laryngeal trajectory will reduce mechanical traction on the anterior aspect of the UES region, thus reducing UES opening (either in duration or size). Collectively these actions result in reduced bolus clearance, which can lead to penetration or aspiration after the swallow. Delayed onset of hyo-laryngeal excursion can lead to material entering the pharynx before the airway is protected, which may result in bolus penetration into the larynx.

Epiglottic Inversion

The epiglottis is a cartilage whose movement is a result of neighboring muscular actions and pressure differentials. Movement of the epiglottis during the swallow can be broken down into two actions: moving the epiglottis to a horizontal position and retroflexion or down-folding of the epiglottis. Although muscle attachments are present and supportive of these actions, the movement of the epiglottis is largely passive (Van Daele, Perlman, & Cassell, 1995). Nonetheless, individuals without an epiglottis can produce a successful and safe swallow (Leder, Burrell, & Van Daele, 2010). However, epiglottic inversion aids in airway protection and emptying the vallecula.

The epiglottis is shaped like the tongue of a sneaker where the base or stem of the epiglottis is attached to the superior medial aspect of the thyroid cartilage via the thyroepiglottic ligament. This site is referred to as the petioles. The midline of the epiglottis is attached to the hyoid bone via the lateral and median hyoepiglottic ligaments. Due to the attachment between the epiglottis to both the hyoid bone and thyroid cartilage, movement of the epiglottis from its vertical rest position to the horizontal plane occurs in conjunction with the superior hyo-laryngeal trajectory. (See the actions of the epiglottis and hyoid in Box 1.48.) As the structure elevates it comes in contact with the tongue base, which further supports the downward movement of the epiglottis. Of these two actions, hyo-laryngeal trajectory and tongue base retraction, it is the posterior trajectory of the tongue base toward the pharyngeal wall that is of paramount importance in displacing the epiglottis to the horizontal plane.

Further, the epiglottis is covered with a mucous membrane that further supports its attachment to the hyoid and then projects to the arytenoids giving rise to the aryepiglottic folds. These serve as the lateral border of the laryngeal aditus. The epiglottis is pitted with small slits which support epiglottic folding to achieve a horizontal position.

The second aspect of the epiglottic down-folding includes the inversion of the upper 1/3 of the epiglottis below the horizontal plane over the laryngeal aditus. This action is timed with the passage of the bolus through the pharynx and results from passive biomechanical forces as a result of hyo-laryngeal trajectory and the resultant tension placed on the hyoepiglottic ligaments. This action is further supported by approximation of the thyrohyoid muscle, as well as bolus propulsion on the superior surface of the epiglottis.

Small muscles, such as the thyroepiglottic and the aryepiglottic muscles, are not consistently present, nor do these muscle fibers terminate on the epiglottis, but rather course to the pharynx and tongue base. Thus, these muscles are unlikely sources of support for epiglottic down-folding (Van Daele et al., 1995).

Respiration and Airway Protection

During the swallow, the pharynx changes from a conduit for breathing to a passageway for the bolus. When this occurs the airway must be protected from the bolus. Optimal and redundant airway protection is achieved through three overlapping actions — moving the airway entrance away from the bolus flow pathway, closing the internal airway, and covering and reducing the airway entrance (laryngeal vestibule closure). If airway protection is unsuccessful and the bolus (or a piece of the bolus) penetrates into the larynx, then the goal of the larynx is to expel the foreign body before it gets into the lungs.

Hyo-laryngeal trajectory resulting from contraction of suprahyoid muscles moves the airway entrance out of the bolus flow pathway. Through contraction of the suprahyoid muscles, the hyoid (and therefore the laryngeal vestibule) is pulled in a superior and anterior direction. Reduced superior-anterior movement of the hyo-laryngeal complex will increase the risk of poor airway protection.

Internal closure of the airway is achieved on several levels. In addition to epiglottic inversion (previously discussed), there is a forward tilting of the arytenoids towards the base of the epiglottis, tightening of the aryepiglottic folds, and approximation of the true vocal folds. These actions, combined with the retroflexion of the epiglottis over the laryngeal vestibule (airway entrance), provide redundancy in the protection of the airway. If laryngeal vestibule closure is lacking (i.e., poor epiglottic down-folding or reduced arytenoid tilting toward base of the epiglottis), then an individual is at risk for penetration of material into the laryngeal vestibule.

The sequence of events to close and protect the airway is one of some debate. There are some that suggest that the internal structure of the larynx closes from bottom to top noting that any penetration would be encouraged away from the lungs in this closure pattern. However, the timing of internal laryngeal closure may be linked to gas exchange. To that end, especially for individuals with poor lung function leading to reduced oxygenation, although the arytenoids make contact early in the laryngeal closure, the glottis remains slightly ajar until later in the swallow. Here the progression is more likely arytenoid tilting toward the base of epiglottis (passive activity from turning off the posterior cricoarytenoid muscle), epiglottic down-folding, and then glottal contact (Van Daele, McCulloch, Palmer, & Langmore, 2005). True cord approximation (activation of the thyroarytenoid muscle, and perhaps the lateral cricoarytenoid muscle) does not occur until at least 50% of maximal hyo-laryngeal elevation is achieved.

Tight glottal closure results in cessation of respiration (a.k.a. deglutitive apnea). For a single swallow, respiratory cessation lasts about half of a second. During sequential gulping, deglutitive apnea can last up to five or more seconds. For individuals with respiratory compromise that may result in reduced oxygen saturation in the blood. In this scenerio, the onset and duration of swallow-related respiratory cessation may be reduced to improve oxygen saturation.

UES Opening

At the bottom of the pharynx is the UES, which must open to allow passage of the bolus into the esophagus (Box 1.49). This opening is the result of both active and passive forces, as well as pharyngeal driving pressure. Anatomical coupling between the anterior fibers of the UES region and the back of the cricoid cartilage, dictates that movement of the larynx will pull on the anterior fibers of the UES while the posterior aspect of the UES stays anchored to the posterior pharyngeal wall. Thus, as the suprahyoid muscles engage the hyoid in an upward and forward motion, the UES is passively pulled open due to the mechanical traction from the hyo-laryngeal trajectory.

Inside the UES region, tonically active cricopharyngeal muscle fibers must turn off to allow full opening of the region. Timely muscular relaxation (during superior hyo-laryngeal trajectory) allows for further compliance of the UES fibers such that the passive forces can impose maximal opening during traction. Maximal UES opening occurs during near-maximal superior and anterior trajectory of the hyo-laryngeal complex. This inferior opening in the pharynx (UES opening) at the head of the bolus leads to a change in pharyngeal pressure, which further aids in widening of the UES. Once the bolus passes through the UES, the pharyngeal swallow is complete and the UES returns to its pre-swallow (hypertonic) status.

Esophageal Phase

Once the bolus enters the esophagus, there is a peristaltic wave (downward moving circular contraction) that carries the bolus through the esophagus at approximately 3 cm/sec. This primary peristalsis takes approximately 10 seconds to traverse the entire length of the esophagus in a healthy adult. At the bottom of the esophagus the LES relaxes to allow passage of the bolus into the stomach.