Response classes in the dorsal cochlear nucleus and its output tract in the chloralose-anesthetized cat.

Neurons in the dorsal cochlear nucleus (DCN) can be classified into three major physiological classes on the basis of responses to pure tone and broadband noise stimuli. A circuit diagram that associates these classes with different cell types has been proposed. According to this proposal, type II cells are inhibitory interneurons that respond well to tones and poorly to broadband noise, type IV cells are projection neurons with the opposite behavior, and type III cells are an inhomogeneous class with intermediate properties. To test the associations proposed, I compared the response type distribution in the DCN with its output tract, the dorsal acoustic stria (DAS), in chloralose-anesthetized cats. Axonal recordings in the DAS showed type III and IV responses as in DCN, but no type II responses. Compared with reports in decerebrate animals, fewer type IV neurons were encountered having sustained inhibition that generated strongly nonmonotonic responses to tones in both DCN and DAS. The presence of type II responses in the nucleus, but not in the output tract, offers strong support for the proposed association with DCN interneurons. On the other hand, the distinction between type III and IV responses needs refinement because the differences are only graded and because both types of responses occur in DAS, which shows that they are both associated with projection neurons.