Sound--How Important It is to Hear!

Two of my favorite quotations regarding the abstract nature of sound are as follows:

"The brain is composed of 100 billion electrically active cells called neurons, each connected to thousands of its neighbors. Each neuron relays information in the form of miniature voltage spikes, which are converted into chemical signals that bridge the gap to other neurons. Most neurons send these signals many times per second;  if each signaling event were to make a sound as loud as a pin dropping, the cacophony from a single human head would blow out all of the windows."^{1  }

The second is: "At any given instant, in any given cell, millions of molecular actions are taking place. Were they not noiseless, the din emanating from the center of a cell's ceaseless tempest of surveillance, commands, and determined activity would be painful to the ear of some imaginary creature infinitely small enough to listen to it. The sound made by one of the body's organs would be intolerable, and as for a whole man-well he could be heard from the next county."²

Both of these quotations, in addition to being extremely well written, relate to the incessant action occurring within us, about which we know little. Apparently, our sense of hearing is not designed to be omnificent. Hearing is an extraordinary sense despite its inability to detect the sound of the routine continual activities of the brain or other body organs. Audible sound, however, constantly surrounds us and informs us about many objects in our world. And our ability to determine the sources of sounds is one of our most important biological traits.³ Unfortunately, hearing loss is the most common sensory defect in humans, affecting normal communication in more than 10% of people aged 65 or older.⁴  Thus, the mechanism of hearing loss is a subject with which we aging individuals should be acquainted. This article is a follow up to my previous publication pertaining to the sense of smell,⁵ and it very briefly describes the anatomy of the ear; the complicated biomechanics of sound; and the prevalence and causes of hearing loss.

Ear anatomy

The auditory system is composed of three anatomical compartments: the outer, middle and inner ear. Sound waves impinging on the head are captured by the outer ear and conveyed through the external auditory canal to the tympanic membrane. Vibrations of the tympanic membrane, caused by airborne sound waves, are transmitted through the middle ear to the inner ear by a chain of movable bones. These bones, or ossicles, consist of the malleus, which is connected to the tympanic membrane; the stapes (the smallest bone in the body), which is attached at its base to the oval window of the vestibule; and the incus, which is situated between the malleus and stapes and articulates with both.⁶ (See Figure 1).  The tympanic membrane is held in place by fibers and cartilage situated in a bony groove between the outer and middle ear. The major function of the middle ear is to provide an effective and efficient means to deliver sound to the inner ear, where the neural process of hearing begins.⁷

The inner ear consists of the bony and the membranous labyrinths that are filled with a fluid called perilymph. It contains three major cavities: the vestibule, the cochlea and the semicircular canals. The cochlea processes auditory signals, whereas our sense of equilibrium depends on the vestibular apparatus composed on the three semicircular canals. ⁸ The inner ear is an evolutionary triumph of miniaturization, owing its success to its complement of hair cells that are responsible in the cochlea for our sensitivity to sound. Evolutionary success of hair cells is evidenced by the persistence of structurally similar inner ear anatomy in all vertebrates.⁹ The volume of the human nervous system devoted to the auditory and vestibular systems also testifies to the ear's importance. The 32,000 hair cells of the two cochleae and the total of 134,000 cells found bilaterally in the organs of equilibrium forward their signals to the large cochlear and vestibular nuclear complexes of the brainstem's medulla and pons. Destruction of hair cells results in loss of equilibrium and profound deafness.

Biomechanics of Sound

Sound may be defined in physical terms; it requires vibration of an object. Any object with the properties of inertia and elasticity may vibrate and produce sound. If we hear the vibration, the sound is audible. A vibrating object causes a wave motion in air, which then causes the eardrum to vibrate, starting the process of hearing. Sound can travel through any elastic medium that has inertia ( a force that is exerted on an object to make it move). Thus sound can travel through air, water, steel, etc, but not through a vacuum.^{10  }Once a sound's pressure wave has traveled from its vibrating source, it will eventually encounter a person's outer ear and the process of hearing begins. Sound analysis is quite complicated and includes frequencies, amplitudes, and phases of various sinusoids. These subjects are beyond the scope of this article.

Hearing Loss

There are a number of conditions that lead to hearing loss. The most common cause in childhood is otitis media with effusion, in which the middle ear and mastoid air spaces are filled with serous fluid. It has been estimated that approximately 4% of people under 45 years of age and 29 percent of those 65 or older have a handicapping loss of hearing. In most cases, hearing loss is a multifactorial disorder caused by both genetic and environmental factors.¹¹ There are three types of hearing loss:

• Conductive hearing loss-when hearing loss is due to problems with the ear canal, ear drum or middle ear and its malleus, incus and stapes.
• Sensorineural hearing loss-when hearing loss is due to problems of the inner ear, also known as nerve-related hearing loss.
• Mixed hearing loss -refers to a combination of conductive and sensorineural hearing loss. This means that there may be damage in the outer or middle ear and in the inner ear (cochlea) or auditory nerve.

Hearing loss can be classified as genetic or nongenetic, prelingual or postlingual, and syndromic or nonsyndromic. Syndromic hearing loss refers to particular combinations of hearing loss with one or a few specific other anomalies. Most cases of monogenetic hearing loss are nonsyndromic (only hearing is affected). Hearing loss can be caused by environmental factors including infection, acoustic or cerebral trauma affecting the cochlea, or ototoxic drugs such as aminoglycoside antibiotics. Morever, hearing loss may be genetic, resulting from the mutation of a single gene or from a combination of mutations in different genes and environmental factors. Presently, little is known about the many genes involved in hearing but progress is being made. ¹²

Final Thoughts

Audiologists (specialists who treat hearing problems) agree that we are experiencing a national epidemic of hearing impairment. At present, 50 million Americans suffer some degree of hearing loss-17% of the population. And hearing loss is not exclusively a product of growing old. The usual onset is between the ages of 19 and 44, and in many cases the cause is unknown.¹³ Fortunately, hearing loss can be treated, but not cured, through hearing aids or cochlear implants. There is hope; however, research in gene therapy, molecular therapy, stem cell technology, and the regeneration of hair cells is advancing at a breathtaking rate.

References

1. Eagleman D. "What our brains can teach us." The New York Times, 22 February 2013.
2. Nuland S. The Wisdom of the Body. Alfred A. Knopf, New York, 1997.
3. Yost WA. Fundamentals of Hearing, Fifth Edition. 2007; Academic Press, San Diego.
4. Willems PJ. "Genetic causes of hearing loss." NEJM 2000; 342:( 15):1101-05.
5. Sherman M. "The sense of smell-dogs or us." Regulatory Affairs Focus, December 2012.
6. Op cit 4.
7. Op cit 3.
8. Hudspeth AJ. "How the ear's works work." Nature 1989; 341:397-404
9. Ibid.
10. Op cit 3.
11. Nadol JB. "Hearing loss." NEJM 1993; 329:1092-1102.
12. Op cit 4.
13. Bouton K. Shouting Won't Help: Why I-and 50 Million Other Americans Can't Hear. Sarah Crichton Books, Farrar, Straus & Giroux, New York, 2013.