Response properties of units in the dorsal cochlear nucleus of unanesthetized decerebrate gerbil

Abstract

1. The electrophysiological responses of single units in the dorsal cochlear nucleus of unanesthetized decerebrate Mongolian gerbil (Meriones unguiculatus) were recorded. Units were classified according to the response map scheme of Evans and Nelson as modified by Young and Brownell, Young and Voigt, and Shofner and Young. Type II units have a V-shaped excitatory response map similar to typical auditory nerve tuning curves but little or no spontaneous activity (SpAc < 2.5 spikes/s) and little or no response to noise. Type I/III units also have a V-shaped excitatory map and SpAc < 2.5 spikes/s, but have an excitatory response to noise. Type III units have a V-shaped excitatory map with inhibitory sidebands, SpAc > 2.5 spikes/s, and an excitatory response to noise. Type IV-T units typically also have a V-shaped excitatory map with inhibitory sidebands, but have a highly nonmonotonic rate versus level response to best frequency (BF) tones like type IV units, SpAc > 2.5 spikes/s, and an excitatory response to noise. Type IV units have a predominantly inhibitory response map above an island of excitation of BF, SpAc > 2.5 spikes/s, and an excitatory response to noise. We present results for 133 units recorded with glass micropipette electrodes. The purpose of this study was to establish a normative response map data base in this species for ongoing structure/function and correlation studies. 2. The major types of units (type II, type I/III, type III, type IV-T, and type IV) found in decerebrate cat are found in decerebrate gerbil. However, the percentage of type II (7.5%) and type IV (11.3%) units encountered are smaller and the percentage of type III (62.4%) units is larger in decerebrate gerbil than in decerebrate cat. In comparison, Shofner and Young found 18.5% type II units, 30.6% type IV units, and 23.1% type III units using metal electrodes. 3. Two new unit subtypes are described in gerbil: type III-i and type IV-i units. Type III-i units are similar to type III units except that type III-i units are inhibited by low levels of noise and excited by high levels of noise whereas type III units have strictly excitatory responses to noise. Type IV-i units are similar to type IV units except that type IV-i units are excited by low levels of noise and become inhibited by high levels of noise whereas type IV units have strictly excitatory responses to noise. Type III-i units are approximately 30% of the type III population and type IV-i units are approximately 50% of the type IV population. 4. On the basis of the paucity of classic type II units and the reciprocal responses to broadband noise of type III-i and type IV-i units, we postulate that some gerbil type III-i units are the same cell type and have similar synaptic connections as cat type II units. 5. Type II and type I/III units are distinguished from one another on the basis of both their relative noise response, rho, and the normalized slope of the BF tone rate versus level functions beyond the first maximum. Previously, type II units were defined to be those ...

Responses of Dorsal Cochlear Nucleus Neurons to Signals in the Presence of Modulated Maskers

“…Principal cells in the DCN can also express a type III response pattern, and the inhibition observed in this study, in general, was not as extensive as what has been reported in the cat, with the majority of units expressing type III or type IV-t rather than type IV response patterns. The relative paucity of type IV units is a common finding in rodents and seems to be a genuine species difference (Davis et al, 1996;Stabler et al, 1996;Gdowski and Voigt, 1997;May, 2003). Type III units are better suited than type IV units to convey the signal information in a CMR-like paradigm, because they do not have a pronounced central inhibitory area (Shofner and Young, 1985;Spirou and Young, 1991;Nelken and Young, 1994;Young and Davis, 2002) and are therefore unlikely to get strong inhibitory input from type II cells.…”

A Modeling Study of Notch Noise Responses of Type III Units in the Gerbil Dorsal Cochlear Nucleus

“…This recovery was seen in 11 of 15 units in the paper of Parsons et al 15 Similar responses to NN have been found in type IV units in cat by Spirou and Young. 2 This demonstrated the sensitivity of type III units in gerbils to the NNs. Thus, type III units in gerbils are capable of detecting spectral notches as do type IV units in cats.…”

“…To illustrate the functional role of type III units in the gerbil DCN, the conceptual model from Davis et al 2 was chosen (see Fig. 2).…”

“…18 The gerbil DCN has fewer type IV units than the cat DCN (11% type IV units in gerbil vs. 31% in cat) 2,17 and is comprised primarily of type III units (62%). 2 As the principal units in gerbil DCN, type III units undoubtedly play an important role in gerbil sound localization.…”

“…18 The gerbil DCN has fewer type IV units than the cat DCN (11% type IV units in gerbil vs. 31% in cat) 2,17 and is comprised primarily of type III units (62%). 2 As the principal units in gerbil DCN, type III units undoubtedly play an important role in gerbil sound localization. In this study, we used the same computational model to fit the physiological data obtained from the gerbil DCN by Parsons et al 15 Despite the fact that type III units have quite different responses to tones than type IV units, this study shows that by modifying the connectivities within the neuron groups we can simulate both type III and type IV units as notch detectors.…”