19.1. Mechanisms of Deglutition

Swallowing or deglutition is the transport of a substance from the mouth to the stomach. It is an important part of eating and drinking. The portion of food or drink that is transported in one swallow is called bolus. Deficient swallowing is called dysphagia and it may allow some of the bolus to enter the airways causing choking or pulmonary aspiration.

Diversity of mechanisms

Vertebrates have incorporated a variety of mechanisms to facilitate the transport of food from the mouth into the esophagus. Several groups make use of multiple mechanisms simultaneously.

Fishes use the water flow produced by ram ventilation or the buccal pump to bring food into the mouth. The flowing water exits through the gill openings and does not enter the esophagus. Food items in the mouth are moved into the esophagus by contractions of the pharyngeal musculature and movement of the pharyngeal jaws, when present.

The mouths of many fishes and amphibians are lined by a ciliated epithelium that employs mucociliary transport to move any particles in the mouth toward the esophagus. Touch receptors in the oral mucosa stimulate the beating of the cilia when a particle contacts the epithelium. The entire lining of the mouth therefore works as a conveying belt transporting food into the esophagus. Frogs can transport and entire cricket from the mouth into the esophagus through mucociliary transport alone.

Many frogs have relatively large eyes that protrude on the surface of the head and can be retracted for protection. When the eyeballs are retracted into the think skull, however, they intrude into the roof of the mouth, forming a convexity that reduces the free internal volume of the mouth. The animals actively use this to facilitate food transport into the esophagus. When a frog swallows, it both retracts its eyeballs and contracts the floor of the mouth to push the food contained in the mouth posteriorly.

Many birds use gravity to facilitate swallowing. In events such as a seagull swallowing a fish or a stork swallowing a frog, swallowing consists largely of the bird lifting its head with its beak pointing up and guiding the prey with the tongue and jaws so that the prey slides down the pharynx and esophagus.

Swallowing in humans

The entire process takes about 4 to 8 seconds for solid food and about 1 second for very soft food and liquids. Although this sounds quick and effortless, deglutition is, in fact, a complex process that involves both the skeletal muscle of the tongue and the muscles of the pharynx and esophagus. It is aided by the presence of mucus and saliva. Swallowing consists of three phases: an oral, pharyngeal and esophageal phase. It involves many smooth muscles of the pharynx and esophagus. The oral phase is voluntary and controlled by the cerebral cortex whereas the autonomic nervous system (ANS) coordinates the pharyngeal and esophageal phases.

Phases

Oral phase

Posterior displacement of the bolus

During the oral phase of swallowing, food has already been mechanically broken down and mixed with saliva at the mouth, forming a bolus. The tongue propels the bolus posteriorly into the pharynx. The superior longitudinal muscle elevates the apex of the tongue to make contact with the hard palate and the bolus is propelled to the posterior portion of the oral cavity. Once the bolus contacts the palatoglossal arch of the oropharynx, it triggers the pharyngeal phase, which is involuntary. Touch receptors initiating this reflex are scattered over the base of the tongue, the palatoglossal and palatopharyngeal arches, the tonsillar fossa, uvula and posterior pharyngeal wall.

Pharyngeal phase

For the pharyngeal phase to work properly all other exit routes from the pharynx must be occluded, including the nasopharynx and the larynx. When the pharyngeal phase begins, other activities such as chewing, breathing, coughing and vomiting are inhibited.

Closure of the nasopharynx

The soft palate is tensed by the tensor palatini muscle and elevated by the levator palatini muscle. The closure of the passage is helped by simultaneous approximation of the walls of the pharynx to the posterior free border of the soft palate. This movement is carried out by the palatopharyngeus muscle and the upper part of the superior pharyngeal constrictor muscle.

The pharynx, larynx and hyoid are elevated to receive the bolus

The pharynx and larynx are pulled upward and forward by the stylopharyngeus, salpingopharyngeus and palatopharyngeus muscles. The hyoid is also elevated by the digastric and stylohyoid muscles, further lifting the pharynx and larynx with it. The entrance into the airways is closed by: 1) Retroversion of the epiglottis, which covers the entrance; 2) Adduction of the aryepiglottic folds. The aryepiglotticus muscle contracts, causing the arytenoids to appose each other and bring the aryepiglottic folds together; 3) Adduction of the false vocal folds; 4) Adduction of the true vocal folds by contraction of the lateral cricoarytenoids and the oblique and transverse arytenoid muscles.

