M. Charles Liberman, Ph.D.
Professor of Otology and Laryngology, Harvard Medical School
Director, Eaton-Peabody Laboratories, Massachusetts Eye and Ear Infirmary
The inner ear is connected to the brain by 1) sensory neurons carrying information about incoming sound as well as 2) feedback pathways sending control signals from the brain back to the ear. These sensory (afferent) and feedback (efferent) pathways are each, in turn, composed of two fundamentally different kinds of neurons. Our research aims to understand why four different neuronal pathways are needed and what role each plays in hearing and deafness. We use a variety of approaches from systems neuroscience to cell and molecular biology.
For example, one line of research showed that sound-evoked activity in the efferent pathway to the outer hair cells protects the ear from acoustic injury and that a simple non-invasive test of efferent reflex strength can predict noise vulnerability. A subsequent study, using a transgenic mouse model, showed that overexpression of the receptor which mediates peripheral effects of this efferent pathway increases the resistance of the ear to noise damage.
Another line of research showed that loss of the efferent pathway to the sensory neurons in the inner hair cell area disrupts the normal binaural balance in excitability of the sensory fibers from the two ears, suggesting the role of this feedback circuit is to enable the precise inter-aural comparison of sound intensity that underlies accurate localization of high-frequency sounds in space.
Caption: The efferent innervation of cochlear hair cells and sensory neurons, as revealed by immunostaining for vesicular acetylcholine transporter (synaptic terminals: green) and neurofilament 200 (axons: red).
1. Maison, S.F., Liberman, M.C. 2000. Predicting vulnerability to acoustic injury with a non-invasive assay of olivocochlear reflex strength. Journal of Neuroscience 20, 4701-4707.
2. Maison, S.F., Luebke, A.E., Liberman, M.C., Zuo, J. 2002. Efferent protection from acoustic injury is mediated via alpha9 nicotinic acetylcholine receptors on outer hair cells. Journal of Neuroscience 22,10838-46.
3. Maison, S.F., Rosahl, T.W., Homanics, G.E., Liberman, M.C. 2006. Functional role of GABAergic innervation of the cochlea: phenotypic analysis of mice lacking GABA(A) receptor subunits alpha 1, alpha 2, alpha 5, alpha 6, beta 2, beta 3, or delta. Journal of Neuroscience 26, 10315-26.
4. Darrow, K.N., Maison, S.F., Liberman, M.C. 2006. Cochlear efferent feedback balances interaural sensitivity. Nature Neuroscience