The cochlear component of the cranial nerve VIII begins in the cochlea of the inner ear. Peripheral fibres of bipolar neurones synapse with sensory hair cells, pass through the spiral ganglion where their cell bodies reside, and then continue on as the cochlear nerve through the internal auditory meatus to the brainstem.
The nerve enters at the level of the cerebellopontine angle in the medulla. Each axon divides at this point to send fibres to the anterior and posterior cochlear nuclei in the medulla and caudal pons respectively. A number of acoustic strial pathways direct second-order neurones to the trapezoid body and the olivary nucleus before heading on to the superior olivary nucleus (SON) on the ipsi- or contralateral side. From the SON, axons ascend as the lateral lemniscus to the inferior colliculus on the same side. Collateral fibres involved with arousal are given off to the reticular activating system on route.
The inferior colliculus relays information to:
From the thalamus, the auditory radiation ascends in the internal capsule to cortical areas 41 and 42 in the superior temporal gyrus - the auditory cortex.
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