There are three parts to swallowing which allows food from the mouth to enter the stomach and thus commence with the processes of digestion and absorption. The esophageal stage of swallowing is the last part and follows the pharyngeal stage which is preceded by the voluntary stage. The esophageal stage of swallowing, like the pharyngeal stage, is involuntary and coordinated by the autonomic nervous system.
The esophagus itself is a narrow muscular tube extending from the throat (pharynx) to the stomach. Its main function is to conduct food from the mouth to the stomach.This is done by two types of peristaltic contractions :
- Primary peristalsis
- Secondary peristalsis
Primary peristalsis is a continuation of the peristaltic wave that originates in the pharynx. It extends all the way to the stomach and is sufficiently effective to push the food from the pharynx to the stomach in less than 10 seconds. This occurs faster when assisted by the action of gravity if a person is standing or sitting upright.
Secondary peristalsis is the backup motility mechanism of the esophagus. It is triggered by distention of the esophagus due to presence of food (bolus ~ ball of food). Any food that has not passed into the stomach by the primary peristalsia will be pushed by repeated secondary peristaltic waves.
The passage of food through the esophagus is fairly rapid since food does not have to delay its transit in the esophagus for mechanical or chemical digestion. To aid with this speedy movement, the inner lining of the esophagus has simple mucus glands. The mucus secreted from these glands lubricate the surface between the bolus and the inner esophageal lining (mucosa).
Control of Esophageal Motility
Since primary peristalsis is a continuation of the peristaltic wave from the pharynx, it is essentially controlled by the areas that initiate and propagate the wave at this point. Impulses triggered by the presence of food in the mouth (tactile) sends feedback to the medulla oblongata through the trigeminal and glossopharyngeal nerves. The swallowing center located in the lower pons and medulla then sends motor impulses to the pharynx and upper esophagus through cranial nerves V (5 – trigeminal), IX (9 – glossopharyngeal), X (10 – vagus) and XII (12 – hypoglossal).
The secondary peristaltic waves are triggered by intrinsic reflexes of the enteric nervous system. It is backed up by motor impulses from the medulla which travels to the esophagus via the glossopharyngeal and vagus nerves. The ability of the myenteric plexus to initiate and maintain movement via the secondary peristaltic wave means that esophageal swallowing can continue independently without input from the central nervous system.
Movement of Food through the Lower Esophageal Sphincter (LES)
Similar to the peristaltic reflex in intestinal motility, the part of the esophagus distal to the bolus (ahead of the bolus in the direction of movement) relaxes, while the proximal part constricts. This is known as receptive relaxation and is propagated by the inhibitory neurons of the myenteric plexus. As food moves through the esophagus, this reflex extends well beyond the area immediately ahead of the bolus and may cause reflex relaxation all the way to the stomach and duodenum in preparation for the entry of food.
The lower end of the esophagus where contents empty into the stomach has a circular muscle sphincter known as the lower esopahgeal sphincter (LES aka gastroesophageal sphincter, cardiac sphincter). This sphincter is constantly constricted to prevent the gastric contents from entering and damaging the esophagus due to the action of gastric secretions, particularly hydrochloric acid. The sphincter’s protective function is backed up by the extension of the distal portion of the esophagus into the stomach. This acts somewhat like a valve. The receptive relaxation wave also relaxes the LES and allows the rapidly moving food to enter the stomach quickly.