Upper Esophageal Sphincter

Overview

The upper esophageal sphincter (UES) is a muscular ring located at the junction of the pharynx and the esophagus. It serves as a critical barrier, regulating the passage of food and liquids from the oropharynx into the esophagus while also preventing the entry of air into the esophagus during breathing. The UES plays an essential role in swallowing, protecting the airway, and ensuring that food moves efficiently into the stomach.

Location

The UES is located at the cervical part of the esophagus, where the esophagus meets the pharynx. Specifically:

• It lies at the level of the C5–C6 vertebrae, just below the cricoid cartilage in the neck region

• It is positioned at the junction of the hypopharynx and the upper part of the esophagus

• The UES is located above the sternocleidomastoid muscle and anterior to the cervical spine

Structure

The UES is primarily composed of a combination of skeletal muscle and connective tissue. Key structural features include:

• Skeletal muscle: The sphincter is made up mainly of striated muscle fibers, which are under voluntary control

• Crico-pharyngeus muscle: The primary muscle forming the UES, it is a continuation of the inferior pharyngeal constrictor muscle and contracts to form the sphincter

• Fibrous connective tissue: The UES is supported by a fibrous capsule that helps maintain the integrity of the sphincter during swallowing and at rest

• Neurological control: The UES is controlled by both voluntary and involuntary (reflex) neural mechanisms, primarily through the vagus nerve (cranial nerve X)

Function

The primary function of the UES is to regulate the passage of food and liquids from the pharynx into the esophagus, while also preventing the entry of air. Its functions include:

• Regulating swallowing: The UES opens during swallowing to allow food to pass from the mouth into the esophagus

• Protecting the airway: The UES remains closed at rest to prevent food, liquids, or saliva from entering the trachea and lungs

• Preventing reflux: It helps prevent gastric contents from moving backward into the pharynx during swallowing or gastric reflux

• Coordinating peristalsis: The UES works in conjunction with the esophageal muscles to coordinate the movement of swallowed food through the esophagus

Physiological Role(s)

The UES plays a key role in several physiological processes:

• Swallowing (deglutition): The UES opens reflexively when food or liquid is swallowed, allowing the bolus to move from the oropharynx to the esophagus

• Airway protection: The UES maintains a tight closure to protect the airway from aspiration and prevent food or liquid from entering the trachea during breathing

• Gastroesophageal coordination: The UES works with the lower esophageal sphincter (LES) to prevent reflux and regulate pressure within the esophagus

• Coordination with the pharynx: The UES helps ensure smooth transitions between the pharynx and esophagus during swallowing

Clinical Significance

The UES is involved in various clinical conditions that affect swallowing, airway protection, and digestion:

• Dysphagia: Difficulty swallowing can result from dysfunction of the UES, leading to food getting stuck or aspiration into the airway

• Zenker's diverticulum: A pouch that forms in the pharyngeal wall, often due to UES dysfunction, leading to regurgitation and aspiration

• Achalasia: A condition in which the UES fails to relax properly during swallowing, leading to difficulty swallowing food and liquids

• UES dysfunction and reflux: Inadequate closure of the UES can contribute to acid reflux or aspiration pneumonia

• Stroke or neurological damage: Damage to the brain or vagus nerve can impair UES function, leading to swallowing difficulties and risk of aspiration

• Endoscopic evaluation: Disorders of the UES are often assessed through imaging or endoscopy, with studies like manometry measuring the pressure and function of the sphincter during swallowing

Diagnosis of UES-related disorders typically involves clinical examination, swallowing tests, and imaging studies. Treatment may include speech therapy, botulinum toxin injections (for achalasia), or surgical interventions for structural issues like Zenker’s diverticulum.