Anatomy

The hearing nerve, which courses from the inner ear (cochlea) to the brain stem, is known as the Cochlear Nerve or Cranial Nerve VII (Cranial Nerve #7). This nerve is only 15-20 mm long (less than an inch) and must extend from the ear through a narrow tunnel (the internal auditory meatus) to reach the brain stem. This meatus or tunnel is critical, as it is the site for curious growths that affect hearing and balance. At the brain stem level, the CNVII joins many other neural pathways. The information in the hearing nerve is divided many times as it is passed through various neural centers on the way to the brain cortex.

Damage and Disease

Damage to the hearing nerve is rare. It can occur with skull fractures or serious concussions. Damage can also result from brain infections (encephalitis or meningitis), although there are typically more severe and global traumas associated with these types of infections.

A not-so-common finding involves acoustic tumors which many times can arise from the nerve coverings of the CNVII. These tumors are rarely malignant but can affect hearing to a varying degree. Both the nerve that supplies motor function to the face – CNV (CN#5) and the balance nerve – Vestibular Nerve (a part of CNVII) – pass through the internal auditory meatus. When the tumors get large enough, they can weaken muscle function of half of the face. These growths can also cause problems with dizziness and balance.

Function

The obvious function of the hearing nerve is to transmit hearing information to the brain. However, there are likely other important duties that are accomplished during the neural-chemical passing of information up the pathway.
In nature, there is a saying that “form follows function.” The multiple branching of the hearing nerve into many other nerves would seem to indicate that there is a reason for all these nerves carrying essentially the same information. It is likely that the pathways themselves organize the hearing information into some sort of order so that the brain can process this information more effectively. Or, perhaps, the hearing information in the nerves acts to trigger reflexes of control for “noise” or specific word recognition – or some other yet-to-be-discovered action that makes human hearing so spectacularly effective.

Treatments

Disorders of the hearing, balance and facial nerves are rare. Treatments vary depending upon the cause but for the most part involve surgical removal of the lesion or repair of the injury. Accurate diagnosis is critical for selecting the appropriate treatment.