Hearing – Survival & Society

Our normal activities require hearing, yet we take hearing for granted. This is largely because the ear does its job so well that we do not pay attention to it. Hearing is the only sensory system that allows us to know what is going on everywhere in our environment – we don’t have to be looking at the twig that is snapping to know there is something behind us in the dark. This ability imparts tremendous survival advantages for all animals. In addition, human social structures rely on speech communication which requires the sensitive, rapid processing of acoustic energy that the normal inner ear provides.

A Brief History of Hearing – From Early Anatomy to Active Hearing

Like other sensory organs that allow mankind to experience the environment, the ear has long held a special fascination for scientists. In mammals, the inner workings of the ear are encased in the hardest bone of the body. It contains the smallest bones, the smallest muscles, and the smallest, yet one of the most elegant organs of the body, the cochlea (part of the inner ear). Progress in understanding the structure and function of this tiny, relatively inaccessible organ has been slow and the milestones broadly spaced. By the 15th century, the presence of the ear drum and two of the three bones of the middle ear had been noted. Almost 300 years would pass before Domenico Cotugno would find that, in contrast to the air filled middle ear, the inner ear is fluid-filled. A century later Ernst Reissner described the presence of two distinct fluid compartments in the cochlea. Improvements in microscopic methods during the 19th century led to Alfonso Corti’s painstaking description of the cells comprising the sensory receptor organ of the inner ear.

Between 1877 and 1900 three machines were invented that greatly enhanced the role that hearing would play in our daily lives. Alexander Graham Bell gave us the telephone, Thomas Edison the phonograph, Nikola Tesla and Guglielmo Marconi, the radio. The manufacture and marketing of these inventions required many years but the public was fascinated with the capturing and transmitting of voices, music, or other sounds over great time and distances. This new sound technology had an instant and monumental impact upon our cultural imaginations that initiated the revolution in communication technologies so central to the history of the 20^th century. With the added importance of hearing to our daily lives came investigations during the late 19^th and first half of the 20^th century by scientists such as Hermann von Helmholtz and Georg von Békésy. Their work led to the concept of the ear as an elegant, but essentially passive device for converting the mechanical energy of sound into electrical signals to the brain. These studies culminated in von Békésy winning the 1961 Nobel Prize in Medicine and Physiology. Significant progress was made during the next two decades particularly with regard to cochlear fine structure and the cellular mechanisms for converting mechanical signals to changes in electrical potentials. But, the prevailing view of cochlear function continued to be that of a passive mechanical receptor for sound-evoked pressure changes in the cochlear fluids. However, as early as 1948 there were suggestions that an active mechanism might be necessary to explain the exquisite frequency resolving powers of the cochlea. Our understanding of how the ear works entered an exciting phase about 20 years ago when it was discovered that the inner ear actually makes sounds. The remainder of this chapter will provide a contemporary overview of how hearing works and describe what is known about the inner ear cells that make sounds and contribute to what is now considered active hearing. The first step in this overview requires a description of the mechanical energy we call sound.

Sound – Mechanical Vibrations – Pressure Waves

The sensory organs of the eye, ear, tongue and skin are each sensitive to specific forms of energy. The nose and tongue detect chemical energy, the eye detects light energy, the skin detects heat and mechanical energy. Sound is a form of mechanical energy. Mechanical forces can be steady, like the weight of this journal in your hand, or they can vibrate, like your car when it goes over speed bumps. Sound is generated by mechanical vibrations (such as a vibrating piano string). This sets up small oscillations of air molecules that in turn cause adjacent molecules to oscillate as the sound propagates away from its source. Sound is called a pressure wave because when the molecules of air come closer together the pressure increases (compression) as they mover further apart the pressure decreases (rarefaction). Since a pressure wave consists of a molecular disturbance, sound waves cannot travel through a vacuum. The velocity of sound in air is around 1,100 ft/sec which is why dividing the seconds between seeing lightning and hearing thunder by 5 gives a rough measure in miles of how far the lightning is from you. Sound waves travel fastest in solids, slower in liquids and slowest in air. Sound vibrations extend from a few cycles per second to millions of cycles per second. Human hearing is limited to a range of between 20 to 20,000 cycles per second. Sound at a vibration rate of greater than 20,000 cycles per second is called ultrasound. Other mammals can hear ultrasound, some such as whales approach 100,000 cycles per second. Physicians now use imaging techniques based on ultrasound (mechanical vibrations at millions of cycles per second) to examine the unborn child.

A sound is characterized by its frequency and intensity. The frequency of a sound contributes to its pitch and is measured by counting the number of cycles per second in the vibration. Intensity is a measure of loudness. If you have ever played a piano, you know where middle C is on the keyboard (see Figure 1). If the piano is properly tuned, middle C has a frequency of 256 cycles per second, high C (7 white keys to the right) has a frequency of 512 cycles per second. People with normal hearing can tell the difference between two sounds that differ by less 0.5%. In order to appreciate how small a difference in frequency this is you need only realize that middle C differs from C sharp (the black piano key immediately to its right of C) by more than 5%.

The intensity of a sound is a measure of its loudness and reflects how tightly packed the molecules of air become during the compression phase of a sound wave. The ear can detect sounds where the vibration of the air at the ear drum is less than the diameter of a hydrogen molecule. The ear has the ability to discriminate intensities over a 100,000 fold difference in energy. Still louder sounds can cause pain and damage the structures of the inner ear.

