In the Pursuit of Connectomes: Combining in vivo Recordings with automated Reconstructions of complete 3D Neuron Morphology

Soma location, dendrite morphology and synaptic innervation are key determinants of neuronal function. Unfortunately, conventional functional measurements of sensory-evoked activity in vivo yield limited structural information. In particular, when trying to infer mechanistic principles that underlie perception and behavior, interpretations from functional recordings of individual or small groups of neurons often remain ambiguous without detailed knowledge of the underlying network structures. I will present a novel reverse engineering approach that allows investigating sensory-evoked signal flow through individual and ensembles of neurons within the context of their surrounding neural networks. To do so, spontaneous and sensory-evoked activity patterns are recorded from individual neurons in vivo. In addition, the complete 3D dendrite and axon projection patterns of such in vivo characterized neurons are reconstructed using a custom- designed semiautomatic tracing pipeline. Our semiautomatic tracing system rivals manual tracings from human experts in both accuracy and completeness. Specifically, the combination of these in vivo filling and reconstruction approaches recovered total axonal lengths far greater than previously observed. For example, intracortical axonal lengths of most pyramidal neurons in rodent sensory cortex exceed 10 centimeters per neuron. The axon of an individual thalamocortical neuron can even reach a length of up to 25 centimeters. Further, our latest developments allow recording and reconstructing of connected pairs in vivo. This approach has been largely used for preparations in vitro, for example resulting in various detailed computer models that relate the structure of an individual neuron to its measured function after somatic current injections. Unfortunately, the typical thickness of an in vitro brain slice is 300µm. Consequently, in vitro tracings usually suffer from cut off dendrites and axons, restricting connectivity measurements to close-by neurons. Our approach overcomes these limits and allows reconstructing the connectivity of neurons separated even by millimeters. Further, our latest developments allow recording and reconstructing of connected pairs in vivo. This approach has been largely used for preparations in vitro, for example resulting in various detailed computer models that relate the structure of an individual neuron to its measured function after somatic current injections. Unfortunately, the typical thickness of an in vitro brain slice is 300µm. Consequently, in vitro tracings usually suffer from cut off dendrites and axons, restricting connectivity measurements to close-by neurons. Our approach overcomes these limits and allows reconstructing the connectivity of neurons separated even by millimeters