The structures of the ear, both external and internal, play important roles in the processing of sounds. Each different structure is involved in its own particular, critical step in the conversion of sound to hearing information that generates neural signals for the brain to interpret.
The External Ear
The process of hearing begins with the collection of sound. The outside, clearly visible portion of the ears, known as the pinnae or auricles, help to direct sound waves into the ear canal. While humans can not substantially move the pinnae in the direction of a sound of interest without moving the head, other animals (cats for example) can and do this as an aid to determining the direction of the sound.
The Structures of the Middle Ear
The Tympanic Membrane – As sound waves travel into the ear canal, their progress is stopped once they “run into” the first of the specialized membranes of the ear, the tympanic membrane (or ear drum). When the sound waves strike the tympanic membrane they cause it to vibrate. The frequency with which it vibrates depends upon the frequency of the sound. Frequency is measured using the units of cycles per second, otherwise known as Hertz (Hz), low pitch sounds are of a low frequency, while high pitch sounds are of high frequency.
The Ossicles – The middle ear contains the ossicles, the three smallest bones in the human body, the malleus, the incus, and the stapes. They are also known commonly as the hammer, the anvil and the stirrup. The malleus is essentially attached on one end to the tympanic membrane. Its other end is attached to the incus and the other end of the incus is attached to the stapes. The stapes is attached at its opposite end to another specialized membrane, the oval window, which is part of the cochlea, the essential organ of hearing.
The cavity in which the ossicles reside is also the same cavity to which the Eustachian tubes are connected. The Eustachian tubes allow for the equalization of pressure in the middle ear. Inflammation of these structures of the middle ear is known as otitis media.
The Inner Ear
The conversion of sound waves to neural signals is finally accomplished in the inner ear, in the structure known as the cochlea. When sound waves strike the tympanic membrane, it vibrates, which causes the ossicles to vibrate. When the end of the stapes vibrates, it causes vibration of the oval window. The vibration of the oval window causes pressure waves to occur inside the cochlea, which is filled with a specialized fluid. Thus, sound waves in air have now been converted to pressure waves in a fluid.
The cochlea is a spiral shaped tissue, somewhat resembling the shell of a snail, with a remarkable organization. It has what is referred to as a “tonotopic map”. What this means is that different regions of the cochlea respond to sounds of different frequencies. Highest pitched, high frequency sounds are sensed at the base of the cochlea nearest to the oval window. Low frequency sounds are sensed at the opposite end near the tip of the spiral of the cochlea.
Converting Waves to Neural Signals
Inside the cochlea are very specialized cells with microscopic hairs on one end that project into the fluid-filled space. These hair cells respond to the waves of pressure in the inner ear fluid by moving. The movement of these hairs causes what is known as “mechanotransduction” to take place. The movement of the hairs on the ends of these cells stimulates the opening of special channels in the cells that allow ions to flow across the cell membrane and the cell now sends an electrical signal to the neurons to which it is coupled.
These signals travel through the auditory nerve back to selected neural pathways in the brainstem. From there the signals are transmitted to the cells of the auditory cortex of the brain and the concept of hearing is generated. A remarkable feat of engineering, the hearing apparatus allows most humans, at the peak of function, to hear sounds between 20 Hz and 20,000 Hz.
To learn more about hearing and watch a fun animated video of how hearing works visit the US National Institute of Deafness and Other Communication Disorders (NIDCD).
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