Semester

Spring

Date of Graduation

2003

Document Type

Dissertation

Degree Type

PhD

College

School of Medicine

Department

Physiology, Pharmacology & Neuroscience

Committee Chair

George A. Spirou.

Abstract

One of the key elements in auditory perception is the localization of sounds in space. The major cues used for localizing sounds in the azimuthal plane have long been recognized as interaural differences in time of arrival of a sound and amplitude differences between the two ears (Rayleigh 1907; Thompson 1878). High frequency sounds are reflected by the head and thereby produce interaural level differences (ILDs) that are used for localization. The head does not reflect low frequency sounds and so interaural timing differences (ITDs) are used. One of the cell groups of the auditory brainstem, the medial superior olive (MSO), functions in sound localization by comparing ITDs between the two ears. The MSO is defined as a binaural group of cells because it integrates input from the cochlear nucleus (CN) from each ear. Afferent nerve fibers from the ipsilateral CN are restricted to dendrites oriented laterally and inputs from the contralateral CN are segregated to medially oriented dendrites (Stotler 1953). At low to moderate sound levels, activation from each cochlear nucleus is below action potential threshold and MSO neurons only generate action potentials when inputs from both sides arrive within a short temporal window called the coincidence detection window.;Several cellular specializations exist along the auditory pathway that aid MSO cells in their ability to detect changes in ITD. These specializations include large nerve terminals and distinct organelle complexes located within terminals, which facilitate fast, well-timed inhibitory inputs to MSO cells. Very little is known about the role of inhibition in sound localization and proper understanding of its role depends on knowledge of the cells that impinge on the MSO and the pharmacology and kinetics of synaptic transmission in MSO cells. Also, the membranes of MSO cells contain specific voltage-gated potassium channels (Kv), these channels are known to affect membrane electrical properties, but how these channels influence ITD sensitivity is unknown. The main goal of my research was to understand these cellular specializations that contribute to neural processing of ITDs.

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