Swallowing 1989). The anatomic areas where swallowing takes

Swallowing refers to the process by which food or liquid
substances are moved from the oral cavity into the stomach. Although a
seemingly simple and primitive process, the swallowing sequence actually
requires complex coordination of over 30 nerves and muscles (Matsuo &
Palmer, 2008). While combining the enjoyment of consuming food with the inherent
need for nutrients and hydration it is critically important that the airway is
kept clear at all times. While most swallows occur involuntarily as a result of
salivation, clusters of voluntary swallows can be evoked during feeding (Dodds,
1989).

 

The
anatomic areas where swallowing takes place are the oral cavity, pharynx,
larynx and oesophagus. As the bolus travels through each of these it comes in contact
with various bony/cartilaginous structures offering support, striated and
smooth muscle components aiding movement and neural elements which innervate
the musculature (Dodds, 1989).  

 

Previously,
normal swallowing in humans was described using a three-stage sequential model
consisting of an oral, pharyngeal and oesophageal stage. The oral stage was
later subdivided into the oral preparatory and oral propulsive stages resulting
in 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 were
said to be involuntary. In contrast, Nerurkar (2017) describes the events as “synchronous
with interaction between voluntary and involuntary aspects merged and unclear” (p.43).

 

Picture
below shows models: https://www.researchgate.net/figure/41452819_fig1_Four-sequential-model-and-process-model-are-illustrated-in-diagrams-showing-progression

 

As can
be seen from the diagram above it is the first two stages of swallowing which
differ for liquids and solids.

 

Oral Preparatory
Stage

This consists
of bolus entry and retainment within the oral cavity. The liquid bolus is
either held on the tongue or on the floor of the mouth. It is surrounded by the
hard palate and dental arches. Posteriorly, the soft palate is tensed and
lowered while the tongue is raised. These structures come together to seal the
cavity and prevent leakage into the oropharynx prior to swallowing (Matsuo
& Palmer, 2008).

 

Oral
Propulsive Stage

With the
mouth closed, the soft palate is raised and contracted to prevent nasal
regurgitation. The tip of the tongue is raised while the posterior part is
lowered, the tongue moves upward to increase the tongue-palate contact. An
anteroposterior contraction of the tongue moves the bolus against the hard
palate and finally the movement of the tongue base transfers the bolus to the
pharynx (Nerurkar, 2017). This process does not relay on gravity as it is
driven by the tongue.  

 

Process Model-Stage
I Transport

When food is
not swallowed whole the sight, smell, hunger, thirst and electrolyte balance
play a significant part in initiating salivary flow and facilitating muscular adjustment
for proper reception of the bolus (Nerurkar, 2017). As food is taken into the
mouth it is moved to the post-canine region and placed on the occlusal surface
for processing (Matsuo & Palmer, 2008).  

 

Process
Model-Food Processing

The food is
pushed between the teeth by the intrinsic and extrinsic muscles of the tongue
and the buccinators in the cheek until it has broken down into finer particles
and has mixed with saliva forming a bolus (Nerurkar, 2017). Continuous
mastication and mixing with saliva reduce the size of particles while softening
them which makes them the optimal consistency for swallowing.

 

The cyclic
movements of the jaw are in synchronization with movements of the tongue,
cheek, soft palate and hyoid bone. Jaw movements are quite large in the vertical
direction. Large movements of the tongue are seen in the vertical and anterior-posterior
direction. Due to constant movement of the soft palate and tongue there is no
barrier between the oral cavity and the pharynx. These movements pump air into
the nasal cavity via the pharynx. The aromas of chewed food are then detected
by chemoreceptors in the nose.

 

When a portion of food
ready to be swallowed it may be moved to the vallecullae, this is known as
stage II transport.  The mechanism of
transport to the pharynx is the same as is described in the oral propulsive stage
with a liquid bolus however transport and food processing stages can be
simultaneous i.e. food can accumulate in the vallecullae while chewing
continues in the oral cavity (Matsuo & Palmer, 2008) (Nerurkar, 2017).