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How We Hear

Intricate, Complex and Exquisitely Engineered!

In his book Neuroscience, Dale Purves, Ph.D. writes: "The auditory system is one of the engineering masterpieces of the human body. At the heart of the system is an array of miniature acoustical detectors packed into a space no larger than a pea. These detectors can faithfully transduce vibrations as small as the diameter of an atom, and they can respond a thousand times faster than visual photoreceptors."  The auditory system is divided into two main parts: peripheral and central. 

The Peripheral Auditory System

The peripheral auditory system has three parts: the outer ear, the middle ear, and the inner ear. Each part plays a very specific role in converting sound waves into electrical impulses that  travel along the nerve pathways to the hearing centers of the brain.

Sound Transmission: The Process of Converting Energy

The outer ear directs sound waves (changes in air pressure) to the external auditory canal where they are funneled to the tympanic membrane (eardrum). The tympanic membrane is the structure that marks the beginning of the middle ear. When the air pressure hits the eardrum, it converts sound waves into mechanical energy, and causes the three tiny bones that are attached to the eardrums (also called ossicles: malleus, incus and stapes) to move back and forth. The stapes is attached to the oval window, which is the beginning structure of the inner ear.  When the stapes moves, it creates waves of pressure within the fluid-filled cochlea.

The Cochlea

The cochlea is coiled structure within the temporal bone of the skull. It resembles a snail shell. Deep within are rows of sensory hair cells that are stimulated by the movement of the inner ear fluid. These hair cells that follow the two and a half turns of the cochlea are tuned to be frequency-specific or tonotopic. The highest frequencies are located at the base of the cochlea, while the lowest frequencies are at the top or apex. The auditory nerve branches from the central core of the cochlea to the structures within.

Within the Cochlea

Inside the cochlea are three compartments or ducts: the scala tympani, the scala vestibuli and the scala media. The scala tympani and scala vestibuli contain fluid called perilymph which responds to  changes in pressure based on the movement of the stapes in the oval window.  The scala media contains another type of fluid called endolymph. It is compressed by the waves of fluid and stimulates the Organ of Corti located on the basilar membrane at the base of the scala media. 

The Organ of Corti

The Organ of Corti depends on both mechanical, chemical and electrical reactions to stimulate  two types of tiny hair cells (steroecilia). These hair cells are located under, and embedded within the tectorial membrane. The hair cells are responsible for sharpening  and amplifying  sound as well as sending the stimuli to the auditory nerve through electrical impulses where they are processed at a series of waystations  along the neural pathways to the auditory centre of the brain. 

Learn More about How We Hear: "Auditory Transduction"

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