This phase of the swallowing reflex is controlled through cranial nerves V, X, XI and XII. The respiratory center of the medulla is directly inhibited by the swallowing center for the brief time that it takes to swallow, causing deglutition apnea.

Opening of the auditory tube

The auditory (Eustachian) tube is and air-filled passage that connects the nasopharynx to the middle ears but it remains collapsed most of the time. The actions of the levator palatini, tensor palatini and salpingopharyngeus muscles in the closure of the nasopharynx and elevation of the pharynx open the auditory tube. This equalizes the pressure between the nasopharynx and the middle ear. It does not contribute to swallowing, but happens as a consequence of it. This is why swallowing helps alleviate pressure in the ears when a plane is taking off or landing.

Closure of the oropharynx

The oropharynx is kept closed by the tongue, preventing the bolus from returning into the mouth. The tongue is held in position by the palatoglossus muscle, the styloglossus muscle and the intrinsic muscles of tongue.

Bolus is moved through the pharynx

The bolus moves down towards the esophagus by pharyngeal peristalsis which takes place by sequential contraction of the superior, middle and inferior pharyngeal constrictor muscles. The lower part of the inferior constrictor is normally closed and only opens for the advancing bolus. Gravity plays only a small part in swallowing and it is possible to swallow solid food with the body upside down.

Esophageal phase

Esophageal peristalsis

Like the pharyngeal phase of swallowing, the esophageal phase of swallowing is under involuntary neuromuscular control. The upper esophageal sphincter relaxes to let food pass. The bolus is then pushed down by the striated constrictor muscles of the pharynx, then by peristalsis of the esophageal walls. Finally, relaxation of the lower esophageal sphincter allows the bolus to enter the stomach.

Relaxation phase

The larynx, pharynx and hyoid move down together at first, mostly by elastic recoil. Then the larynx and pharynx move down in relation to the hyoid also by elastic recoil.

The descent of the larynx

Swallowing in humans is a rather complicate process and it frequently fails, causing chocking or aspiration of fluids into the lungs. It makes one wonder why do the digestive and respiratory systems have to share the oropharynx? And why do we not see dogs, cats or horses choking when they drink water?

It turns out that while the mechanism of swallowing described above is common to vertebrates, the anatomy of the pharynx is modified in humans. Our unique feature is having a descended larynx in which the epiglottis does not contact the soft palate. The entire oropharynx fits into the gap between the epiglottis and the soft palate in humans.

Other mammals have the entire larynx positioned closer to the nasopharynx and the epiglottis rests in contact with the soft palate. The epiglottis is hinged ventrally and forms a dorsal opening into the larynx. In this position:

• Air can flow directly between the nasopharynx and the larynx.

• Large food particles in the mouth hit the epiglottis and are blocked until the epiglottis is retroverted, covering the entrance of the larynx.

• Fluids and small food particles can follow the vallecula and go around the larynx as it is thinner than the pharynx. This morphology raised the interesting possibility that mammals other than humans could possibly drink fluids without interrupting their breathing. Recent assessments through live imaging techniques have failed to confirm simultaneous drinking and breathing though. The animals execute instead a rhythmic alternation between airflow and swallowing.
  
  

Humans have a permanently descended larynx. Our newborns start with the general mammal condition but the larynx descends rapidly as the baby grows. In men, the increase in male sex hormones that takes place during puberty causes the larynx to become larger and descend further than in women.

The descended larynx has other effects in addition to making swallowing more challenging. The low position of the larynx renders the trachea shorter and the pharynx longer. The shorter trachea tends to facilitate vocalization by reducing the attenuation and delay that the duct imposes to air pressure changes that are generated at the lungs and are converted into voice at the larynx. A larger pharynx allows for more cavity resonance, emphasis in lower frequencies of voice and potentially a wider range of resonance patterns. This translates into louder and lower pitched voice with a potentially greater variety of producible vowels.

Vocal communication is extremely elaborate in humans. The advantages that the descended larynx confer for vocal communication may have generated the selective forces that drove the descent of the larynx. Other vocal primates also benefit from a lowered larynx but they only lower it during vocalization, whereas we have the larynx permanently lowered and have to elevate it to swallow. The physical association between loud and deep voice with large body size and the common involvement of men in territorial disputes may explain the enlargement and increased descent in the male human larynx at sexual maturation.

