Page 22 - 2017Spring

William E. Brownell
Postal:
Department of Otolaryngology- Head and Neck Surgery Baylor College of Medicine One Baylor Plaza Houston, Texas 77030 USA
Email:
brownell@bcm.edu
What Is Electromotility? -
The History of Its Discovery and Its Relevance to Acoustics
Experiments on an inner ear sensory cell revealed that it converts electri- cal energy directly into mechanical energy at acoustic frequencies.
Introduction
Thirty-five years ago, the members of the Physiological and Psychological Acous- tics Technical Committee of the Acoustical Society of America were at a pivotal juncture in understanding hearing. The original description of the ability of the ears to produce sound had been published in The Journal of the Acoustical Soci- ety of America a few years earlier. Remarkable differences in neural circuitry and structural specializations in the hearing organ (Figures 1-4) were being described. It was a propitious time to take a close look at the outer hair cell (OHC), which was then the most structurally and functionally mysterious cell in the ear. We dis- covered that OHCs undergo rapid changes of cell shape in response to electrical stimulation, something we now call OHC electromotility (http://acousticstoday. org/OHCEM1).1 OHC electromotility revolutionized the hearing sciences by re- vealing the active process responsible for the sounds coming from the ear. The history behind the discovery, the subsequent biophysical investigations, and the role of electromotility in hearing are the topics of this narrative.
Origins: Mammalian High-Frequency
Hearing and the Outer Hair Cell
The ability to detect and analyze high-frequency sounds that other animals could not hear is likely to have provided early mammals a strong survival advantage (Allman, 1999). The mammalian cochlea (Figures 1 and 2) houses a mechanosen- sitive sensory hair cell that is structurally and functionally different from the sen- sory hair cells found in other vertebrates. The defining characteristic of the newly evolved OHCs is their electromotility. The role of OHC electromotility is linked to the nature of sound that Pythagoras and his fellow Greeks, inspired by stringed musical instruments, reasoned was a vibration. They introduced the concept of the octave that eventually led to quantification and a physical understanding of tone. The study of acoustics progressed from these early beginnings, but exploration on the biological basis of hearing lagged. Two millennia passed before the structural organization of the mammalian inner ear began to emerge. Its structure provided clues as to how the ear works, particularly after it was realized that the inner ear is fluid filled.
Finding the Organ of Corti:
Hair Cells in a Fluid-Filled Cavity
The inner ear is difficult to study because it is small and encased in bone. Gabri- ello Fallopio described the openings between the middle ear and the cochlea in a
1 This is the first video recording of outer hair cell electromotility. An explanation of the video and links to other videos are provided in the supplementary material at the end of the article.
20 | Acoustics Today | Spring 2017 | volume 13, issue 1 ©2017 Acoustical Society of America. All rights reserved.