The basis of hearing and speech comprehension lies in the frequency map first seen at the level of the basilar membrane. This map is preserved throughout the ascending auditory system up to and including the auditory neocortex. Although the structural adaptations in the auditory brainstem that underlie frequency maps are known, very little is understood regarding the anatomical substrate for these maps in the auditory thalamus and AI. Recently, we confirmed the existence of two functionally distinct regions in the rabbit MGV; the pars lateralis (LV) and pars ovoidea (OV). In contrast to other species, these regions can be easily distinguished immunocytochemically and anatomically in the rabbit, making this species an excellent model for studying structural/functional relationships in the auditory nervous system. A laminar architecture in the rabbit MGV is visible with routine Nissl stains. Mapping studies have revealed that the laminae underlie the MGV frequency map. Focal groups of MGV neurons within a laminae project in a patchy or clustered manner to AI. Combined anatomical and electrophysiological studies indicate that these thalamic laminae appear to innervate separate functional regions coding frequency and binaurality in the auditory cortex. These data support the existence of multiple divergent channels linking the auditory thalamus and neocortex. We have developed a model that explains how patchy TC projections from the LV can account for discontinuous (e.g., binaurality), as well as continuous (e.g., frequency), maps in AI. This model will be tested using a combination of anterograde labeling and electrophysiological methods in anesthetized NZW rabbits.
In specific aim 1, a combination of anterograde labeling and electrophysiological mapping will be used to compare the neural circuits that project from the OV and LV to AI.
In specific aim 2, double anterograde labeling and mapping studies will be used to determine how different laminae project to AI and contribute to cortical maps.
In specific aim 3, we will apply techniques similar to those in specific aim 2 to compare thalamocortical projection patterns and functional topography from two physiologically distinct locations in a single LV lamina. These studies have the potential for revealing the structural basis of functional maps in AI and have important implications for understanding auditory cortical plasticity in normal, aging and impaired human ears. ? ?
| McMullen, N T; Velenovsky, D S; Holmes, M G (2005) Auditory thalamic organization: cellular slabs, dendritic arbors and tectothalamic axons underlying the frequency map. Neuroscience 136:927-43 |
