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 20th century. With the added importance of hearing to our daily lives came investigations during the late 19th and first half of the 20th 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.
Next chapter: Sound - Mechanical Vibrations - Pressure Waves
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