Everyday Muscle.

Inferior Pharyngeal Constrictor

Its lowest fibers (cricopharyngeus) form a sphincter — dysfunction here causes Zenker's diverticulum.
Function
Constricts lower pharynx during swallowing
Origin
Oblique line of thyroid cartilage and cricoid cartilage
Insertion
Pharyngeal raphe
Innervation
Vagus nerve (CN X) via pharyngeal plexus and recurrent laryngeal nerve