Oral Stage


The Oral stage begins once the bolus formation has been completed, and the bolus is ready to be swallowed. The oral stage is characterised by the propulsion of the bolus into the pharynx, the sealing of the nasopharynx, the elevation of the hyoid bone and larynx, and the closure of the airway (Groher & Crary, 2010). The following diagram is a representation of the bolus (the black area) during the oral stage.




Oral.jpg


Airway closure


The first thing to occur in the oral stage is the closure of the airway. This happens immediately before the initial propulsion of the bolus towards the pharynx. Many mechanisms are involved in the protection of the airway during the swallow; at the oral stage, active respiration ceases, and the true vocal cords approximate (Groher & Crary, 2010).



Propulsion of the bolus


The tongue cradles the bolus by raising its anterior and lateral edges, and pressing them against the hard palate. At the moment of swallow, the bolus is then propelled posteriorly by the tongue. This is achieved primarily through the contraction of extrinsic tongue muscles; digastricus, myohyoid and the geniohyoid. During this process, the posterior tongue is also raised by the contraction of the palatoglossus muscle (Groher & Crary, 2003).


The posterior tongue plays the most important role in delivering the bolus into the pharynx due to the pressure that it creates on the bolus. When the base of the tongue approximates with the posterior pharyngeal wall and the velum, positive pressure is placed on the tail of the bolus, encouraging it to move further into down into an area with relatively low pressure; the hypopharynx (Groher & Crary, 2010).



Sealing of the nasopharynx


When the bolus is propelled towards the pharynx, it is vital that it is prevented from entering the nasopharynx. The closure of the nasopharynx is achieved by the elevation of the velum, which occurs when the tensor veli palatini and palatopharangeal muscles are contracted (Groher & Crary, 2003).



Elevation of the hyoid bone, and decent of the epiglottis


At the point of swallow, the digastric, mylohyoid, and gehiohyoid muscles contract. Due to their attachment to the hyoid bone, the hyoid is elevated in a superior and anterior plane (Groher & Crary, 2003). As the larynx is attached to the hyoid, it is also raised in a similar fashion, until it sits under the base of the tongue. As the larynx is elevated, the epiglottis descends to cover the entrance to the airway. In this position, the epiglottis helps direct the bolus towards the oesophagus, and prevents it from entering the larynx. Laryngeal elevation also helps create negative pressure in the hypopharynx, which aids the bolus’ transit towards the oesophagus (Groher & Crary, 2010). Importantly, the elevation of the hyoid bone contributes to the relaxation of the cricopharyngeus muscle, which will allow the bolus to enter the oesophagus in the pharyngeal stage (Groher & Crary, 2010).




References

Groher, M., & Crary, M. (2003). Introduction to adult swallowing disorders. Philadelphia, PA: Butterworth-Heinemann.

Groher, M., & Crary, M. (2010). Dysphagia: Clinical management in adults and children. Maryland Heights, MO: Mosby Elsevier.