utriculus

The utriculus, or utricle, is a membranous sac located in the vestibule of the inner ear. It forms part of the vestibular apparatus, together with the saccule and the semicircular canals, and contributes to the sense of balance and spatial orientation in many vertebrates, including humans.

Structure and contents: The utriculus contains a sensory epithelium called the macula utriculi. This surface is covered by a gelatinous layer embedded with otoconia, small calcium carbonate crystals. Hair cells and supporting cells comprise the macula; their hair bundles respond to mechanical movement when the head changes position or experiences linear acceleration. The utricle connects to the brain via the vestibular nerve, transmitting signals related to head movement.

Function: The utricle detects linear acceleration and head tilt in the horizontal plane, providing information about motion and orientation relative to gravity. This input supports reflexes that stabilize gaze and posture, including vestibulo-ocular and vestibulo-spinal reflexes. In combination with the saccule, the utricle contributes to a broader sense of head position in three-dimensional space.

Clinical relevance: Dysfunction of utricular input can contribute to imbalance, dizziness, or vertigo. Otolith function can be assessed with specialized tests such as vestibular-evoked myogenic potentials (VEMP), which help evaluate otolith organ integrity and vestibular pathways.

Etymology and terminology: The term utriculus is Latin for “little bag” and is used historically; in modern English, the term utricle is more common, with utriculus appearing in some anatomical writings and Latinized forms.