Final answer:
The esophagus contains layers of smooth and skeletal muscle fibers. In the upper third, it consists of skeletal muscle, in the middle third a combination of both types, and in the lower third, it is smooth muscle. The circular lower esophageal sphincter located at the end of the esophagus helps regulate the passage of food into the stomach.
Step-by-step explanation:
The fibers of the esophagus include both smooth and skeletal muscle fibers. These fibers are organized in specific layers that aid in the process of peristalsis, which is the series of wave-like movements that push the food bolus from the mouth toward the stomach. In the upper third of the esophagus, the muscularis is primarily composed of skeletal muscle. In the middle third, there is a transition from skeletal to smooth muscle fibers. Finally, in the lower third, the muscularis is exclusively smooth muscle. The circular muscle layer of the muscularis contracts to propel the bolus forward, while relaxation of these muscles allows the esophagus to expand and receive the food bolus.
At the lower end of the esophagus, there is a circular muscle known as the lower esophageal sphincter, which acts as a valve, opening to let food pass into the stomach and closing to prevent the backflow of food and stomach acids. This sphincter works in coordination with the peristaltic movements of the esophagus, a process that is involuntary and controlled by the medulla oblongata.