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Callan, Alexander R.; Heß, Martin; Felmy, Felix; Leibold, Christian ORCID: 0000-0002-4859-8000 (18. November 2020): Arrangement of excitatory synaptic inputs on dendrites of the medial superior olive. In: Journal of Neuroscience, No. JN-RM-1055-20
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Neurons in the medial superior olive (MSO) detect 10 microsecond differences in the arrival times of a sound at the two ears. Such acuity requires exquisitely precise integration of binaural synaptic inputs. There is substantial understanding of how neuronal phase locking of afferent MSO structures, and MSO membrane biophysics subserve such high precision. However, we still lack insight into how the entirety of excitatory inputs is integrated along the MSO dendrite under sound stimulation. To understand how the dendrite integrates excitatory inputs as a whole, we combined anatomical quantifications of the afferent innervation in gerbils of both sexes with computational modeling of a single cell. We present anatomical data from confocal and transmission electron microscopy showing that single afferent fibers follow a single dendrite mostly up to the soma and contact it at multiple (median 4) synaptic sites, each containing multiple independent active zones (the overall density of active zones is estimated as 1.375 per μm2). Thus, any presynaptic action potential may elicit temporally highly coordinated synaptic vesicle release at tens of active zones, thereby achieving secure transmission. Computer simulations suggest that such an anatomical arrangement boosts the amplitude and sharpens the time course of excitatory postsynaptic potentials by reducing current sinks and more efficiently recruiting subthreshold potassium channels. Both effects improve binaural coincidence detection as compared to single large synapses at the soma. Our anatomical data further allows for estimation of a lower bound of 7 and an upper bound of 70 excitatory fibers per dendrite.