Swallowing (deglutition) is a rapid, coordinated sequence of muscular and neurological actions. This process safely moves food and liquid from the mouth to the stomach while simultaneously protecting the airway. It involves the precise control of over 25 pairs of muscles in the mouth, throat, and esophagus, orchestrated by a complex network of nerves. The act of swallowing transitions from a conscious, voluntary action to a series of quick, involuntary reflexes.
The Anatomy Essential for Swallowing
Safe swallowing depends on the coordinated function of several distinct structures in the head and neck. The tongue manipulates food and forms it into a cohesive mass called a bolus. The soft palate moves upward during the swallow to seal off the nasal cavity, preventing food or liquid from entering the nose.
The pharynx, or throat, serves as a common passageway for both food and air. The epiglottis, a flexible flap of cartilage at the base of the tongue, is a primary safety mechanism. When a swallow is triggered, the larynx elevates and moves forward, causing the epiglottis to fold down and cover the entrance to the trachea (windpipe). This temporary closure blocks the airway, ensuring the bolus is directed solely toward the esophagus.
The upper esophageal sphincter (UES) is a ring of muscle at the top of the esophagus that is typically closed at rest. This sphincter must relax and open rapidly to allow the food bolus to pass from the pharynx into the esophagus. The UES then closes quickly behind the bolus, defending against food entering the airway or refluxed material moving back up.
The Three Phases of Swallowing
The movement of food from the mouth to the stomach is divided into three sequential phases. The first is the Oral Phase, which is the only phase under voluntary control. During the oral preparatory stage, food is chewed and mixed with saliva to form the bolus. The propulsive stage involves the tongue pushing the bolus backward toward the throat.
Once the bolus passes a certain point, it triggers the Pharyngeal Phase, an involuntary reflex lasting about one second. This rapid sequence involves the elevation of the larynx and the closure of the airway by the epiglottis. This is accompanied by a momentary stop in breathing known as deglutition apnea. The pharyngeal constrictor muscles then contract in a sequential, wave-like motion to squeeze the bolus downward.
The Esophageal Phase begins when the bolus passes through the upper esophageal sphincter and into the food pipe. This phase is entirely involuntary and is characterized by peristalsis, a series of rhythmic, wave-like muscular contractions. Peristaltic waves propel the food bolus down the length of the esophagus toward the stomach. The lower esophageal sphincter at the bottom of the esophagus then relaxes to allow the bolus entry into the stomach.
Understanding Dysphagia
Dysphagia is difficulty swallowing, which occurs when any part of the complex swallowing mechanism is compromised. It is a symptom of an underlying medical issue affecting the muscles or nerves involved in swallowing.
One common cause is a neurological event, such as a stroke, which damages the brain centers coordinating the swallowing reflex. Other conditions affecting the nervous system can also weaken the necessary muscles. These include Parkinson’s disease, amyotrophic lateral sclerosis (ALS), and muscular dystrophies. Structural obstructions, such as those caused by head and neck cancers or chronic acid reflux (GERD), can also lead to dysphagia.
Aging contributes to swallowing difficulty due to sarcopenia, the loss of muscle mass and strength in the throat. The most serious consequence of dysphagia is aspiration, where food, liquid, or saliva accidentally enters the airway instead of the esophagus. Aspiration can be “silent,” occurring without noticeable symptoms like coughing or choking. Repeated aspiration introduces bacteria into the lungs, leading to aspiration pneumonia. Dysphagia also poses risks of dehydration and malnutrition if discomfort causes an individual to reduce their overall food and liquid intake.
Therapeutic Approaches and Management
Management of swallowing disorders is overseen by a Speech-Language Pathologist (SLP). The SLP conducts an assessment to pinpoint the specific area of the swallowing mechanism that is impaired. Treatment plans are designed to address deficits and improve the safety and efficiency of eating and drinking.
One common approach involves diet modification, which changes the texture of food and the thickness of liquids to make them easier to control. Foods may be pureed or mechanically soft, while liquids are often thickened to slow their flow. This modification reduces the risk of liquids entering the airway.
Therapeutic exercises are also used to strengthen weakened muscles and improve coordination and timing. Specific exercises, such as the effortful swallow, instruct the patient to squeeze all the swallowing muscles tightly to increase the force of propulsion. Other techniques may focus on increasing the strength of the tongue muscles or improving the range of motion of the larynx.