Swallowing refers to the process by which food or liquidsubstances are moved from the oral cavity into the stomach. Although aseemingly simple and primitive process, the swallowing sequence actuallyrequires complex coordination of over 30 nerves and muscles (Matsuo &Palmer, 2008). While combining the enjoyment of consuming food with the inherentneed for nutrients and hydration it is critically important that the airway iskept clear at all times. While most swallows occur involuntarily as a result ofsalivation, clusters of voluntary swallows can be evoked during feeding (Dodds,1989).
Theanatomic areas where swallowing takes place are the oral cavity, pharynx,larynx and oesophagus. As the bolus travels through each of these it comes in contactwith various bony/cartilaginous structures offering support, striated andsmooth muscle components aiding movement and neural elements which innervatethe musculature (Dodds, 1989). Previously,normal swallowing in humans was described using a three-stage sequential modelconsisting of an oral, pharyngeal and oesophageal stage.
The oral stage waslater subdivided into the oral preparatory and oral propulsive stages resultingin a four-stage model which adequately describes the swallowing of liquids. In addition,the process model was established for solid food. (Matsuo & Palmer, 2008). Dodds(1989) described the oral phase to be voluntary whereas the later stages weresaid to be involuntary.
In contrast, Nerurkar (2017) describes the events as “synchronouswith interaction between voluntary and involuntary aspects merged and unclear” (p.43). Picturebelow shows models: https://www.researchgate.net/figure/41452819_fig1_Four-sequential-model-and-process-model-are-illustrated-in-diagrams-showing-progression As canbe seen from the diagram above it is the first two stages of swallowing whichdiffer for liquids and solids. Oral PreparatoryStageThis consistsof bolus entry and retainment within the oral cavity.
The liquid bolus iseither held on the tongue or on the floor of the mouth. It is surrounded by thehard palate and dental arches. Posteriorly, the soft palate is tensed andlowered while the tongue is raised. These structures come together to seal thecavity and prevent leakage into the oropharynx prior to swallowing (Matsuo& Palmer, 2008). OralPropulsive StageWith themouth closed, the soft palate is raised and contracted to prevent nasalregurgitation.
The tip of the tongue is raised while the posterior part islowered, the tongue moves upward to increase the tongue-palate contact. Ananteroposterior contraction of the tongue moves the bolus against the hardpalate and finally the movement of the tongue base transfers the bolus to thepharynx (Nerurkar, 2017). This process does not relay on gravity as it isdriven by the tongue. Process Model-StageI TransportWhen food isnot swallowed whole the sight, smell, hunger, thirst and electrolyte balanceplay a significant part in initiating salivary flow and facilitating muscular adjustmentfor proper reception of the bolus (Nerurkar, 2017). As food is taken into themouth it is moved to the post-canine region and placed on the occlusal surfacefor processing (Matsuo & Palmer, 2008). ProcessModel-Food Processing The food ispushed between the teeth by the intrinsic and extrinsic muscles of the tongueand the buccinators in the cheek until it has broken down into finer particlesand has mixed with saliva forming a bolus (Nerurkar, 2017).
Continuousmastication and mixing with saliva reduce the size of particles while softeningthem which makes them the optimal consistency for swallowing. The cyclicmovements of the jaw are in synchronization with movements of the tongue,cheek, soft palate and hyoid bone. Jaw movements are quite large in the verticaldirection. Large movements of the tongue are seen in the vertical and anterior-posteriordirection. Due to constant movement of the soft palate and tongue there is nobarrier between the oral cavity and the pharynx. These movements pump air intothe nasal cavity via the pharynx. The aromas of chewed food are then detectedby chemoreceptors in the nose.
When a portion of foodready to be swallowed it may be moved to the vallecullae, this is known asstage II transport. The mechanism oftransport to the pharynx is the same as is described in the oral propulsive stagewith a liquid bolus however transport and food processing stages can besimultaneous i.e. food can accumulate in the vallecullae while chewingcontinues in the oral cavity (Matsuo & Palmer, 2008) (Nerurkar, 2017).