utricles

Utricles are the two membranous sacs located in the vestibule of the inner ear, forming part of the otolith organs of the vestibular system. Each ear contains a utricle, positioned alongside the saccule and the semicircular canals, and together they help regulate balance and spatial orientation. The sensory epithelium of the utricle, called the macula utriculi, consists of hair cells embedded in a gelatinous otolithic membrane that contains otoconia, or calcium carbonate crystals.

The macula utriculi is oriented horizontally, making it sensitive to linear acceleration in the horizontal plane and head tilt relative to gravity. When the head moves, inertia causes the otoconia to slide within the membrane, bending the hair bundles of the hair cells. This mechanical stimulus is transduced into neural signals that travel via the vestibular portion of the vestibulocochlear nerve (CN VIII) to the brainstem and cerebellum, where they contribute to postural control and the vestibulo-ocular reflex.

Clinical relevance: Dislodged otoconia from the utricle can migrate into a semicircular canal, most commonly the posterior canal, leading to benign paroxysmal positional vertigo (BPPV). Vestibular dysfunction involving the utricle can contribute to vertigo, disequilibrium, or abnormal nystagmus, reflecting impaired processing of linear accelerations and head orientation.