Auditory Efferent System New Insights From Cortex To Cochlea

The main function of the sensory systems is the transducing of external stimuli into bioelectrical signals, which are conducted through afferent pathways from sensory epithelia to the brain. However, it is known that descending projections are ubiquitous in the different sensory modalities, and in the case of auditory efferents connect the cerebral cortex with sensory receptor cells. Several functions have been attributed to the efferent system, including protection to acoustic trauma, unmasking of auditory stimuli in background noise, balance of interaural sensitivity and some cognitive functions like modulation of cochlear sensitivity during selective attention to auditory or visual stimuli. In addition there is evidence of a possible involvement of the efferent system in the etiology or treatment of some clinical pathologies like tinnitus. In this e-book, entitled “Auditory Efferent System: New Insights from Cortex to Cochlea”, we aimed to give an overview of the advances concerning the descending projections from the auditory cortex to subcortical nuclei and the olivocochlear system. In addition, different theoretical proposals of efferent functions are presented. We think that this e-book is an important contribution to the understanding of the efferent system in mammals, merging auditory-cortex literature with studies performed in the olivocochlear system.

The main function of the sensory systems is the transducing of external stimuli into bioelectrical signals, which are conducted through afferent pathways from sensory epithelia to the brain. However, it is known that descending projections are ubiquitous in the different sensory modalities, and in the case of auditory efferents connect the cerebral cortex with sensory receptor cells. Several functions have been attributed to the efferent system, including protection to acoustic trauma, unmasking of auditory stimuli in background noise, balance of interaural sensitivity and some cognitive functions like modulation of cochlear sensitivity during selective attention to auditory or visual stimuli. In addition there is evidence of a possible involvement of the efferent system in the etiology or treatment of some clinical pathologies like tinnitus. In this e-book, entitled "Auditory Efferent System: New Insights from Cortex to Cochlea", we aimed to give an overview of the advances concerning the descending projections from the auditory cortex to subcortical nuclei and the olivocochlear system. In addition, different theoretical proposals of efferent functions are presented. We think that this e-book is an important contribution to the understanding of the efferent system in mammals, merging auditory-cortex literature with studies performed in the olivocochlear system.

In contrast to the level of interest which is paid to the orga nization of meetings about the structure and function of the audi tory periphery, the central auditory system has received little attention in the last several years. However, much recent data accu mulated during this period has provided auditory physiologists with new ideas about the function of the central auditory system. The successful exploration of new anatomical tracing techniques (triti ated aminoacids, horseradish peroxidase, 2-deoxyglucose) together with the collection of electrophysiological data obtained with intra cellular and extracellular recordings from the receptors and neurones in the auditory pathway have considerably deepened our understanding of central auditory function. Particular interest was concentrated upon the development of the auditory system under normal conditions and in conditions ofaudi tory deprivation. Although, from the methodological point of view, the conditions of reversible auditory deprivation are complicated, promising new data appeared in this field. Similarly the specific ability of the auditory system to encode communication signals and speech sounds has been examined in many laboratories allover the world. A very fruitful method. based upon the results of electrical stimulation of cochlear nerve fibres in experimental animals, is the application of neuroprostheses in deaf patients. At the present time, the method still does not meet all requirements and many improvements will be necessary. Undoubtedly the exploration of the results of recent physiological experiments may help in the further improvement of neuroprostheses.

Since the last symposium on "Neuronal Mechanisms of Hearing" held in Prague in 1980 and published in the volume of the same name (J. Syka and L. Aitkin, Eds. , Plenum Press, 1981), remarkable progress has been achieved in the understanding of the auditory system. A variety of new ideas and new methods have emerged. This progress can be easily documented by comparing the volume based on the 1980 Symposium with the program for the 1987 Symposium. For example, there were 45 contributions to auditory physiology in each symposium but there were 27 contributions focusing on anatomy in 1987 as compared to 7 in 1980, and perhaps most telling, there were 12 contributions to the neurochemistry of the system in 1987 while there were only 3 in 1980. In terms of percentages of contributions, neuroanatomy rose from 13% to 32% and neurochemistry (or chemical anatomy) rose from 5% in 1980 to 14% in 1987. These increases in the numbers and proportions of anatomical and neurochemical contributions undoubtedly reflects the increasing availabil ity and rising expertise in the new neuroanatomica1 and biochemical techniques most notably, tract-tracing by exploitation of axonal transport or by intracellular micro-injection methods, and neurotransmitter identifi cation by use of immunocytochemistry or receptor-binding techniques. New ideas have emerged on the function of cochlear hair cells particularly in connection with olivococh1ear bundle stimulation and supported by findings of contractile proteins in outer hair cells.

Efferent sensory systems have emerged as major components of processing by the central nervous system. Whereas the afferent sensory systems bring environmental information into the brain, efferent systems function to monitor, sharpen, and attend selectively to certain stimuli while ignoring others. This ability of the brain to implement these functions enables the organism to make fine discriminations and to respond appropriately to environmental conditions so that survival is enhanced. Our focus will be on auditory and vestibular efferents, topics linked together by the inner ear connection. The biological utility of the efferent system is striking. How it functions is less well understood, and with each new discovery, more questions arise. The book that is proposed here reflects our vision to share what is known on the topic by authors who actually have made the observations.

The symposium that has provided the basis for this book, "Plasticity of the Central Auditory System and Processing of Complex Acoustic Signals" was held in Prague on July 7-10, 2003. This is the fourth in a series of seminal meetings summarizing the state of development of auditory system neuroscience that has been organized in that great world city. Books that have resulted from these meetings represent important benchmarks for auditory neuroscience over the past 25 years. A 1980 meeting, "Neuronal Mechanisms of Hearing" hosted the most distinguished hearing researchers focusing on underlying brain processes from this era. It resulted in a highly influential and widely subscribed and cited proceedings co-edited by professor Lindsay Aitkin. The subject of the 1987 meeting was the "Auditory Pathway - Structure and Function". It again resulted in another important update of hearing science research in a widely referenced book - edited by the late Bruce Masterton. While the original plan was to hold a meeting summarizing the state of auditory system neuroscience every 7 years, historical events connected with the disintegration of the Soviet Empire and return of freedom to Czechoslovakia resulted in an unavoidable delay of what was planned to be a 1994 meeting. It wasn't until 1996 that we were able to meet for the third time in Prague, at that time to review "Acoustical Signal Processing in the Central Auditory System".

This is a graduate-level text on the neurobiology of hearing, covering the structure and function of the central auditory pathway of all mammals.

There has been substantial progress in understanding the contributions of the auditory forebrain to hearing, sound localization, communication, emotive behavior, and cognition. The Auditory Cortex covers the latest knowledge about the auditory forebrain, including the auditory cortex as well as the medial geniculate body in the thalamus. This book will cover all important aspects of the auditory forebrain organization and function, integrating the auditory thalamus and cortex into a smooth, coherent whole. Volume One covers basic auditory neuroscience. It complements The Auditory Cortex, Volume 2: Integrative Neuroscience, which takes a more applied/clinical perspective.