Topology of the central nucleus of the mustache bat's inferior colliculus: correlation of single unit properties and neuronal architecture.

The central nucleus of the mustache bat's inferior colliculus was studied in Golgi, Nissl, and fiber stained preparations; the neuronal organization and cytoarchitecture were correlated with the tonotopic maps revealed by single cell recordings. Three divisions of the central nucleus were defined by anatomical and physiological criteria: the anterolateral, medial, and dorsoposterior divisions. In horizontal sections, the anterolateral division has pronounced, semicircular fibrodendritic laminae. The dendritic fields of adjacent neurons form rostro-caudally-oriented laminae related to the tonotopic sequence. The neurons in the medial division are similar in size and arrangement, but here the laminar orientation follows the lateral-to-medial axis, with less curvature. The dorsoposterior division has many small disc-shaped and stellate neurons and a different, somewhat less fully expressed, laminar orientation. Each division has a unique frequency representation and tonotopic organization which conform to the pattern of dendritic orientation. In the anterolateral division, frequencies from about 10 kHz to about 59 kHz are represented, whereas the frequency representation in the medial division ranges from about 65 kHz to 110 kHz, and higher. The dorsoposterior division has an isofrequency organization in which the best frequency is characteristic for each bat, ranging from 60 to 64 kHz and varying by only +/- 300 Hz. This frequency corresponds to the dominant echo frequency of the bat's echolocation signals. We suggest that the dorsoposterior division is a hypertrophied isofrequency lamina, with many neurocytological features comparable to the isofrequency laminae in the central nucleus of other mammals.