In search of a periodic table of the neurons: Axonal‐dendritic circuitry as the organizing principle

Abstract

Summary No one knows yet how to organize, in a simple yet predictive form, the knowledge concerning the anatomical, biophysical, and molecular properties of neurons that are accumulating in thousands of publications every year. The situation is not dissimilar to the state of Chemistry prior to Mendeleev’s tabulation of the elements. We propose that the patterns of presence or absence of axons and dendrites within known anatomical parcels may serve as the key principle to define neuron types. Just as the positions of the elements in the Periodic Table indicate their potential to combine into molecules, axonal and dendritic distributions provide the blueprint for network connectivity. Furthermore, among the features commonly employed to describe neurons, morphology is considerably robust to experimental conditions. At the same time, this core classification scheme is suitable for aggregating biochemical, physiological, and synaptic information.

An update to Hippocampome.org by integrating single-cell phenotypes with circuit function in vivo

“…We also mined early articles for in vivo phase-locking information for CA1 neuron types [ 6 , 29 , 70 , 71 ] and more recent publications obtained in drug-free conditions [ 7 , 11 ]. We then proceeded to search for articles that cited these works and performed PubMed searches for each of the subregions of the hippocampal formation and the entorhinal cortex, looking for phrases such as “theta,” “gamma,” and “ripple.” Finally, we updated the search to incorporate any missing references reporting in vivo single-cell firing where: (a) the cell identity was morphologically validated [ 72 ]; (b) the cell was recorded simultaneously with respect to LFP signals; and (c) information about the cell intrinsic firing pattern and during ongoing LFP was reported. We data mined articles that quoted phase-locking values, contained tables of values, or depicted phase-locking behavior in diagrams or figures.…”

Weighing the Evidence in Peters’ Rule: Does Neuronal Morphology Predict Connectivity?

“…It may thus be possible to progressively refine this 40-year-old connectivity rule by introducing appropriate corrections for different orders of specificity [25,52]. To a first approximation, certain connections are based on the axonal–dendritic coincidence [53]. Describing the network in this fashion is useful to expose the backbone architecture underlying the main signal propagation paths and local processing unit blocks [40].…”

Reconstruction of 1,000 projection neurons reveals new cell types and organization of long-range connectivity in the mouse brain

“…However, these methods average across large groups of neurons, including multiple cell types, and obscure fine-scale spatial organization. Mapping brain-wide connectivity at the single neuron level is crucial for delineating cell types and understanding the routing of information flow across brain areas, but very few complete morphological reconstructions of individual neurons are available, especially for long-range projection neurons (Ascoli and Wheeler, 2016;Svoboda, 2011).…”