ABSTRACT
The primary afferents have their cell bodies in the spiral ganglion located in the modiolus. In humans, about 30 000 type I afferents from inner hair cells provide the bulk of the output from the cochlea. Three-quarters of the hair cells send their output to only about 3000 type II afferents. The nature of type II bipolar cell signalling is unknown. Auditory afferents fire spontaneously. Auditory afferents have a dynamic range of about 30 dB beyond which further increase in sound pressure level (SPL) has no additional effect. The full range of SPL is signaled by afferents with different sensitivities. Cells with the same characteristic frequency may differ in threshold SPL by 70 dB. The sensitivities of afferents can be modified by efferents which have their cell bodies in the superior olivary complex. Medial olivocochlear neurons synapse with outer hair cells, releasing acetylcholine which hyperpolarizes them, thereby reducing their cochlear amplification effect on the basilar membrane.
