Specializations for fast signaling in the amniote vestibular inner ear

Synopsis

During rapid locomotion, the vestibular inner ear provides head-motion signals that stabilize posture, gaze, and heading. Afferent nerve fibers from central (striolar) and peripheral (extrastriolar) zones of vestibular sensory epithelia use temporal and rate encoding, respectively, to emphasize different aspects of head motion: central- and striolar-zone afferents adapt faster to sustained head position and favor higher stimulus frequencies, reflecting specializations at each stage from motion of the accessory structure to spike propagation to the brain. One specialization in amniotes is an unusual nonquantal synaptic mechanism by which type I hair cells transmit to large calyceal terminals of afferent neurons. Its reduced synaptic delay suggests that it may have evolved to serve reliable and fast input to reflex pathways that ensure stable locomotion on land.