What is the Difference Between Upper and Lower Esophageal Sphincter

The main difference between upper and lower esophageal sphincter is that the upper esophageal sphincter (UES) is a high pressurize zone that occurs at the transition of the pharynx and the cervical esophagus, whereas the lower esophageal sphincter (LES) is a high pressurize zone that occurs where the esophagus meets the stomach.

The upper and the lower esophageal sphincter are two types of sphincters located on both ends of the esophagus.

Key Areas Covered

1. What is Upper Esophageal Sphincter
     – Definition, Structure, Function
2. What is Lower Esophageal Sphincter
     – Definition, Structure, Function
3. Similarities Between Upper and Lower Esophageal Sphincter
     – Outline of Common Features
4. Difference Between Upper and Lower Esophageal Sphincter
     – Comparison of Key Differences

Key Terms

Lower Esophageal Sphincter, Upper Esophageal Sphincter

What is Upper Esophageal Sphincter

The upper esophageal sphincter (UES) is the sphincter that occurs in the upper end of the sphincter. Therefore, it is the inferior pharyngeal sphincter. It is located at the lower end of the pharynx. The main function of the UES is to guard the entrance upper end of the esophagus. Function-wise, the UES prevents the entry of air into the esophagus during breathing. In addition, it prevents the reflux of the esophageal contents into the pharynx. The UES can be identified as a high-pressure zone, extending 3-4 cm vertically.

Figure 1: Upper Esophageal Sphincter

Furthermore, the UES occurs behind the cricoid cartilage, extending both above and below it. Generally, the cricopharyngeus serves as the major contributor to the high-pressure zone of the UES. The high-pressure zone peaks above the cricopharyngeus muscle. The oblique placement of the muscle fibers of the thyropharyngeus and the horizontal placement of the cricopharyngeus muscle fibers form the UES. The muscle fibers of the cricopharyngeus are either slow or fast. Slow fibers are predominant and are responsible for the tonic, while fast muscle fibers undergo phasic contractions.

What is Lower Esophageal Sphincter

The lower esophageal sphincter (LES) is a high-pressure zone connecting the bottom of the esophagus to the stomach. The main function of the LES is to pass food from the esophagus to the stomach. It also prevents the reflux of gastric contents. Besides, there are two types of muscles in the LES: the intrinsic component and the extrinsic component. The esophageal muscle fibers that are intrinsic are under neurohormonal control. In contrast, the extrinsic component of the LES contains diaphragmatic crura and the pharyngoesophageal ligament, providing anatomical support and protection against gastric reflux.

Figure 2: Lower Esophageal Sphincter

Moreover, the intrinsic component of the LES consists of smooth muscles, keeping the sphincter tightly contracted. However, the relaxation of smooth muscles occurs through inhibitory signals, opening the sphincter. This allows the passage of food bolus. But the LES does not possess dilator muscles; therefore, the opening dilation depends on the relaxation of smooth muscles.

Similarities Between Upper and Lower Esophageal Sphincter

• Upper and lower esophageal sphincters are two types of high-pressure zones that occur at the transition sites of the esophagus.
• They contain ring-shaped muscles that sense the food and liquid coming.

Difference Between Upper and Lower Esophageal Sphincter

Definition

The upper esophageal sphincter refers to the high-pressure zone located between the pharynx and the cervical esophagus, while the lower esophageal sphincter refers to a high-pressure zone located where the esophagus meets the stomach and protects the esophagus from the reflux of gastric contents.

Occurrence

The upper esophageal sphincter occurs at the upper end of the esophagus, while the lower esophageal sphincter occurs at the bottom of the esophagus.

Connecting the Esophagus

Moreover, the upper esophageal sphincter connects the pharynx to the esophagus, while the lower esophageal sphincter connects the stomach to the esophagus.

Function

The main function of the upper esophageal sphincter is to pass food from the pharynx to the esophagus, while the main function of the lower esophageal sphincter is to pass food from the esophagus to the stomach.

Types of Muscles

The upper esophageal sphincter contains thyropharyngeus and cricopharyngeus muscle fibers, while the lower esophageal sphincter contains intrinsic and extrinsic muscle fibers.

Conclusion

In brief, the upper and lower esophageal sphincter are two types of sphincters that occur on both ends of the esophagus. Their function is to connect the esophageal ends to the different organs of the alimentary canal. Generally, the upper esophageal sphincter occurs at the esophagus’s upper end, connecting the esophagus to the pharynx. The main function of the upper esophagus is to transfer food from the pharynx to the esophagus. In general, the two types of muscle fibers occurring in the upper esophageal sphincter are thyropharyngeus and cricopharyngeus muscle fibers. In comparison, the lower esophageal sphincter occurs at the lower end of the esophagus, connecting the stomach to the esophagus. Therefore, the main function of the lower esophageal sphincter is to pass food from the pharynx to the stomach. Hence, the main difference between the upper esophageal sphincter and the lower esophageal sphincter is their function.

References:

1. Mittal RK. Motor Function of the Pharynx, Esophagus, and its Sphincters. San Rafael (CA): Morgan & Claypool Life Sciences.
2. Rosen RD, Winters R. Physiology, Lower Esophageal Sphincter. [Updated 2023 Mar 17]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing.

Image Courtesy:

1. “5.2 Basic Concepts – Nursing Advanced Skills” By WI Technical collages Open Press – Own work (CC BY 4.0) via Commons Wikimedia
2. “Gastric Anatomy” By MIT OpenCourseWare – Own Work (CC BY-NC-SA 2.0) via Commons Wikimedia