Inferior pharyngeal constrictor

The inferior pharyngeal constrictor is the lowermost and thickest of the three pharyngeal constrictor muscles. It forms the lower part of the circular muscular wall of the pharynx, surrounding the laryngopharynx.

Origin

The inferior pharyngeal constrictor muscle consists of two parts, named according to their sites of origin:

1. Thyropharyngeal part

• Arises from the oblique line of the thyroid cartilage and the tendinous arch between the thyroid and cricoid cartilages.

• Fibres run posteriorly and medially, overlapping the lower part of the middle constrictor.

2. Cricopharyngeal part

• Arises from the lateral surface of the cricoid cartilage.

• Its fibres encircle the upper end of the oesophagus in a nearly horizontal fashion, forming a muscular sphincter known as the upper oesophageal sphincter (cricopharyngeal sphincter).

Insertion

Most fibres curve posteriorly and medially to meet their counterparts in the median pharyngeal raphe. The lowest cricopharyngeal fibres blend directly with the circular muscle fibres of the esophagus, rather than inserting into the raphe.

Innervation

• Thyropharyngeal part: Supplied by the pharyngeal branch of the vagus nerve (CN X) via the pharyngeal plexus, containing motor fibres from the cranial part of the accessory nerve (CN XI).

• Cricopharyngeal part: Receives additional innervation from the recurrent laryngeal and external laryngeal nerves.

Action

• The thyropharyngeal fibers constrict the lower part of the pharynx during swallowing, pushing the bolus toward the esophagus.

• The cricopharyngeal fibers act as a functional sphincter, remaining tonically contracted between swallows to prevent air from entering the esophagus and to stop esophageal contents from regurgitating into the pharynx. The sphincter relaxes momentarily during swallowing to allow passage of the bolus. Dysfunction or incoordination of this segment can lead to oropharyngeal dysphagia or Zenker’s diverticulum — a mucosal pouch that protrudes through the Killian’s dehiscence (a weak area between the thyropharyngeal and cricopharyngeal parts).