Describe the mechanism and disorders related to the process of swallowing in humans.

Introduction:

The act of swallowing can be considered as a complex activation of the muscles involved in an orderly sequence as orderly like that of the muscles of a Balle dancer.

Swallowing is initiated either by voluntary cortical drive or via sensations via the peripheral nervous system. Once the act of swallowing is initiated it continues in an orderly manner. After the initiation the whole act of swallowing is not under voluntary control. The neural network responsible for this reflexive phase of swallowing is known as the central pattern generator.

Components of central pattern generator:

Brain stem

Tractus solitarius

Nucleus ambiguus

Reticular formation

Deglutition is defined as the act of swallowing which causes the food bolus / liquid to the stomach via the mouth, pharynx, and oesophagus. Normal deglutition involves a complex series of voluntary and involuntary muscular contractions. For better understanding the process of deglutition is divided into three phases:

Oral

Pharyngeal

Oesophageal

Oral phase:

This phase of swallowing is voluntary in nature. This phase is subdivided into oral preparatory phase, and oral propulsive phase.

During the oral preparatory phase the bolus is processed in such a way that it is rendered swallowable. This phase involves chewing of the food mixed with saliva making it into a bolus which can be smoothly swallowed.

During the oral propulsive phase the muscles of the tongue plays an important role in in propelling the food into the oropharyx. When the bolus reaches the oropharynx the involuntary phase of deglutition begins.

Cranial nerves involved during the oral phase of deglutition include:

Trigeminal, Facial and Hypoglossal nerves. The cerebellum controls these cranial nerve inputs.

Disorders involving oral phase of swallowing:

Disorders involving this phase of swallowing is usually due to impaired tongue control. These patients have difficulty in chewing food and initiating swallow. These patients also have difficulty in holding liquids inside the oral cavity. Excess liquid inside the oral cavity starts to drool. When attempt is being made to swallow liquid the oropharyngeal reflexes are not initiated causing aspiration.

In patients with paralysis involving the facial nerve the lip closure is not precise and complete causing difficulties in holding the bolus inside the oral cavity. These patients also have food stasis in the lateral sulcus due to poor cheek muscle tone.

In paralysis involving the hypoglossal nerve the patient is unable to form bolus inside the mouth. Unless a bolus is formed swallowing act is not possible / becomes difficult. Due to reduction in tongue thrust the bolus cannot be propelled into the oropharynx. Due to incomplete contact between the tongue and palate the bolus cannot be efficiently propelled to the oropharynx.

In Parkinson's disease repeated tongue rolling may cause difficulty in propelling the bolus into the oropharynx.

Disorders involving oral phase of swallowing may cause a delay in the oral phase of swallowing.

Pharyngeal phase:

This phase involves a rapid sequence of overlapping events. This phase involves propulsion of bolus from the pharynx into the upper oesphagus through the cricopharyngeal sphincter. This phase is totally involuntary and reflexive capable of progressing as soon as it is initiated.

Critical events involved in this stage include the laryngeal protective sphinteric mechanism which is vital in preventing aspiration of bolus. These events include:

a. The soft palate rises

b. The hyoid bone and larynx moves upwards and forwards

c. The vocal folds adducts and come close to midline

d. The epiglottis folds backwards protecting the airway

e. The tongue pushes backwards pushing the bolus towards the cricopharyngeal sphincter

f. The pharyngeal wall constricts to facilitate backward movement of bolus

g. The cricopharyngeal sphincter relaxes due to forward movement of hyoid bone and larynx facilitating easy passage of bolus into the upper oesophagus.

This phase of swallowing lasts for about a second and it involves the motor and sensory components of 9^th and 10^th cranial nerves.

Disorders involving this phase of swallowing could case severe feeding impairments. In normal persons during swallow, small amounts of ingested food could be retained in the vallecula and pyriform sinus. In patients with disorders involving the pharyngeal phase of swallowing large amounts of food material may get retained in the vallecula / pyriform fossa causing aspiration when the patient attempts a clearance swallow.

Velopharyngeal closure which occurs during this stage helps in preventing nasal regurgitation of food. Imperfect velopharyngeal closure as it occurs due to palatal paralysis / cleft palate / submucosal cleft palate may cause nasal regurgitation during this phase of swallowing. This scenario could also occur when the posterior pillar of tonsil is injured while performing tonsillectomy.

Presence of severe cervical osteophytes could cause severe swallowing disability involving this phase.

In patients with imapired laryngeal elevation due to paralysis of the elevators could lead to cricopharyngeal muscle spasm leading on to aspiration.

Reduced laryngeal closure will cause food spillage into the larynx.

Pharyngeal muscle paralysis on both sides will cause coating of the lateral pharyngeal wall thereby hindering the swallowing process. Ultimately this could lead to an increase in the pharyngeal bolus transit time.

Esophageal phase:

In the esophageal phase of swallowing the bolus after crossing the cricopharyngeal sphincter traverses the oesophagus facilitated by the peristalsis of oesophageal musculature. The lower esophageal sphincter relaxes on initiation of the swallowing process. When the bolus enters the stomach the lower esophageal sphincter closes preventing reflux of gastric contents into the oesophagus. This phase of swallowing is involuntary and reflexive being controlled by the medulla. This phase usually lasts less than 20 seconds.

Disorders involving this phase could lead to retention of food inside the oesophagus. This retention could be caused by:

1. Mechanical obstruction due to presence of tumors
2. Motility disorders
3. Lower oesophageal sphincter failure