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Li, Lu (2016): Intrinsic structure of the inferior colliculus. Dissertation, LMU München: Graduate School of Systemic Neurosciences (GSN)
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Abstract

The inferior colliculus (IC) is a major integrative center within the central auditory system, which contains three main subdivisions, i.e. the central nuclei (ICc), the external cortex (ICx) and the dorsal cortex (ICd). The central nucleus is characterized by the fibro-dendritic laminae constructed from the dendrites and axons of its two principal cell types: the disc-shaped cells and the stellate cells. Whereas in the ICc ascending inputs converge from all auditory nuclei upstream, the external and dorsal cortex converge the descending inputs from all auditory nuclei downstream. Additionally, the external cortex is receiving ascending inputs from multisensory systems. The IC plays an important functional role in the auditory system. A large amount of work has been done on the cell morphology and electrophysiology; however, less research work was done to study the intrinsic network, which is important to understand the computations performed by the inferior colliculus. Recently, the existence of excitatory and inhibitory intrinsic innervations has been proven in the ICc; however, it is still unclear whether the amount of intrinsic innervation and orientation depends on the location, the morphology of the cell or its intrinsic membrane parameters. Besides, the intrinsic inputs of other IC subdivisions are still unknown. This thesis was a phenomenological study of the intrinsic innervation of the inferior colliculus neurons. We used whole-cell patch clamp technique combined with laser scanning photostimulation of caged glutamate in order to determine the organization of intrinsic inputs of the ICc and the ICx neurons in the Mongolian gerbil. Our approach specifically aimed not only to study the intrinsic innervations and their regional differences, but also to find the correlations of intrinsic innervation with electrophysiological and morphological parameters. The excitatory and inhibitory intrinsic inputs of neurons from the ICc and ICx were measured. In addition, the electrophysiology and morphology of the neurons were recorded. We were the first to find that in case of excitation, ICc cells had two preferred orientations that were arranged both in the isofrequency axis and in the tonotopic axis, and yet the innervation of ICx cells only oriented along the outer boundary of the IC. In contrast, the inhibitory innervations of both ICc and ICx neurons dispersed in different directions. Moreover, we found that the intrinsic innervations were regionally heterogeneous in their excitation-inhibition balance. Whereas cells in the dorso-medial ICc cells had a balanced intrinsic excitation and inhibition, excitation was prevalent in the ventro-lateral ICc and in the ICx. Besides, we compared the amount of inputs in paired regions. Thus we found that in the lateral ICc, there existed a narrow region spanning along the outer boundary of the ICc where cells received significantly smaller amounts of excitatory inputs than in the adjacent region (ICx). In addition, we found that ICx cells received more excitatory inputs than the ICc cells. Moreover, we found that the amount of excitatory intrinsic innervations was correlated with the apparent surface of the cells, and the amount of inhibitory intrinsic innervations was correlated with the membrane conductance of the cells. Thus we found that the dorso-medial ICc cells varied most in their membrane conductance, whereas the ICx cells were most similar in their membrane conductance and the lateral ICc cells were most similar in their apparent cell surface.