Neuronal Architecture and Functional Organization of the Rabbit Auditory Thalamus

The ventral division of the medial geniculate body (MGV) is the primary tonotopicallyorganized nucleus of the auditory thalamus. Previous studies have suggested a close association between the anatomical structure of the MGV and its observed functional organization, but direct correlative studies are lacking. In the present study, regional differences in the cytoarchitecture of the rabbit MGV were described. These different regions were shown to have distinct frequency organizations. The central portion of the rabbit MGV is characterized by a laminated cytoarchitecture that is formed from the orderly arrangement of highlyoriented neuronal cell bodies. In this region, there exists a steep frequency gradient that extends across the dorso-ventral axis of the nucleus, orthogonal to the cellular laminae. This frequency gradient is marked by a discontinuous and stepwise-progression of best frequency. In regions lacking a laminated cytoarchitecture, a steep frequency gradient is absent. In addition, the morphology and basic response properties of individual cells were studied with the juxtacellular labeling technique. Two morphological types of projection neurons as well as two types of putative intemeurons were identified on the basis of dendritic thickness, cell soma size and spine morphology. Both types of tufted projection neurons had a variety of different response properties, but the Onset pattern and summation response to binaural stimulation predominate. Quantitative spatial analyses demonstrated that the dendritic fields of both types of tufled neurons are highly oriented. Further, for neurons within the laminated portion of the nucleus, the major axis of orientation of the dendritic fields are parallel to the cellular laminae and orthogonal to the frequency gradient. Departures