Content Reviewers:Viviana Popa, MD, Andrew Horne, MSc, BSc (Hons), Kaitlin Marshall,B.Sc. (Hons), M.Sc.
Contributors:Zachary Kevorkian, MSMI, Jennifer Montague, PhD, Sam Gillespie, BSc, Alexandru Duhaniuc, MD
Do you know that feeling when you listen to your favorite song and you start dancing along? Well, you can thank the eighth cranial nerve for that! CN eight, or the vestibulocochlear nerve, transmits special sensory information, related to balance and hearing, from the inner ear to the brain.
The vestibulocochlear nerve emerges laterally at the cerebellopontine angle, which is the junction between the pons, medulla and cerebellum. Once it emerges from the cerebellopontine angle, it enters the internal acoustic meatus where it separates into its two branches: the cochlear nerve and the vestibular nerve. The cochlear nerve carries information about hearing, and the vestibular nerve carries information about movements of the head which aids in balance.
Now, let’s look at the pathway of sensory information, starting with the auditory pathway. This begins in the cochlea, which is a bony tube that spirals on itself, resembling the shape of a snail shell.
If we uncoil this structure and look within it in cross-section, there are three fluid-filled cavities called the scala vestibuli, the cochlear duct, and the scala tympani. The scala vestibuli is connected to the middle ear through the oval window, and contains perilymph. The cochlear duct is filled with endolymph, and houses the organ of Corti, which is our organ of hearing and contains our hearing receptors, or hair cells. Finally, the scala tympani is connected to the middle ear through the round window, and it also contains perilymph.
The auditory pathway does begin in the cochlea, but first the sound waves have to get from the outside world to the cochlea and this requires the help of some very small structures. The sound vibrations first travel through the external ear and vibrate the tympanic membrane which sends a cascade of vibrations through the ossicles in the middle ear which eventually transmit these vibrations against the oval window.
When these vibrations hit the oval window, they transfer over to the perilymph inside the scala vestibuli, forcing the perilymph fluid into motion. Vibrations are then transferred to the cochlear duct which contains the organ of corti. The organ of Corti contains inner hair cells that activate in response to these vibrations and generate action potentials. These action potentials travel to the spiral ganglion. The spiral ganglion contains the cell bodies of the first order bipolar neurons of the cochlear nerve.
These bipolar neurons then send central projections that form the cochlear nerve which transmit signals to the cochlear nuclei located in the medulla, which synapse with second order neurons in the sensory pathway. From the cochlear nuclei, the neurons pass bilaterally to both the inferior colliculi located in the midbrain, and to the medial geniculate bodies located in the thalamus.
The inferior colliculi are important relay centres in the auditory pathway. They process auditory information by discriminating pitch, rhythm and the specific location of a sound and then send projections to the medial geniculate bodies. The medial geniculate bodies are where synapsing with third order neurons occur, and from here these third order neurons travel to the auditory cortex within the temporal lobe where the sensory information is processed for us to interpret.