April 1, 2021

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This article was originally published as a chapter in the book “Design and Catastrophe: 51 Scientists Explore Evidence in Nature"

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Have you ever considered what life would be like in this world without sound? No music, no birdsongs, no sounds of laughter or the voices of our loved ones. No warnings of danger from a train whistle or the barking of a dog. Those who struggle with a hearing impairment have perhaps a deeper understanding of the importance and value of the gift of sound. But have you ever wondered what gives someone the ability to hear the familiar sounds of life? The design of the structures and mechanisms required for hearing is truly amazing.[1]

Sound is a vibration that is transmitted through air, liquids, and solids. These vibrations, or sound waves, occur at different amplitudes and frequencies. The amplitude of the sound wave relates to the volume of the sound. Sound waves for loud sounds have a higher amplitude, while soft sounds come from sound waves with a lower amplitude.

Sound wave frequency relates to the pitch. A rapid rate of vibration, or high frequency sound wave, results in a high-pitched sound. A low frequency sound wave has a slower rate of vibration and results in a low-pitched sound.

The external ear is designed to funnel sound waves to the tympanic membrane, or eardrum, which then vibrates at the same frequency as the sound wave. In the middle ear, three tiny bones, called ossicles, transmit these vibrations from the eardrum to the cochlea, which is in the inner ear.

The cochlea is shaped like a snail shell and is filled with a fluid called perilymph. This spiral-shaped tube contains a smaller tube called the cochlear duct, like the inner tube of a bicycle tire. The cochlear duct is formed by two membranes—the basilar membrane and the vestibular membrane—and also contains fluid, called endolymph.

When vibrations from the eardrum are transmitted to the cochlea, it causes waves in the perilymph and endolymph. These waves cause distortions of the basilar membrane of the cochlear duct. Different tones cause distortions in different areas of this membrane. High-pitched tones cause distortions of the basilar membrane near the base of the spiral-shaped cochlea, while low-pitched tones cause distortions near the apex.

The cochlear duct contains some intricate structures. Hair cells on the basilar membrane have tiny hair-like protrusions, called microvilli, at one end. These microvilli touch another membrane, the tectorial membrane, which is stationary. As the basilar membrane moves with the sound vibrations, the microvilli bend. This transmits a signal to a nearby nerve cell, which in turn transmits a signal to the auditory portion of the brain.

The mechanism through which these signals are transmitted is fascinating. When the microvilli of a hair cell bend in a certain direction, they pull on tiny springs that are attached to the gates of potassium channels. This causes the gated potassium channels to open so that potassium ions can pour into the cell from the surrounding fluid. This changes the electrical charge within the hair cell, causing voltage-gated calcium channels to open.

As calcium ions pour into the hair cell, the electrical charge within the cell changes even more. This causes an increased release of neurotransmitters from the hair cell. These neurotransmitters act on nearby nerve cells, which then send electrical signals called action potentials to the brain.

The change in the electrical charge of the hair cell also opens voltage-gated potassium channels, causing potassium ions to leave the cell so it will be ready to start the process all over again.

Now let’s consider what happens in the brain so that we can perceive sounds from the world around us. The action potentials that are generated in the cochlea are transmitted through the auditory nerve pathways to the part of the brain known as the auditory cortex. These nerve pathways are complex with many different connections.

Action potentials that are generated near the base of the cochlea stimulate nerve cells in a specific part of the auditory cortex. These nerve cells, or neurons, interpret the electrical signals as a high frequency, or high-pitched sound. Other action potentials generated near the apex of the cochlea stimulate different neurons, which interpret them as a low frequency or low-pitched sound.

Many of the sounds that we hear are very complex. We are able to recognize different voice qualities or the distinctive sounds of a violin, trumpet, or flute. We can simultaneously hear combinations of many sounds, like the exquisite music of a symphony orchestra. We can even distinguish the direction from which a sound is heard and focus on the sounds most important to us, like a mother tuning in to her child’s voice.

We can also perceive the volume of the sound. High-amplitude sound waves from loud sounds cause greater vibration of fluid in the cochlea and of the basilar membrane, which results in more intense stimulation of the hair cells. This causes more frequent action potentials to be transmitted to the auditory cortex, which is interpreted as a louder sound.

The design of the ear and cochlea is truly amazing, but without the auditory nerves and cortex we would still be unable to hear the sounds around us or understand what they mean. How the brain interprets the electrical signals from the cochlea as noise, beautiful music, or the familiar voices of our loved ones is a mystery we have barely begun to explore.[2]

If any part of this amazing system does not work properly, we are unable to hear. The eardrum and ossicles must be able to transmit sound waves to the cochlea. The fluid, membranes, and hair cells of the cochlea must function properly in order to open the potassium and calcium channels and generate action potentials.[3]The auditory nerve pathways must be intact and the auditory cortex of the brain must function normally in order for us to perceive and understand the sound waves that surround us.

The gift of hearing is the result of an integrated system of complex structures that work in coordination to transmit signals to the brain. These signals are then processed to give meaningful sensory input that is valuable for our lives. The intricate design required for this amazing gift speaks to us of a Designer and suggests that this Designer is an audiophile who understands the importance of sound and hearing for our happiness.

NOTES

[1] For a general introduction to human hearing, see C VanPutte, J Regan, A Russo, R Seeley. Seeley’s Anatomy & Physiology, 12th ed. New York: McGraw Hill; 2019, pp. 534–548.

[2] S Frühholz, W Trost, SA Kotz. The sound of emotions—towards a unifying neural network perspective of affective sound processing. Neuroscience & Biobehavioral Reviews 2016; 68:96–110.

[3] T Moser, F Predoehl, A Starr. Review of hair cell synapse defects in sensorineural hearing impairment. Otology & Neurotology 2013; 34(6):995-1004.

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Lucinda Hill Spencer is a professor of biology at Southern Adventist University. She earned her MD and MPH from Loma Linda University. She teaches an origins course for biology majors and contributed to the development of origins curriculum resources, some of which are available online at www.southern.edu/academics/ academic-sites/faithandscience/Origins-Curriculum-Resources.