How We Hear

The auditory system is one of the human body’s most complex and delicate sensory systems. When functioning normally, the auditory system processes and transforms acoustical energy through the intricate structures of the outer, middle and inner ear and then on to the auditory center of the brain, where it can be identified as sound. The entire process occurs within a split-second timeframe.

The Human Auditory System (See Enlarged View)

The process of hearing begins with the outer ear, which collects sound (acoustic) energy and directs it through the ear canal to the eardrum. The incoming waves of sound energy cause the eardrum to vibrate, setting into motion the malleus, incus and stapes (also known as the hammer, anvil and stirrup) bones that make up the ossicular chain of the middle ear chamber, which is connected to the Eustachian tube*. The middle ear’s anatomical structure and conductive motion combine to amplify the sound by appx. 2.3 decibels (dB) and transform it effectively into a fluid (hydraulic) vibration inside our inner ear.

   
The complex hearing and balance functions of the inner ear are all contained within an area smaller than the diameter of a dime.
 

When the stirrup - the third bone in the middle ear - vibrates against the oval window membrane that leads to the inner ear, fluids within the spiral-shaped cochlea of the inner ear chamber are set into motion. This hydraulic energy causes the oscillation of thousands of tiny sensory hair cells that reside on the cochlea’s basilar membrane. The motion of the cells triggers chemical-electrical signals that are transmitted through the cochlea’s nerve fibers to the brain along the auditory nerve pathway. The brain can then translate the impulses of energy into recognizable sound patterns.

The Vestibular Labyrinth: a Balancing Act

In addition to housing our hearing organ, our inner ear chamber contains the vestibular labyrinth, a system that helps us to maintain our sense of balance and gravity. It is comprised of three semicircular canals and the otolith organ. The semicircular canals detect our angular motion and our body’s orientation in space, while our otolith organ senses linear motion, such as acceleration. Our gravitational sense is detected by tiny calcium crystals located inside the otolith’s gravity receptors. Together, the vestibular system helps us to keep objects in visual focus as we move and maintains our balance in motion. (See also Dizziness)

As with the cochlea, hydraulic movement in the vestibular labyrinth stimulates thousands of tiny sensory cells. This sensory cell stimulation results in the transmission of electrical impulses to our brain, providing us with information pertaining to the position of our head and its movement. When this system malfunctions, dizziness or unsteadiness can result.

*Footnote:   Eustachian tube

The middle ear chamber is connected to the back of the nose by the Eustachian tube, a small channel that serves to maintain equalization of pressure between the middle ear chamber and the outside atmosphere.

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