The task of all hearing organs is to analyze environmental sounds and transmit the results of that analysis to the brain. The brain interprets the ear’s analysis. All sensory organs have specialized sensory cells that convert an environmental signal into electrical energy. The change in electrical energy is then converted to a type a digital code that is transmitted to the brain. The human auditory system performs an analysis of sound entering the ear prior to the conversion to the neural code. The inner ear first determines how much energy is contained at the different frequencies that make up a specific sound. The cochlea is designed so that it is most sensitive to a specific frequency (say middle C) at one location and most sensitive to another frequency (say high C) at another. These different locations then transmit information to the brain. If the brain receives an increase in activity from the middle C location it then knows that the original sound contained energy at that frequency. This “mapping” of frequency information is just one of several strategies that the ear uses to code incoming information. The frequency analysis of environmental sounds begins in the external ear.

Getting Sound to the Inner Ear, the Analysis of Sound Begins

Millions of years ago marine animals had hearing organs that allowed them to detect sounds in the water. When land dwelling animals evolved they now had to detect environmental sounds that traveled through the air. This created a special challenge because their inner ears continued to be fluid filled. When sound passes from one media to another (as, in this example, from air to water) some energy is reflected by the surface and does not pass to the new media. In order to reduce these reflections and maximize the transfer of sound energy from the air filled environment to the fluid filled inner ear, land animals evolved external ears as sound collectors and middle ears as mechanical force amplifiers. It is fascinating that the tiny bones in the middle ear appear to have evolved from gills that were no longer needed.

The outer portion of the external ear reflects sound towards the ear canal (see Figure 2). Once in the ear canal, the pressure waves are aligned so they strike the ear drum at right angles. The reflection of sounds of different frequency is not the same and as a result the relative amplitude of some frequencies is greater than others. The result is that the relative amplitude of different frequencies at the ear drum differs, even if sound begins at the same intensity for all frequencies. Modification of the original sound by the external ear is a type of analysis that your brain learns to interpret. The frequency composition of familiar sounds aids your auditory system in determining where a sound is coming from. You can perform a simple experiment to appreciate the “frequency analysis” performed by the external ear. Cupping your hands over your ears and bending the top of your ears down changes the energy of different frequencies at your ear drum. Ask a friend to snap their fingers or clap their hands behind you before and after bending your ears. The sound will appear to come from different locations, particularly if the sound is coming from above or below your head, because your brain attempts to analyze the ear’s input to the brain based on the normal frequency pattern.

The middle ear bones conduct sound from the ear drum to the fluids of the inner ear. The ear drum is bigger than the oval window. The decrease in the area of these two membranes leads to an increase in pressure (pressure is equal to force divided by area and as the area gets smaller the pressure increases). The middle ear bones act as mechanical levers and further increase the pressure of the sound at the entrance to the cochlea. All of this is necessary to maximize the sound energy that gets to the fluids of the inner ear. There is a tube (called the eustachian tube) that connects the middle ear to the nose. It’s purpose is to allow the air pressure in middle ear to be equal to the air pressure in the environment. The pressure balance allows the ear drum to vibrate freely. Sometimes when you have a cold the tube is blocked and the middle ear pressure can not be balanced. You may have experienced the discomfort and even pain that can result if you are rapidly changing altitude (as when an airplane is landing). The freedom of movement of the middle ear bones can be reduced by certain diseases which leads to hearing loss. Any problem in the outer or middle ear that leads to a reduction of the sound energy entering the inner ear leads to what is called a conductive hearing loss. Many of these problems can be corrected either though medicine or surgery and contrast with the long term hearing problems that arise from damage to the structures in the inner ear.

The Inner Ear – A Closed Shop with a Division of Labor

The inner ear contains the sensory systems of balance and hearing. Its location is close to the center of the skull and it is encased in the hardest bones in the body which make it one of the best protected sensory systems. This protection reflects the importance of the hearing and balance for survival. The organs of balance are much older than those of hearing and evolved with the earliest multicellular organisms. All vertebrate balance organs are similar in structure and function. The organ of hearing evolved from one of the balance organs and this heritage is retained when it buds off from the balance organ early in fetal development. The basic structure of the human inner ear is present in the fetus at 6 months and hearing begins before birth. The auditory portion of the inner ear of mammals differs structurally from that of birds, reptiles and fish but its function in all animals is the same – to tell the brain how much energy is contained in an environmental sound and at what frequencies that energy is located.

The Inner Ear Battery – One Group of Cells Powers Another Group

The inner ear is divided into two fluid filled chambers – one inside the other. Figure 3 illustrates the basic organization of both the organs of hearing and balance. The fluid in the two chambers differs on the basis of the kind of salt that each contains. The fluid in the outer or bony chamber is filled with a sodium salt solution (called perilymph) that resembles the salt composition in the blood or the fluids found in the brain. The inner or membranous chamber is filled with a potassium salt solution (endolymph) that resembles the fluid that is normally found inside the cells of the body. Specialized cells that line parts of the membranous chamber and “pump” potassium into the membranous chamber maintain the difference in concentration between the two chambers. The difference in the chemical composition of these two fluids provides chemical energy (like a battery) that powers the activities of the sensory cells.

This division of labor is