vestibule

The vestibule is part of the bony (osseous) labyrinth of the inner ear within the temporal bone, situated between the cochlea anteriorly and the three semicircular canals posteriorly. Inside the vestibule lie the utricle and saccule, membranous sac-like structures filled with endolymph. Each contains a specialized sensory epithelium called the macula, composed of hair cells with stereocilia embedded in an otolithic membrane containing calcium carbonate crystals (otoconia). The utricle's macula is oriented horizontally and detects horizontal linear acceleration (e.g., forward movement in a car), while the saccule's macula is oriented vertically and detects vertical linear acceleration (e.g., in an elevator) and static head tilt relative to gravity. These structures are innervated by the vestibular branch of the vestibulocochlear nerve (CN VIII), which relays information to the vestibular nuclei in the brainstem for balance, posture, and coordination with eye movements (vestibulo-ocular reflex). Clinically, the vestibule is relevant in conditions such as benign paroxysmal positional vertigo (BPPV), where otoconia dislodge from the utricle and migrate into the semicircular canals (most commonly the posterior canal), causing brief episodes of vertigo triggered by head position changes (diagnosed via Dix-Hallpike maneuver, treated with Epley maneuver). Other relevant conditions include vestibular neuritis, labyrinthitis, and Meniere's disease, which involves endolymphatic hydrops affecting the entire labyrinth including the vestibule. Understanding vestibular anatomy is essential for interpreting caloric testing and diagnosing central versus peripheral causes of vertigo.