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Reason and Revelation Volume 24 #9

What's that You Said?

by  Matthew Vanhorn

Many people recognize “Can you hear me now?” as a catch phrase made popular by commercial broadcasts of the Verizon® Wireless phone company. Hearing a response via a cellular phone requires much more than good wireless phone service, however. It requires the most complex auditory receptors on Earth—the ear. Even though we have invented amazing technologies, we have not been able to match the receptive design of the ear.

The ear is divided into three parts: the outer ear, the middle ear, and the inner ear. The process of hearing begins with vibrations in the air. These vibrations are captured and enhanced by the outer (external) ear, which is comprised of two parts—the pinna and the external auditory canal. Part of the external ear, called the concha, intensifies the sound waves. The intensified sound next enters the external auditory canal, which is the area from the external ear to the ear drum. The ear drum is so sensitive that it can perceive vibrations on even a molecular level. A faint high note may cause the eardrum to vibrate back and forth by less than the diameter of a single hydrogen atom. Still, this vibration will be transformed into nerve impulses within the inner ear, and will be registered in the brain. Even among the lowest notes, the inner ear will detect a motion of the eardrum that amounts to less than the wavelength of visible light. An amazing aspect of the eardrum is that after recognizing the tiniest vibrations, it can return quickly to its regular state within five thousandths of a second. This recovery rate is extremely important; if the eardrum could not return to its regular state so quickly, every sound entering the ear would echo. The sound waves are amplified by the eardrum, and they then proceed to the middle ear region.

Ear Illustration

The middle ear has the smallest bones in the human body: the malleus, incus, and stapes (a.k.a., the hammer, stirrup, and anvil). The middle ear contains a sort of buffer that reduces exceedingly high levels of sound. This buffer is provided by two of the body’s smallest muscles, which control the malleus, incus, and stapes. These involuntary muscles contract, thus reducing the intensity of the vibration of loud noises before they reach the delicate inner ear. As a result, humans are able to hear shockingly loud sounds at a moderate volume.

The middle ear must maintain a vital equilibrium. The air pressure inside the middle ear must be the same as the pressure beyond the ear drum (the atmospheric pressure). Thus, the ear has been equipped with a three-and-a-half-centimeter-long canal. This canal, known as the Eustachian tube, is a hollow tube that extends from the inner ear to the oral cavity, and allows a controlled exchange of air between the middle ear and the outside environment. Another interesting feature of the auditory canal is the wax that it constantly secretes. The ear contains about 4,000 wax-producing glands. This wax, which contains antiseptic properties, keeps bacteria and insects out. The cells on the surface of the auditory canal are aligned in a spiral form directed toward the outside, ensuring that the wax always flows toward the outside of the ear.

All of these processes occur within the outer and middle ear and control only the mechanical portion of sound waves. These mechanical motions are turned into sound in the region known as the inner ear. The inner ear contains the most critical part of the hearing mechanism—the organ of Corti, located in the snail-shaped cochlea. The cochlea is an organ of the inner ear that is filled with a liquid called perilymph. The winding interior of the cochlea is studded with thousands of hair-like structures called stereocilia. When the middle ear receives signals from the eardrum, such as a ringing telephone, the perilymph fluid transmits the signal to the auditory nerve and the brain.

The vibrations in the liquid of the cochlea cause waves. The inner walls of the cochlea are lined with stereocilia, which move in perfect synchronization with the motion of the perilymph. When the stereocilia sense a vibration, they move and push each other in sequence, like dominos falling in a line. These tiny hairs vibrate at incredible speeds—up to 20,000 times per second! This motion opens channels in the membranes of the cells, allowing the flow of ions into the cells. When the stereocilia move in the opposite direction, these channels close again.

The perpetual motion of the stereocilia produces electrical signals, which are forwarded to the brain by the auditory nerve. This change from pressure to electrical waves is called transduction. The brain now interprets the signals, and assesses the pitch, volume, and meaning of the sounds. Whereas a grand piano has 240 strings and 88 keys, the inner ear has 24,000 “strings” and 20,000 “keys,” which enable us to hear an incredible variety and range of sounds.

The inner ear actually can be thought of as two organs: the cochlea, which assists in hearing, and the semicircular canals, which serve as balance organs. The semicircular canals detect acceleration in the three perpendicular planes. They utilize hair cells similar to the stereocilia of the organ of Corti. These hair cells detect movements of the fluid in the canals caused by angular acceleration about an axis perpendicular to the plane of the canal. Tiny floating particles assist in the process by stimulating the hair cells as they move within the fluid. These signals of motion then are transmitted to the brain via nerve impulses, and are processed there by the cerebellum.

The fact that all parts of the ear are necessary to produce hearing should be obvious when one considers the complex chain of mechanical and electrochemical processes involved. In order for the ear to function, each component must be in perfect order. If any one of these mechanisms is taken away, hearing fails. Additionally the organs of the inner ear provide balance, which allows humans to stand upright. Could such amazing complexity arise by chance? Hardly. “The hearing ear and the seeing eye, The Lord has made both of them” (Proverbs 20:12).

REFERENCES

Gillen, Alan L. (2001), Body by Design (Green Forest, Arkansas. Master Books).

Yahya, Harun (2003), Darwin Refuted (New Delhi, Goodward Books).



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