The inner ear that houses inner hair cells (blue) with their presynaptic ribbons (white) and a recorded spiral ganglion neuron (yellow). Image credit: Lina María Jaime Tobón and Tobias Moser (CCBY 4.0)
From the rustling of falling leaves all the way to a roaring jet engine, our sense of hearing allows us to recognise a wide range of sounds that vary in pitch and intensity. Two groups of inner ear cells, known as the inner hair cells and the spiral ganglion neurons, perform this feat by encoding sounds into signals that can be processed by the nervous system.
This mechanism relies on inner hair cells detecting sound vibrations and then causing spiral ganglion neurons to ‘fire’ electrical signals that can be relayed to the brain. Each inner hair cell connects to multiple spiral ganglion neurons through contact points called synapses, where information corresponding to specific sounds is transmitted. For any given pitch, different groups of spiral ganglion neurons encode different sound intensities (that is, loud versus soft sounds). Whether this is because these groups are fundamentally different in some way or because the synapses between neurons and hair cells have different properties remained to be elucidated.
To investigate this question, Jaime Tobón and Moser examined the mechanisms underpinning sound encoding in the inner ear of mice, using tissue preparations containing inner hair cells and spiral ganglion neurons with fully intact synapses.
Measuring the electrical properties on both the inner hair cell and spiral ganglion neuron side of individual synapses revealed differences in the levels of spontaneous activity of the synapses. Synapses with higher spontaneous activity detected softer stimuli, whereas those with lower rates responded only to stronger stimulation.
Each type of synapse formed at different locations on the surface of inner hair cells, and they had different electrical properties that mirrored the firing diversity of the spiral ganglion neurons. In other words, it is the inner hair cell ‘side’ of the synapses that dictates the different responses of the neurons connected to them. To diversify the response of their synapses, the inner hair cells relied on variations in synaptic properties (such as voltage-dependent activation thresholds and the coupling of calcium channels and vesicular release sites) that determine how sensitive a cell is to an electric signal, and how quickly and efficiently it can react to it.
These results shed new light on the biological mechanism of sound encoding, a process fundamental to our sense of hearing. In the future, Jaime Tobón and Moser hope that this knowledge may eventually inform the development of better aids and treatments for hearing loss patients.
