Guest Post By: Seth Horowitz, Ph.D., Neuroscientist and author The Universal Sense
When we think about hearing (if we think about it at all), we tend to focus on its ephemerality. Sound comes from vibrating air molecules moving so gently that we can’t feel them (unless we’re standing dangerously close to a speaker), inducing motion in micron scale tufted cells waving in a fluid filled inner ear, needing to go through complicated processing to bringing out powerful cognitive, emotional or even physical responses from a listener. But what we think of as a soft interface between air and fluid will actually reflect away most sounds without something to bridge the divide. Something that, based on its stiffness and structure, can act as a natural or induced amplifier and overcome the normal difference in impedance that lets us hear air borne sounds in our fluid filled ears. And while James Wheldon Johnson’s old song is wrong and the ear bones (ossicles) are not connected to the headbone (skull), bones are critical to normal hearing.
Hearing airborne sounds requires a tremendous amount of amplification, and much of it depends on lever action by the ossicles, the three tiny bones that link the air outside the eardrum to the fluid in the cochlea via the oval window. The malleus (Latin for “hammer”) attaches to the eardrum which has an approximate surface area of 55-60 square millimeters. The innermost surface of the malleus articulates with the much smaller incus (anvil) which then passes the pressure onto the stapes (stirrup) whose faceplate contacts the oval window with a surface area of only 3 – 3.5 square millimeters. This allows the three bones to provide 22 times more pressure to the inner ear than received at the eardrum, while still responding fast enough to maintain the exquisite timing needed for proper pitch discrimination. But despite their rigidity compared to the other elements of the peripheral auditory system, these bones are delicate and subject to all the other woes that precise skeletal joints are heir to, ranging from dislocation to arthritis. While many clinical treatments have emerged to treat damage to the ossicles, they still remain critical and highly vulnerable elements in the hearing pathway and pathology or injury can have serious and sometimes permanent effects on detection of airborne sounds.
But we hear with more than just our ears, as you can tell if you go to a concert for the deaf or watch Evelyn Glennie perform. Due to her severe hearing loss, she often performs with her feet bare to pick up vibrations from the stage and her body placed precisely to pick up vibrations directly from the instruments. Like her, your entire body is sensitive to vibrations and your skeleton can act as a series of rigid low frequency transducers. In humans, this pathway is limited to detecting (not hearing) very loud low frequency vibrations (or, more often, a pathway to induce vibroacoustic disease as often experienced by heavy machinery operators). However, it is a remnant of the earliest way vertebrate animals detected sounds when they emerged onto the land hundreds of millions of years ago. Many non-avian and non-mammalian land animals still rely on transmission of lower frequency sound through skeletal pathways, called the “extratympanic pathway” that transmit vibrations through their limbs to their shoulder girdle and finally to their skull and ears. But this evolutionary “remnant” has provided us with an opportunity for overcoming some forms of damage to our tympanic pathway. By vibrating our skull, some hearing aids such as the Baha® bone anchored system or Advanced Brain Technologies’ wearable Bone Conduction System called WAVES™ use this lower frequency pathway transmit vibrations to the inner ear directly to overcome some of the drastic effects of damage to the tympanic system. So while it seems counter intuitive, our densest bodily structures are critically important for maintaining one of our most fluidic and delicate sensory systems, and highlight how no one system is ever truly isolated from the rest of our physiological makeup.