Nathaniel T. McMullen
Professor of Cell Biology & Anatomy, Neurology,
Speech and Hearing Science, and Neural Systems, Memory and Aging.
Ph. D., Ohio University
Neurobiology of Hearing
E-mail: natemcm@email.arizona.edu
RESEARCH INTEREST:
The Neurobiology of Hearing: Dendritic and Axonal Circuits Underlying Frequency Maps in the Auditory Central Nervous System
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 and Holmes MG. 2004. Auditory thalamic organization: Cellular slabs, dendritic arbors and tectothalamic axons underlying the frequency map. Neuroscience, Manuscript in preparation.
Velenovsky DS, Holmes, MG, Sinex DG and McMullen NT. 2004. Laminar organization of tectothalamic bands and synaptic nests in the rabbit auditory thalamus. Association for Research in Otolaryngology Annual Meeting, February 22, Daytona Beach, Florida.
Velenovsky DS, Cetas JS, Price RO, Sinex DG and McMullen NT. 2003. Functional subregions in primary auditory cortex defined by thalamocortical terminal arbors: an electrophysiological and anterograde labeling study. Journal of Neuroscience 23(1):308-316.
Cetas JS, Price RO, Velenovsky DS, Crowe JJ, Sinex DG and McMullen NT. 2003. Dendritic orientation and laminar architecture in the rabbit auditory thalamus. J Comp Neurol 458:307-317.
Cetas JS, Price RO, Velenovsky DS, Crowe JJ, Sinex DG and McMullen NT. 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.
Cetas JS, Price RO, Velenovsky DS, Sinex DG and McMullen NT. 2001. Frequency organization and cellular lamination in the medial geniculate body of the rabbit. Hear Res 155:113-123.
Danzer S, McMullen NT, and Rance NE 2001. Testosterone modulates neuronal morphology in the arcuate nucleus of adult rats. Brain Research 890:78-85.
Danzer S, Price RO, McMullen NT and Rance NE 1999. Sex steroid modulation of neurokinin B gene expression in the arcuate nucleus of adult male rats. Molecular Brain Research 66: 200-204.
Cetas JC, deVenecia RK and McMullen NT 1999. Thalamocortical axons of Lorente de No: medial geniculate axons that project to primary auditory cortex have collateral branches to layer I. Brain Research 830: 203-208.
Danzer SC, McMullen NT and Rance NE. 1998. Dendritic remodeling of arcuate neuroendocrine neurons following orchidectomy. J Comp Neurol 390:234-246.
de Venecia RK, Smelser CB and McMullen NT 1998. Parvalbumin is expressed in a reciprocal circuit linking auditory neocortex and medial geniculate body in the rabbit. J Comp Neurol 400:349-362.
