Description
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Neuroscience investigates the structure and functioning of neural systems. This endeavor is challenging since neural systems are composed of small, fragile components. When studying these components, only a limited level of access is available. This constrains the range of addressable questions. To investigate certain questions, it is therefore useful to choose specialized systems which allow greater access to some variables.
The present thesis addresses three questions, two related to the transmission of action potentials between neurons (Ch. 2 and 3) and one related to the representation of acoustic stimuli in neuronal responses (Ch. 4). Which systems are suited to address these questions? To study the transmission of action potentials, access to the sending and the receiving neurons is required. To study the representation of acoustic stimuli, access to an identified population of neurons is advantageous. If the questions are to be addressed in the central nervous system in vivo, the synapses of Held (Held, 1893) suggest themselves as a model system due to their size and good identifiability.
The synapses of Held are extraordinarily large which allows access to the presynaptic and the postsynaptic side simultaneously. Since the synapses of Held are large enough to elicit an action potential on the postsynaptic side, properties of action potential transmission can be studied. Further, they are especially suitable for in vivo studies since the pre- and postsynaptic activity can be recorded with a single, extracellular electrode. The resulting signal not only carries information about the underlying processes but also has an easily identifiable signature. In contrast to other systems, this signature allows to record from a single population of neurons.
The present focus is on the most eminent synapse of Held, the calyx of Held, which contacts the principal cells of the medial nucleus of the trapezoid body (MNTB). For comparison the endbulbs of Held were also studied which contact the spherical bushy cells of the anteroventral cochlear nucleus (AVCN). These nuclei are in prominent locations of the afferent neuronal pathways serving the localization of sound sources (Fig. 1.1). Since this task relies on comparably small differences between the information from both ears, properties of action potential transmission, e.g. reliability or timing, will influence the processing in later stages of the auditory system.
