FIGURE 1. Supporting the auditory pathway.

a, After passing through the outer and middle ear, sound waves reach the cochlea in the inner ear. b, In the mature ear, sound vibration causes depolarization of the membrane of the inner hair cells. This opens voltage-gated calcium channels, allowing Ca²⁺ to flood into the cell to trigger release of the neurotransmitter glutamate. Tritsch et al.³ find that, in the immature ear, Ko¨lliker's organ releases ATP, which binds to P2X receptors on inner hair cells to cause depolarization and calcium influx, so mimicking the effect of sound input. In both cases, glutamate activates receptors on the afferent fibres, triggering electrical action potentials that propagate along the nerve fibres to the brain.