Auditory & Vestibular Systems

The outer, middle and inner ear.

The auditory and vestibular systems are key components of our sensory system. The auditory system provides us with hearing, and the vestibular system provides us with balance and equilibrium. The two systems are often categorized together because the organs involved with both systems are contained in the middle and inner ear.

Auditory Anatomy

The name cochlea comes from the Latin for "snail" because of its coiled shape.

While we often think of our ear as the cartilage that sticks out from the side of our head, this portion--the outer ear--mainly serves to focus sound waves from the outside world in toward the "business end." The true work of the auditory system occurs inside the head, where the sound waves work their way to the eardrum and three small bones called the hammer, anvil, and stirrup. The vibrations of these middle ear structures travel into the cochlea, in the inner ear, where the sound waves convert into nerve impulses. From here, the impulses move along the auditory nerve and eventually to the brain.

Vestibular Anatomy

The two main parts of the vestibular system are deep within the skull, in the inner ear. The semicircular canals are fluid-filled, so that as our head moves, nerve sensors within those canals detect fluid movement. The otolithic system behaves similarly, except that it exists more for vertical and horizontal movements, whereas the semicircular canals detect mostly rotational movements.

Nerve Receptors

Receptors called hair cells are located on a membrane in the cochlea. These serve as receptors for the auditory system. The hair cells are the point of conversion of the sound in wave form (motion of liquid in the inner ear) to electrical form.

Pathologies

Partial or complete deafness can occur from damage to any of the structures along the auditory pathway, whether the damage is to any of the sound wave conductive structures or to one of the nerves. Tinnitus, or "ringing in the ears," is the presence of phantom sound, whether caused by partial deafness or wax buildup. The major pathology associated with the vestibular system is vertigo, also associated with a wide variety of causes ranging from the flu to a tumor.

Types of Deafness

Deafness falls into two main categories, depending on which part of the conductive system is affected. Conductive deafness affects the structures of the ear that conduct the sound waves toward the nervous system--for instance, the conductive bones may wear down and no longer work. Sensorineural deafness affects the actual nerves that carry the sound information to the brain. Someone with conductive deafness can benefit from a hearing aid, as this will amplify sound waves to be picked up by the functional auditory nerve. However, someone with sensorineural deafness has better luck with a cochlear implant, which helps stimulate weak auditory nerve fibers.

References

• University of Maryland Medical Center: "Sensorineural Deafness"
• Baylor College of Medicine: "Hearing and Hair Cells"
• University of Maryland Medical Center: "Vestibular System and Vertigo"