The vestibular system

Small, beautifully formed and locked in the skull, the vestibular organs continuously bombard the brain with messages. The messages are quite unlike any others. They tell of accelerations, how the head is rotating and translating and its orientation in space. The messages never stop and cannot be turned off. Even when we are completely motionless, they signal the relentless pull of gravity. Perhaps because of their constant monologue, the vestibular sensation is different to the other senses. There is no overt, readily recognizable, localisable, conscious sensation from these organs. They provide a silent sense. Known as balance organs of the inner ear, the vestibular organs serve this complex motor function at a largely subconscious level, but their role does not stop with balance. They contribute to a surprising range of brain functions, from the highest levels of consciousness to the most automatic reflexes. The value of the vestibular sensory system to brain functions such as perception of self and non-self motion, spatial orientation, navigation, voluntary movement, oculomotor control, and autonomic control, comes from their unique and complete description of head motion and orientation in three dimensions. Two different vestibular organs, the otolith organs and semicircular canals, sense different types of acceleration. Two otolith sensors, the utricle and saccule, sense linear acceleration. Three semicircular canals, the anterior, posterior and horizontal canals, sense rotational movements. These two complimentary signals are necessary for the brain to understand the range of physical situations that we experience, probably the most fundamental of which is to work out which way is up.