Professor

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Address: AHSC 4224
1501 N. Campbell Ave.
PO Box 245044
Tucson, AZ 85724-5044
Phone: (520) 626-2248
E-Mail: natemcm@email.arizona.edu

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Research Interests

Neuroscientists use sensory systems as windows for peering into the structure and function of the central nervous system. In my lab, our 'window' is the auditory system. Hearing, our primary sensory modality for speech and language acquisition, is one of the most important, but least understood, sensory system. We are currently carrying out studies of the auditory midbrain, thalamus and neocortex using electrophysiological and anatomical methods. Our goal is to understand the anatomical and physiological basis of frequency (and other) functional maps in the auditory CNS.

The Organization of the Auditory Thalamus.

We have recently proposed a model of auditory thalamic organization that incorporates cellular laminae and oriented dendritic growth as the basis of the frequency map at this level of the ascending auditory system (Cetas et al., 2001, 2002, 2003). Additional support for this model derives from our study of midbrain projections to the thalamus which form narrow axonal bands closely aligned with cellular layers and dendritic fields of neurons in this structure (McMullen et al., 2004; Velenovsky et al., 2004). These data provide evidence for an extraordinary structural-functional correlation in the auditory thalamus. Electrophysiological and anatomical methods are being applied to characterize these circuits in more detail. The strong relationship between relay neuron dendritic trees and functional (and cellular) thalamic laminae makes the auditory thalamus an excellent model for studying development, plasticity and aging in the auditory central nervous system.

Thalamocortical Patches in Auditory Neocortex

Several features of our acoustic world are "mapped" within primary auditory neocortex. Some, like frequency, are "continuous" maps; others, which include binaurality, threshold/intensity and sharpness of tuning, appear to be mapped in a discontinuous manner. What is the relationship between these cortical maps and the brain circuits that carry acoustic information into the neocortex? In 1993, a former graduate student, Dr. Ron deVenecia, and I described divergent clusters of axonal projections from the auditory thalamus to neocortex using anterograde tracing methods. Since that report, a similar patchy organization of thalamocortical axons has been described in a variety of species including rats, ferrets, cats and monkeys. My colleague, Dr. David Velenovsky, is attempting to determine the physiological counterpart of these patches. Results to date indicate that the patches represent cortical regions with consistent response characteristics related to frequency, binaurality and response pattern (Velenovsky et al., 2003). These results are evidence for the existence of multiple, parallel pathways linking the auditory thalamus and auditory neocortex. The specific circuits formed by these thalamocortical axons and their participation in the formation of various auditory maps are under investigation.

Selected Publications

McMullen NT, Velenovsky DS, Holmes MG. Jan 2005. Auditory thalamic organization: Cellular slabs, dendritic arbors and tectothalamic axons underlying the frequency map. Neuroscience, 136:927-43

Cetas JS, Price RO, Crowe J, Velenovsky DS, McMullen NT. Apr 2003. Dendritic orientation and laminar architecture in the rabbit auditory thalamus. J Comp Neurol, 458:307-17

Velenovsky DS, Cetas JS, Price RO, Sinex DG, McMullen NT. Jan 2003. Functional subregions in primary auditory cortex defined by thalamocortical terminal arbors: an electrophysiological and anterograde labeling study. J Neurosci, 23:308-16

Cetas JS, Price RO, Velenovsky DS, Crowe JJ, Sinex DG, McMullen NT. Mar 2002. Cell types and response properties of neurons in the ventral division of the medial geniculate body of the rabbit. J Comp Neurol, 445:78-96