The Ribbon Synapse Between Type I Spiral Ganglion Neurons and Inner Hair Cells
This chapter provides an overview of the first auditory synapses, in the cochlea where sound is encoded. We review insights into the development, structure, and function of the excitatory ribbon-type synapses between presynaptic inner hair cells and postsynaps on the type I spiral ganglion neurons. They convey all information about sound timing and intensity to the brain, via action potentials in the auditory nerve. Recordings from individual type I spiral ganglion neurons in vivo demonstrate remarkable diversity between neurons in their sound-response properties. Although much has been learned about the representation of acoustic information in the auditory nerve, relatively little is known about the synaptic mechanisms underlying diversity of encoding. The response properties of SGN determined by properties of SGN may be largely determined by the details of the 1:1 connection between each inner hair cell presynaptic active zone and its postsynaptic type I spiral ganglion neuron. This chapter covers (1) synaptogenesis as inner hair cells mature from pattern generators to sound receivers, (2) presynaptic mechanisms governing exocytosis, (3) synaptic transmission to the type 1 spiral ganglion neuron and subsequent action potential generation, and (4) how pre- and postsynaptic heterogeneities may contribute to the diversity of spiral ganglion neuron response properties that enable hearing over a broad range of sound pressure levels. Presynaptic stimulus-secretion coupling appears to operate in a nanodomain regime and the postsynaptic action potential generator is tightly coupled to synaptic input. Thus, opening of a single presynaptic Ca2+ channel may be sufficient to trigger a postsynaptic action potential.