The lack of laryngeal descent in human newborns could be a reflection of special mechanical needs for nursing or of different selective pressure on vocal communication at that age. The fact that milk can flow around the open epiglottis and into the esophagus without entering the larynx raised the question of simultaneous drinking and breathing during nursing in newborns. Again, modern assessments have found evidence of rhythmic patterns of alternation between respiration and swallowing in newborns of humans and other mammals.

Suckling

Nursing in newborns was a major innovation in vertebrates and promoted the diversification of mammals into an entire class of organisms. Nursing allows females to give birth to small offspring and then continue to transfer nutrients to them until they can forage on their own. A key aspect to suckling is the mechanical coupling and transfer of milk from the mammary gland into the stomach of the offspring.

The nipple of the mammary gland, also called teat, is held between the tongue and the palate. The need for effective nursing is likely to have been the selective force that drove the evolution of the palate in mammals. The mandible, the anterior and posterior portions of the tongue, the lips, the buccinator and other muscles of the mouth move in complex rhythmic coordination during suckling.

There has been a long debate about the basic mechanism of suckling, as completely opposite processes seemed plausible. In one hand, milk could be squeezed out of the teat by compression between the tongue and the palate. On the other hand, milk could be extracted out of the teeth by suction produced mainly through lowering of the tongue. Detailed analyzes of live ultrasound imaging during nursing revealed that the tip of the teat retracts when the mandible and tongue are elevated and it protracts when the mandible and tongue are depressed. This conforms with the expectations for milk extraction by suction when the volume of the mouth is increased but not with the expectation for compression when the mandible and tongue are elevated.

Summary

Swallowing is the transport of food from the mouth into the stomach. The general mechanism is peristalsis of the digestive tube, but fishes can help it using mucociliary transport or pharyngeals jaws. Frogs add a muscular tongue and retraction of the eyes and some birds use gravity. Mammals have a palate and a muscular soft palate that protects the nasal cavity from the entrance of food. The epiglottis prevents food from entering the larynx. Its effectiveness is reduced in adult humans, however, because our larynx is permanently descended and the epiglottis does not contact the soft palate. Imperfect swallowing results in chocking or pulmonary aspiration. The human larynx has most likely descended in response to selection for loud and low pitched voice. In newborns, the larynx is not yet descended however and they suckle by producing negative pressures in the mouth to extract milk from the nipple.

Key terms

Swallowing, deglutition, chocking, pulmonary aspiration, eating, drinking, ram ventilation, uccal pump, pharyngeal jaws, mucociliary transport, eye retraction, ocular retraction, oral phase, pharyngeal phase, esophageal phase, tongue, bolus, nasopharynx, oropharynx, laryngopharynx, tensor veli palatini muscle, levator veli palatini muscle, stylopharyngeus muscle, salpingopharyngeus muscle, palatopharyngeus muscle, superior pharyngeal constrictor muscle, middle pharyngeal constrictor muscle, inferior pharyngeal constrictor muscle, medulla oblongata, peristalsis, upper esophageal sphincter, lower esophageal sphincter, descent of the larynx, nursing, suckling.

Figure credits

Figure 1 by Fraser G. J., Hulsey C. D., Bloomquist R. F., Uyesugi K., Manley N. R. and Streelman J. T. - "An ancient gene network is co-opted for teeth on old and new jaws". (2009) PLoS biology, 7 (2): e1000031. doi:10.1371/journal.pbio.1000031, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=37208634

Figure 2 by Dinkum - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=13295531

Figure 3 by Biomedizinische NMR Forschungs GmbH. 2011. CC BY-SA 3.0. http://www.biomednmr.mpg.de. Martin Uecker, Shuo Zhang, Dirk Voit, Alexander Karaus, Klaus-Dietmar Merboldt, and Jens Frahm, Real-time magnetic resonance imaging at a resolution of 20 ms, NMR in Biomedicine 23: 986–994 (2010) DOI:10.1002/nbm.1585

Figure 4 by OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30148430

Figure 5 by OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30148434

Figure 6 Public Domain, https://commons.wikimedia.org/w/index.php?curid=1607066

Figure 7 Public Domain, https://commons.wikimedia.org/w/index.php?curid=504492

Figure 8 by Museum of Veterinary Anatomy FMVZ USP, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=58495209

Figure 9 by BruceBlaus. Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=29600440