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Zusammenfassung:
The sequential activation of neurons has been observed during a range of behaviors and cognitive states and is central to many models of neural circuit function, but the synaptic connections enabling such dynamics are poorly understood. Song production in the zebra finch depends on a cortical region called HVC, which contains a major class of excitatory (HVC(RA)) neurons that fire action potential bursts in a fixed, sequential pattern during singing. These neurons not only project to downstream motor centers but also make numerous connections within HVC and could, therefore, form a sequence-generating synaptic chain. Evidence for this network architecture has, however, been either indirect or inconclusive. Here we employ light and transsynaptic electron microscopy to explore the synaptic connectivity of HVC(RA) neurons. We find that HVC(RA) cells receive the vast majority of their excitatory connections from distal sites on the axons of other HVC(RA) cells; proximal axonal sites, on the other hand, nearly always target inhibitory interneurons. Overall, this connectivity pattern provides evidence for a distributed excitatory synaptic chain supported by local inhibition, characteristic of coupled winner-take-all architectures.
