Anatomy

The entire inner ear is approximately the size of a medium-large green pea. The two parts of the inner ear function in a similar manner to allow for the sense of hearing and the sense of balance. These small, delicate structures are embedded in the petrous pyramid of the temporal bone of the skull.

The balance portion of the inner ear consists of three semicircular canals: the utricle and the saccule. The semicircular canals, so positioned that they are in the three common planes of head movement, allow for the sensations of movements in these planes. Both the utricle and the saccule help the balance portion of the brain keep us upright and gravity-oriented so we don’t fall.

The hearing portion of the inner ear is known as the cochlea – a spiral-shaped tunnel in the ear bone that, if measured, would be about 2 cm long (less than an inch). This tunnel or labyrinth is filled with fluid containing various membranes, which, in turn, support thousands of hair cells. The hair cells are aligned in rows from one end of the “snail shell” cochlea to the other – some in multiple rows on the “outside” of the cochlea (outer hair cells) and some in single rows toward the middle of the cochlea (inner hair cells). These hair cells do not contain hair but have hair-like structures called cilia, which protrude from the tips of these cells.

Movement of the fluids of the inner ear causes the hair cells to move. The tiny cilia are bent, and the hair cell discharges an electrical potential into the hearing nerve.

The anatomy of the inner ear is more complicated than science understands today. The secrets of normal function require discovery, and these discoveries will lead to a better understanding of the problems that can cause hearing loss. Understanding the problems is essential if we are to undertake treatment.

Damage and Disease

Beginning at conception, the formation of the inner ear depends upon many health factors. If the unborn baby is subjected to diseases, there are possibilities that the inner ears may not form correctly. While many factors related to congenital hearing loss have been discovered and corrected, hearing loss at birth is still one of the most common birth abnormalities. Development of the inner ear and associated structures is largely accomplished during the first trimester of pregnancy (first three months). Viral diseases, which infect the mother-to-be during this critical time, may also result in mal-development of the inner ear.

While chemical agents (drugs and medications) can cause damage to the cochlea (and, thus, hearing loss), the most common cause for hearing loss in humans is noise exposure. The noises of an industrial society are many and loud. Constant exposure of the delicate structures of the inner ear to such noises can cause damage of varying degree. Industrial noise exposures are particularly hazardous since they typically occur over long periods of time (6-8 hours per day) on a continual basis (day after day). Damaging noise does not have to be unpleasant to be harmful. Amplified music can be quite pleasant, but very harmful to some ears.

It is commonly thought that simple aging is also a cause for hearing loss originating in the inner ear. There are known changes that occur with aging. These include loss of flexibility (not with limbs or joints, but within the underlying tissues), long-term toxic effects (perhaps due to such exposures as heavy metals or other toxic agents), and long-term effects of diet, circulation or chemicals. There are also underlying genetic effects that likely affect how long our ears (hearing) will last and how susceptible they are to other damaging influences.

Function

The broad function of the inner ear is to transmit vibratory energy (mechanical or hydraulic energy) from the middle ear into neurochemical energy. Neurochemical energy – nerve energy – is required by the brain.

In the past, science considered the cochlea a simple transducer of energy. Recently, research has taught us that the cochlea plays a much more important role than we once thought. The outer hair cells, for example, not only function passively to change hydraulic energy into nerve energy but also actively add to our ability to hear quiet sounds. These hair cells act as tiny amplifiers for the soft sounds we hear. Equally, when these same hair cells work together, they can even start to “interpret” the sounds so that the brain can make more use of the nerve energy.

There are even cells in the inner ear that, when they are damaged or destroyed, prevent us from hearing well in the presence of other noises. These inner hair cells appear to receive information from the outer hair cells and do some interpretation of the sounds before sending their signals off to the brain. Damage to these hair cells limits hearing in situations with background noise.

From a functional viewpoint, the balance portion of the inner ear – the vestibular labyrinth – helps us to control where we move in space and where we are in relationship to gravity. In a similar manner to the cochlea, the sensory receiver cells of the semicircular canals (the utricle and the saccule) allow us to walk without falling, maintain our balance, move our heads without getting dizzy, and otherwise maintain our position.

Treatments

It used to be said that there were no treatments for “nerve” hearing loss (hearing loss that originated in the inner ear). Such is no longer the case. Cochlear implants – the surgical implantation of an electrode into the inner ear – can offer significant improvements for people with severe hearing loss. There are medical treatments for sudden hearing loss that may reverse the loss and restore all or part of the hearing process. There are even implantable hearing aids that, while still new and being proven, promise to reverse or restore hearing almost to normal levels.
While there are many promising surgical and medical treatments for inner ear hearing loss, by far the most successful treatment to date involves the use of hearing aids. Because all hearing loss is not the same and, while one particular hearing instrument may work well for one type of hearing loss, it is important to understand the difference in hearing losses before attempting to correct said losses with any hearing aid.