Cochlear ganglion

Cochlear ganglion (a.k.a. spiral ganglion) is situated in a spiral canal within the bony modiolus, that forms the long axis of the cone-shaped cochlea.

The ganglion comprises of cell bodies of afferent neurons (AF) whose dendrites transmit auditory information from the hair cells of the organ of Corti in the inner ear. The afferent neurons are of two types: 90-95% of them are type-I neurons (AF-I), which are myelinated bipolar neurons. In contrast, only 5-10% are type-II neurons (AF-II), which are unmyelinated pseudo unipolar neurons. AF-I transmit sensory information from the inner hair cells (IHCs), while AF-II from the outer hair cells (OHCs).

Proximal to the cochlear ganglion, the axons of the AF-I and AF-II neurons come out from the base of the spiral cochlear via its bony axis–the modiollus (MD)–forming the auditory portion of the VIII nerve, that combines with the vestibular portion of the VIII nerve, to form the vestibulocochlear nerve.

The cochlear afferents: AF-I and AF-II neurons from the cochlear ganglion, are projected to the cochlear nuclei within the brainstem. The afferents arising from the basal coil of cochlea encode high frequency sounds and end up synapsing in the deeper parts of cochlear nuclei. Whereas, the afferents coming from the apical coil of cochlea encode low frequency sound waves and synapse in the superficial parts of cochlear nuclei. This shows that the tonotopic organization of segregating sound frequencies, early on in the auditory pathway, helps perceive complex sounds.