What is dysphagia?

Dysphagia is the medical term for the symptom of difficulty swallowing, derived from the Latin and Greek words meaning difficulty eating.
Mechanism of swallowing

Swallowing is a complex action.

Food is first chewed well in the mouth and mixed with saliva.
The tongue then propels the chewed food into the throat (pharynx).
The soft palate elevates to prevent the food from entering the posterior end of the nasal passages, and the upper pharynx contracts, pushing the food (referred to as a bolus) into the lower pharynx. At the same time, the voice box (larynx) is pulled upwards by muscles in the neck, and, as a result, the epiglottis bends downwards. This dual action closes off the opening to the larynx and windpipe (trachea) and prevents passing food from entering the larynx and trachea.
The contraction of the muscular pharynx continues as a progressing, circumferential wave into the lower pharynx pushing the food along.
A ring of muscle that encircles the upper end of the esophagus, known as the upper esophageal sphincter, relaxes, allowing the wave of contraction to push the food from the lower pharynx on into the esophagus. (When there is no swallow, the muscle of the upper sphincter is continuously contracted, closing off the esophagus from the pharynx and preventing anything within the esophagus from regurgitating back up into the pharynx.)
The wave of contraction, referred to as a peristaltic wave, progresses from the pharynx down the entire length of the esophagus.
Shortly after the bolus enters the upper esophagus, a specialized ring of muscle encircling the lower end of the esophagus where it meets the stomach, known as the lower esophageal sphincter, relaxes so that when it arrives the bolus can pass on into the stomach. (When there is no swallow the muscle of the lower sphincter is continuously contracted, closing off the esophagus from the stomach and preventing contents of the stomach from regurgitating back up into the esophagus.)
After the bolus passes, the lower sphincter tightens again to prevent contents of the stomach from regurgitating back up into the esophagus. It remains tight until the next bolus comes along.

Considering the complexity of swallowing, it is no wonder that swallowing, beginning with the contraction of the upper pharynx, has been “automated,” meaning that no thought is required for swallowing once swallowing is initiated. Swallowing is controlled by automatic reflexes that involve nerves within the pharynx and esophagus as well as a swallowing center in the brain that is connected to the pharynx and esophagus by nerves. (A reflex is a mechanism that is used to control many organs. Reflexes require nerves within an organ such as the esophagus to sense what is happening in that organ and to send the information to other nerves in the wall of the organ or outside the organ. The information is processed in these other nerves, and appropriate responses to conditions in the organ are determined. Then, still other nerves send messages from the processing nerves back to the organ to control the function of the organ, for example, the contraction of the muscles of the organ. In the case of swallowing, processing of reflexes primarily occurs in nerves within the wall of the pharynx and esophagus as well as the brain.)

The complexity of swallowing also explains why there are so many causes of dysphagia. Problems can occur with:

the conscious initiation of swallowing,
propulsion of food into the pharynx,
closing of the nasal passages or larynx,
opening of the upper or lower esophageal sphincters,
physical blockage to the passage of food, and
transit of the bolus by peristalsis through the body of the esophagus.

The problems may lie within the pharynx or esophagus, for example, with the physical narrowing of the pharynx or esophagus. They also may be due to diseases of the muscles or the nerves that control the muscles of the pharynx and esophagus or damage to the swallowing center in the brain. Finally, the pharynx and the upper third of the esophagus contain muscle that is the same as the muscles that we use voluntarily (such as our arm muscles) called skeletal muscle. The lower two-thirds of the esophagus is composed of a different type of muscle known as smooth muscle. Thus, diseases that affect primarily skeletal muscle or smooth muscle in the body can affect the pharynx and esophagus, adding additional possibilities to the causes of dysphagia.
Odynophagia and globus sensation

There are two symptoms that are often thought of as problems with swallowing (dysphagia) that probably are not. These symptoms are odynophagia and globus sensation.
Odynophagia

Odynophagia means painful swallowing. Sometimes it is not easy for individuals to distinguish between odynophagia and dysphagia. For example, food that sticks in the esophagus often is painful. Is this dysphagia or odynophagia or both? Technically it is dysphagia, but individuals may describe it as painful swallowing (i.e., odynophagia). Moreover, patients with gastroesophageal reflux disease (GERD) may describe dysphagia when what they really have is odynophagia. The pain that they feel after swallowing resolves when the inflammation of GERD is treated and disappears and is presumably due to pain caused by food passing through the inflamed portion of the esophagus.

Odynophagia also may occur with other conditions associated with inflammation of the esophagus, for example, viral and fungal infections. It is important to distinguish between dysphagia and odynophagia because the causes of each may be quite different.
Globus sensation

A globus sensation refers to a sensation that there is a lump in the throat. The lump may be present continuously or only when swallowing. The causes of a globus sensation are varied, and frequently no cause is found. Globus sensation has been attributed variously to abnormal function of the nerves or muscles of the pharynx and GERD. The globus sensation usually is described clearly by individuals and infrequently causes confusion with true dysphagia.