Seeing the anatomy of hearing: New tools and approaches chart the pathways underlying how we process and integrate sound


  • Jeffrey T. Laitman

    Associate Editor, The Anatomical Record, Corresponding author
    1. Center for Anatomy and Functional Morphology, Box 1007, Mount Sinai School of Medicine, New York, NY 10029
    • Center for Anatomy and Functional Morphology, Box 1007, Mount Sinai School of Medicine, New York, NY 10029
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“Dad, turn down the volume! Are you deaf?”

He is telling me to lower the volume? My teenage son, the same one whose idea of educational television is a perpetually tympanic membrane-damaging station called MTV (“Music Television,” for those of you not blessed with protohumans of his age), is admonishing me yet again about the volume being too high. “Yeah, Dad. What's your problem,” chimes in one of my other tax-deductions. “Are you deaf or what?”

This is not good. This is definitely not good. For years I have kicked and screamed to have my children tone down the cacophony of sounds that emanate from their CDs, DVDs, multipods, Gameboys, Gamegirls, or whatever. I have practically had to learn to lip-read in order to comprehend the evening news over the painful din that they have unendingly produced. And now, I am the offender. Something has obviously changed.

As life progresses, we change. No great insight here. I know that I cannot run with the gazelle-like speed of my youth or hit into a baseball with the combination of strength and nimbleness that once was mine. Pepperoni pizzas live more in my dreams than in reality. If there is going to be a lot of traffic, I'll hold off on the coffee. I deal with all these indignities; they are life's natural gifts. But some things are so central, so core, so primeval, that changes to them are like the shifting of the Earth itself.

Our senses are what link us to our world. They are like the strong cables on a mighty bridge; they are always there, true and firm, through sweet summers and harsh winters. Senses, like cables, are not supposed to change. When they do, the bridges to our world shift. Arguably, no sense is at once as core, as subtle, or as complex as that of our hearing. Indeed, few things are as jarringly disturbing as this sense's hushed degradation. And no sense is still as incompletely understood.

The quest to comprehend the anatomy and physiology underlying how we perceive and integrate sound is almost as old as anatomical and scientific investigation itself. Complicating the story, of course, is that hearing is not the provenance of any one simple structure or region. There is no single organ of hearing; the ear itself is a multichambered evolutionary accretion of external and internal bits and pieces of skin and bone, cells and fluids, which have anatomically morphed and migrated over eons. The “wires” connecting and regulating these parts—the various nerves—have similarly shifted and changed as we slithered from our reptilian past and swung through our primate ancestry. The connections of these relays to the regulatory and higher function centers of the brain have likewise moved and meandered. Our auditory world is, seemingly, a forever tangled web.

The hunt to understand the nature of hearing, and untangle this web, has been an incredible journey, replete with great breakthroughs and fascinating digressions. As to the latter, and illustrating some of the early handicaps faced in trying to interpret the nature of hearing, was that for centuries the ear was thought to function in respiration as well as hearing. Indeed, references to the ear and its connections to the nose via what is generally referred to as the “Eustachian” tube today [after Eustachius (1564), who, contrary to popular belief, did not “discover” the tube's existence, but rather described it in detail] can be traced back to the ancient Egyptians. They appear to have thought that the ear functioned in both hearing and respiration, possibly due to their observations on the connections of the tube. The ear's role in respiration was subsequently reinforced and supported in the reports of Alcmaeon in the 6th century B.C. and apparently was held for some time. So much for the wisdom of the ancients.

A pantheon of anatomy's greatest and brightest all held fascination with the ear and its ties to hearing: Vesalius's descriptions, arguably the first accurate ones, of the malleus and incus in the Fabrica (1543); Fallopius's description of the tympanic membrane (1561); Eustachius's detailing of the tube that bears his name (1564); Casserius's monumental comparative study on the ear throughout animals (1600–1601); Duverney's magnificent treatise on the development and diseases of the ear (1683); Valsalva's description of the ear into internal, middle, and external regions (1704); Scarpa's discovery of the membranous labyrinth (1772); Corti's precise descriptions of portions of the cochlea (1851); and Reissner's description of the vestibular membrane (1851) are but a sample of the great minds that have brought their eyes and hands together in an attempt to uncover bits and pieces of the tangled web.

This special issue of The Anatomical Record, entitled “Structure and Function in the Auditory System: From Cochlea to Cortex,” continues this great tradition of exploration by presenting a compilation of cutting edge science and scholarly reviews that depict the excitement and possibilities in auditory science today. The issue is guest-edited by David R. Friedland, an accomplished neuroanatomist and otolaryngologic surgeon, whose own science has spanned developmental and comparative explorations of brain stem nuclei to current work charting the global expression of critical genes in the cochlear nucleus. Indeed, his background and training in both bench science and clinical medicine have given him a special perch from which he has been able to observe both the scientific advancements in his field and their direct medical and surgical applications. He has brought together a truly extraordinary array of contributors that span the breadth of this most complex matrix that seeks to explore the underpinnings of the hearing pathways.

One of Friedland's expressed purposes in undertaking this issue (2006) was to bring to the fore the scope and power of different paths of hearing investigation and the value inherent in each. He has done this in part due to the enormous diversity in auditory research itself, and the fact that there is often a large divide, and lack of familiarity among hearing researchers themselves, as to the good science that sits just on the other side of the membrane, so to speak. The scope of the approaches is indeed impressive. For example, one major theme in the issue is the peripheral-to-central connections of auditory pathways and the ensuing cortical interconnections. Approaches to charting such pathways are demonstrated both by classic detailed comparative neuronal projection studies in rodents and by new work utilizing the power of functional imaging. Some of the functional neuroimaging analyses that focus on music as a tool to understand cortical processing are particularly interesting and will shed new light not only on cortical hearing centers but on the relationship between these and traditionally held language areas. (Indeed, lest one think that such forays into the relationship of music and auditory science are peripheral, it should be remembered that the great von Helmholtz—whose ideas on hearing published in 1863 and 1869 are to some extent the basis of most current theories on resonance—was not only a physician and physicist but an accomplished musician himself. His ideas on sound reception and perception were entwined from the beginning with those on the nature of music.)

The cochlea itself—its structure and evolution, relationships to contiguous temporal bone anatomy, hair cell inhabitants, and connections to the cochlear nuclei—receives considerable attention, again from varied perspectives and approaches. From detailed review of the forces that have shaped the odd marriage of balance and hearing structures in the bony milieu of the temporal bone to the molecular biology of nicotinic acetylcholine receptors and their relationship to hair cells in the organ of Corti, from cochlea hair cell development to the genetics underlying hearing loss, from detailed analyses of multipolar cells in the ventral cochlear nucleus to the differential expression of cytoskeletal genes there, the twists and turns of the cochlea and its connections are unfurled.

While I have overseen a number of special issues for The Anatomical Record, I must admit that the articles in this one held a special interest and attraction for me. The exciting new approaches and pathways portrayed by Friedland and colleagues surely hold promise for untangling pieces of the ever elusive web that is hearing. I wish them the best in their good work and hope that their findings and revelations come quickly. Very quickly.