5.1 From receptor potential to spike

The mechanism that turns the IHC’s depolarization into auditory-nerve spikes runs through the ribbon synapse — a structure unique to a few sensory cells, including IHCs, vestibular hair cells, and retinal photoreceptors. Each IHC has 10–30 ribbon synapses on its basal pole, each one contacting a different auditory-nerve fiber. A ribbon synapse is a specialized structure: a single presynaptic density (the ribbon) that tethers a large array of synaptic vesicles in close proximity to the active zone where Ca²⁺-triggered release occurs.

When the IHC depolarizes, voltage-gated Ca²⁺ channels open near the ribbon, Ca²⁺ flows in, and vesicles release glutamate into the synaptic cleft. The released glutamate binds AMPA receptors on the postsynaptic auditory-nerve terminal. If enough AMPA receptors open quickly enough, the spiral-ganglion neuron’s terminal depolarizes past threshold and the fiber fires a spike.

This sounds like a standard chemical synapse, but it is not. Ribbon synapses can release vesicles continuously at rates approaching 1000 per second per ribbon, with sub-millisecond timing precision. Conventional synapses cannot. The ribbon serves as a vesicle conveyor, keeping the active zone primed even under sustained depolarization. This is what gives the auditory system its exceptional temporal fidelity.

⏳ The history — Wever, Bray, and the cochlear microphonic

In 1930 Ernest Glen Wever and Charles Bray at Princeton placed an electrode on a cat’s auditory nerve and connected it, via an amplifier, to a loudspeaker in another room. When they spoke into the cat’s ear, their words were reproduced in the loudspeaker with startling fidelity. The electrical signal they recorded — which they initially interpreted as a neural response — turned out to be mostly the cochlear microphonic: an extracellular potential generated by hair-cell transduction currents that follows the acoustic waveform cycle by cycle.

The Wever-Bray experiment demonstrated that the cochlea produces an electrical signal that mirrors the acoustic input with remarkable precision. Disentangling the cochlear microphonic (generated by outer hair cells) from the compound action potential (generated by the auditory nerve) took another decade of work by Hallowell Davis and others. The cochlear microphonic remains a clinical tool today — it is recorded during electrocochleography and cochlear-implant surgery.