This review outlines the current state of mesoscale connectomic approaches, from data collection to analysis, written for the novice to this field. Unfortunately, the intersectional nature of this progress presents newcomers to the field with a daunting array of technologies that have emerged from disciplines they may not be familiar with. As a consequence, investigators now enjoy an unprecedented toolbox of reagents that can be directed against selected subpopulations of neurons to identify their efferent and afferent connectomes. Although neural tract tracing techniques have been available to experimental neuroscientists for many decades, considerable methodological advances have been made in the last 20 years due to synergies between the fields of molecular biology, virology, microscopy, computer science and genetics. At the mesoscopic level, distant and local connections between neuronal populations are identified, providing insights into circuit-level architecture. The field of connectomics seeks to map the structure that underlies brain function at resolutions that range from the ultrastructural, which examines the organization of individual synapses that impinge upon a neuron, to the macroscopic, which examines gross connectivity between large brain regions.

The mammalian nervous system is comprised of a seemingly infinitely complex network of specialized synaptic connections that coordinate the flow of information through it. 3CNRS, Hindbrain Integrative Neurobiology Laboratory, Neuroscience Paris-Saclay Institute (Neuro-PSI), UniversitГ© Paris-Saclay, Gif-sur-Yvette, France.2The School of Physiology, Pharmacology and Neuroscience, University of Bristol, Bristol, United Kingdom.1Neurobiology of Vital Systems Node, Faculty of Medicine and Health Sciences, Macquarie University, Sydney, NSW, Australia.Christine Saleeba 1,2вЂ , Bowen Dempsey 3вЂ , Sheng Le 1, Ann Goodchild 1 and Simon McMullan 1*

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