The spiral ganglion is the cochlear sensory ganglion containing the bipolar first-order auditory neurons whose peripheral processes innervate the inner and outer hair cells of the organ of Corti, and whose central processes form the cochlear nerve travelling in the internal auditory canal to the cochlear nuclei in the pons. The spiral ganglion neurons (approximately 30,000 per ear) are arranged in a coil mirroring the cochlear spiral within the modiolus (central cochlear pillar).
The spiral ganglion neurons are the primary target of cochlear implantation: the cochlear implant electrode array is inserted into the scala tympani of the cochlea, with the electrical contacts stimulating the nearby spiral ganglion neurons to convey frequency-coded auditory information to the auditory brainstem. Preservation of spiral ganglion neuron populations (neuron survival) is the primary determinant of cochlear implant benefit. Noise-induced and age-related sensorineural hearing loss (presbycusis) produce cochlear synaptopathy (loss of synaptic connections between hair cells and spiral ganglion neurons) before hair cell death. Ototoxic drugs (aminoglycosides, cisplatin) destroy both hair cells and spiral ganglion neurons.
Cochlear implantation delivers electrical stimulation to the residual spiral ganglion neurons in the scala tympani; patients with greater spiral ganglion neuron survival (from shorter duration of deafness and preservation of cochlear anatomy) achieve better speech recognition scores with cochlear implants; early implantation within 2 years of onset of profound deafness maximises neuron preservation for optimal implant benefit.
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