The inner ear is a delicate structure, generating the sensation of sound thanks to fragile hair cells, detecting the pressure of sound waves. Sensory neurons, known as type I afferents, then transmit the signals from the hair cells to the brain. However, very loud noises can destroy hair cells, permanently degrading our sense of hearing. Type II afferent neurons are relatively insensitive to sound, but respond when outer hair cells, those closest to danger, are damaged; like pain receptors in the skin, their role is to warn the brain, so that we can protect ourselves before more cells are lost. In the rat’s inner ear shown, a type II afferent stretches from the outer hair cells to the brain, where its cell body forms a dark spot near the centre of the picture. Studying how these neurons operate could provide new insights into hyperacusis, a painful hyper-sensitivity to noise.
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