Chapter 5 — The auditory nerve

Analog becomes spikes

At the end of movement 5, we had a cochleagram — a picture of “Hey Dr. Miles!” painted across the basilar membrane in space and time, with the inner hair cells reporting the displacement at each place as a graded electrical receptor potential. The brain does not work in graded potentials. It works in discrete electrical events — action potentials, or spikes. Somewhere between the inner hair cell and the brain, the analog signal becomes a digital one. That somewhere is the auditory nerve.

There are about 30,000 fibers in each human auditory nerve. Each fiber is the axon of a spiral ganglion neuron, which sits in the bony spiral canal of the cochlea and contacts one or a few inner hair cells via the ribbon synapses we discussed in 5.6. Each fiber carries one stream of spikes from its IHC contact point up to the cochlear nucleus in the brainstem. The brain receives 30,000 channels of spike trains. That is the entire bandwidth of human hearing.

This movement asks: what is encoded in those 30,000 spike trains, and how?