Functional Pathways in the Auditory System
Fig. 1: The main ascending pathways in the mammalian auditory system. AN, auditory nerve; CN, cochlear nucleus; SO, superior olive; NLL, nuclei of the lateral lemniscus; IC, inferior colliculus; MGB, thalamus; and A1, cortex. One goal of the lab is to identify functional pathways (patterns of afferent convergence) that link the brainstem auditory nuclei.
The auditory system is unique among the sensory modalities in that it has more than one nucleus between its peripheral sensors (located in the inner ear) and the forebrain. As illustrated in Fig. 1, incoming information about the acoustic environment enters the brain via the auditory nerve (AN), whose fibers terminate in the cochlear nucleus (CN). From there, at least five distinct neural pathways are initiated; these pathways ascend through a multitude of other brainstem nuclei including the superior olive (SO) and nuclei of the lateral lemniscus (NLL) before converging upon the inferior colliculi (IC). The colliculi then provide most, if not all, of the input to the auditory thalamus (MGB) and cortex (A1).
Current research in the laboratory centers on the central nucleus of the inferior colliculus (ICC) because it occupies a pivotal position in the auditory system; it receives direct inputs from most, if not all, of the auditory nuclei in the brainstem and, in turn, provides nearly all of the input to the auditory forebrain. Anatomical evidence suggests that the projections to the ICC form highly organized synaptic domains with both segregated and shared sources of input. In support of this parallel processing model, our recent electrophysiological studies have discovered that ICC units can grouped into three major types based on the patterns of excitation and inhibition evoked by contralateral tones of differing frequency and level. Examples of these ICC response types, labeled type V, I, and O, based on the shape of their excitatory response areas (red fill), are shown in Fig. 2. Several lines of experimental evidence lead to the conjecture that these response types reflect a dominant excitatory input from the medial superior olive, the lateral superior olive and the dorsal cochlear nucleus, respectively. We are now performing experiments designed to provide direct evidence for these functional connections.
Fig. 2: Single units in the central nucleus of the ICC can be grouped into three major types (called types V, I, and O) based on the patterns of excitation (red fill) and inhibition (blue fill) in response to contralateral pure tones. We seek to test the hypothesis that these response types reflect a dominant excitatory input from different lower order nuclei.