Brainstem and Inner Ear Interactions
The auditory nerve carries sound information from the inner ear to centers in the brainstem, which interact with the hair cells by sending information back to them. For instance, sound played into one ear triggers a mechanism which suppresses the oto-acoustic emission of the other ear. In some unusual pathological situations, these emissions are not suppressed and the person is unable to hear or understand sounds when there is a noisy background. Kresge Lab's Charles I. Berlin, PhD, and Linda J. Hood, PhD were among the first to quantify this suppression and its characteristics of time, frequency and intensity. Using a powerful computer program developed at Kresge by Engineer Han Wen, Berlin and Hood have been studying the phenomenon of suppression and persons in who it does not occur. While this investigation is still in its early stages, it has aroused world-wide interest. It promises to help us understand some feedback mechanisms of the nervous system auditory system in general and of the auditory system in particular.
Remarkable progress has been made over the past ten years in the identification of genes for hearing loss, in large part because of new technologies and resources that have become available as a result of the human genome project. The complete nucleotide sequences of the human genome, as well as model organisms, are now in public databases. The mouse, in particular, is an excellent model organism for genetic hearing loss because many deaf inbred strains are available for research. We now know that humans have about 35,000 genes encoding the proteins that are needed for the growth, development and functioning of the organs and tissues of our bodies. Abnormalities (mutations) in more than 100 genes are now known to be associated with hearing loss.
Bronya Keats, PhD, together with other Kresge scientists, worked on the genetic causes of both syndromic and non-syndromic forms of hearing loss. She was particularly interested in the gene that causes Usher syndrome type I in the Acadian population of south Louisiana. One component of her work was to create a mouse that contains the human mutation in this gene to facilitate the development of an effective therapy for this devastating disorder. Another of her projects focused on identifying the genetic causes of deafness in schools for the Deaf.
The process that turns sound waves into nerve impulses occurs in the hair cells of the inner ear, about which so much has been learned in recent years.
There are new developments almost daily from the Molecular Hearing Division. They are isolating individual, living hair cells from the rest of the ear and recording their responses to various stimuli. In these studies, they are asking three questions about hair cells:
What are the molecular mechanisms that allow them to change sound into nerve impulses?
How do drugs affect their functioning?
Do they play a role in tinnitus - the peculiar type of hearing loss in which one hears "ringing" or other noises in the ear?
The outer hair cells normally produce faint sounds, called oto-acoustic emissions, which have been the subject of much interest.