In addition to the cochlea, the inner ear has structures called the vestibular system. The vestibular system includes the three semicircular canals, which sense angular (rotational) acceleration of the head, and the vestibule, which senses linear (straight) acceleration of the head as well as the direction of gravity. The vestibular system senses these things with different types of hair cells. In the semicircular canals, angular acceleration causes endolymph to flow, which stimulates hair cells. In the vestibule, crystals called otoliths are attached to the hair of hair cells, causing the hairs to bend in response to linear acceleration or gravity.
Information from the vestibular hair cells is passed to axons of the vestibulocochlear nerve. These axons enter the lower brainstem to synapse in nuclei called vestibular nuclei. Vestibular nuclei axons then travel, probably bilaterally, in the lateral lemniscus to synapse in the thalamus, which then projects to the insula and neighboring cortex, which is probably where vestibular perception occurs. Vestibular nuclei axons also project to several other areas of the central nervous system for unconscious functions that use this information.
Lesions in vestibular pathways usually cause the symptom of vertigo, which is a perception of movement that is a hallucination (a perception without a stimulus) or an illusion (a misperception of a stimulus). Vertigo can be in any direction, but rotational (“spinning”) is most common from dysfunction of one or more semicircular canals. Nausea and dysequilibrium (feeling unsteady) are also common symptoms of vestibular dysfunction. The vestibular nuclei are also tightly connected to the cerebellum to help coordinate movements, as well as the brainstem nuclei that control eye movements. Nystagmus is a common sign of vestibular dysfunction, which involves involuntary, small, back and forth eye movements in one or more directions. Vestibular symptoms and signs usually appear or worsen with head movements.