Auditory Nerve
Possible synonyms for the auditory nerve are cochlear nerve or acoustic nerve.
The eight cranial nerve
It is a nerve in the head that transmits electrochemical signals that are transduced inside the cochlea of the inner ear and carrying them to the brain stem. This cranial nerve is the auditory component of the vestibule-cochlear nerve. It is called also the 8th cranial nerve.
Orientation in space
The orientation in space is dependent on feedback from the other component of the 8th cranial nerve that is called the vestibular nerve. The sensory information about spatial orientation of the head comes from the semicircular canals. The cochlear nerve is classified as a cranial sensory nerve. The meaning of the electrochemical signals in this nerve is information about sound energy in the environment. These signals are transmitted to the brain via the brainstem.
Acoustic energy
The starting point of most of the acoustic energy is in vibrations of the tympanic membrane. The origin of the auditory nerve is inside the cochlea in the inner ear. The electrochemical current, transmit the auditory signals to the brainstem. Its neurons terminate in the cochlear nucleus that is located in the brainstem.
The Auditory nerve
Auditory nerve fibers are bipolar. The distal part is called the peripheral process and the proximal part is called the axon. Both the peripheral processes of the nerve cells and the axons are myelinated. The estimated number of nerve cells within the cochlear nerve of a healthy man is 30,000.
The Auditory nerve is the first link to the CNS. It's fibers synapse with the hair cells inside the cochlea on the distal side and the cochlear nucleus within the brainstem on the proximal side. The bodies of the cochlear nerve cells are located in the central aspect of the cochlea. They are collectively, called the spiral ganglion.
The central axons emerge from the base of the cochlea. These axons unify in order to from a nerve trunk. The nerve trunk enters the brainstem. Inside it, the nerve fibers makes the first synapsis inside the CNS. The partner is the cell bodies of the cochlear nucleus. Most of the cochlear ganglion cells comes from the inner hair cells, the rest of them come from the outer hair cells.
The signal delivery between the inner hair cells and the neurons is chemical. Glutamate is the molecule that is used as a neurotransmitter. The electrochemical transmission of the acoustic signals in the cochlear nerve end in the cochlear nuclear complex. It is which is located ipsilaterally in the medulla of the brainstem.
Auditory Brainstem Evoked Response (ABR)
Clinical test by evoked potentials called ABR (Auditory Brainstem Evoked Response) can show the electric waves of the transmission via the axons and the ganglions in a living patient. Unfortunately the test cannot distinguish between a tinnitus sufferer and a healthy person.
Cochlear nucleus
The cochlear nucleus is the first 'relay station' of the auditory nervous system and receives mainly ipsilateral afferent input. The 3 sub structures of the cochlear nuclear organization are: (1) The dorsal cochlear nucleus. (2) The anteroventral cochlear nucleus. (3) The posteroventral cochlear nucleus. Every cochlear nuclei is tonotopically organised. It means that the frequency spectrum is preserved.
The relevance to tinnitus, balance dysfunction and meniere disease is the location of a tumor or other space occupying lesions.
Acoustic tumor
Acoustic tumor has 3 names that are synonyms: (1) Acoustic neuroma. (2) Acoustic neurinoma. (3) Acoustic schwannoma. It is located on the vestibulocochlear nerve and may cause tinnitus. In about 70% of the patients with acoustic tumor the complaints are: tinnitus, balance disturbance and hearing loss.
These manifestations can be also Meniere disease symptoms, and the clinician may be tempted to do a wrong diagnosis of a preliminary stage of Meniere disease.
Other cerebellopontine angle findings in imaging of a tinnitus patients may include cholesteatoma, or epidermoids and choroids plexus papillomas or intracranial aneurism.
