How We Hear

Edited by Tim Hain, MD 10/2012

The Ear

The ear serves the important functions of allowing us to hear sounds produced by our environment, as well as maintaining our sense of balance. It is made up of three major parts: the outer, middle and inner ear.

The outer ear consists of the part of the ear that you can see, in addition to the narrow, tube-like opening called the ear canal. At the end of the ear canal is a tightly stretched tympanic membrane, the eardrum, which separates the outer ear from the middle ear.

The inner ear is made up of two components: one involved with hearing, the cochlea (“coke-lee-a”), and the other with balance, the vestibular system. The cochlea is a snail-shaped chamber filled with fluid and lined with sensory receptors in the form of tiny hair cells. It is attached to the auditory, or hearing, nerve that leads to the brain. The vestibular system is made up of a network of tubes (semicircular canals) and sacs (vestibule).

Cross section of an ear
Outer, middle and inner ear. Click on the image for a larger and more detailed diagram.

Like the cochlea, the vestibular system also contains sensory receptors and is attached to the vestibular, or balance, nerve that leads to the brain. The inner ear is also referred to as the labyrinth.

How We Hear

We hear sound when a series of sound waves, or vibrations, pass through our outer, middle and inner ear and reach our brain for interpretation.

Sound waves are first collected in our outer ear, pass through our ear canal and cause our eardrum to vibrate. These vibrations are in turn transmitted to our inner ear by the bones of our middle ear.

Our inner ear plays a vital role in the transformation of these mechanical vibrations into electrical impulses, or signals, which can be recognized and decoded by our brain. When the vibrations reach the cochlea through movement of the bones in the middle ear, the fluid within it begins to move, resulting in back and forth motion of tiny hairs (sensory receptors) lining the cochlea. This motion results in the hair cells sending a signal along the auditory nerve to the brain. Our brain receives these impulses in its hearing centers and interprets them as a type of sound.

For a more in-depth discussion, see The Search for the Mechanisms of Hearing, by Dr. Peter Dallos.

How We Maintain Our Balance

Our balance is maintained through the interaction of the visual, vestibular and sensory systems. These systems function individually and in combination to keep us stable with respect to our postural orientation. We may experience feelings of dizziness, lightheadedness or imbalance in the event of a disturbance of any one or more of these systems.

The visual system contributes to the maintenance of balance by making us aware of our surroundings and the position of our bodies in relation to those surroundings. The vestibular system detects rotational (circular) and linear (back and forth) motion produced when we engage in actions such as stopping, starting or turning. The sensory system keeps track of the movement and tension of our muscles and joints as well as the position of our body with respect to the ground. Upon receiving signals from these systems, the brain processes the information gathered to produce a sensation of stability.

The tubes and sacs within the vestibular system are filled with fluid. When we move our heads, this fluid is set into motion, causing the hair cells of the sensory receptors to bend. This change results in the generation of an electrical impulse which is carried along the vestibular nerve to the brain for interpretation.

Once the brain has interpreted the impulses as head movement, it responds by signaling our eyes to move in a manner that will allow us to maintain clear vision during the motion. Our brain will also send signals to our muscles so that their corresponding action serves to ensure balance regardless of the position our body (sitting, standing, moving).

The vestibular systems of our left and right inner ears must work equally to send uninterrupted impulses to our brain. Balance is disrupted if the signals are unequal, resulting in a type of dizziness called vertigo.