Type I spiral ganglion neuron
The cell bodies of type I spiral ganglion neurons (SGNs) is located within the spiral ganglion in the cochlea. A single dendrite projects to the sensory epithelium of the cochlea and forms a synapse with a single presynaptic ribbon of an Inner Hair Cell (IHC). A single Axon projects to the Cochlear Nuclei via cranial nerve VIII (vestibulocochlear), within the cochlear nuclei, the axon bifurcates and sends projections to the ventral and dorsal cochlear nuclei.
Input: Inner Hair Cells (Glutamate - Excitatory), Lateral Olivocochlear Neurons (ACh, GABA, CGRP, dopamine, serotonin - Inhibitory).
Output: Cochlear Root Neurons and Cochlear Nuclei (Glutamate - Excitatory).
Type I SGNs display "spontaneous" spiking activity at rates of 0.1-100Hz, driven by spontaneous glutamate release at the IHC ribbon synapse. Excitation and Inhibition of IHCs increases and decreases the spike rate respectively.