An outlook on extracellular waveforms produced by the three neuronal compartments

The brain is composed of billions of neurons with virtually endless morphologies and ion channel compositions, resulting in unique extracellular waveforms. Nevertheless, almost all neuronal morphologies can be reduced to a simple architecture made of three principal compartments: 1) the soma and nearby axonal hillock, 2) axonal projections ending in arbors or single synaptic contacts, and 3) dendrites. This review offers a perspective on how these three ubiquitous neuronal compartments can be identified and how they shape the extracellularly recorded waveforms, when spatial considerations are taken into account. This outlook utilizes biophysical modelling to complement existing experimental observations. Modeling has predicted a rich landscape of putative extracellular waveforms based on morphology, channel density, and sequential temporal activation. Recent advances in extracellular in vivo recording, combining low noise with high spatial density of recording sites, have improved the precision of extracellular waveform measurements, particularly in capturing waveforms beyond the classical somatic spikes, and in some cases, combinations originating from different compartments. This review aims to reorganize extracellular waveform heterogeneity by separating signals stemming from three neuronal compartments using three dimensions: amplitude, duration, and spatial extent or footprint. In doing so, we argue for a change of perspective, looking beyond somatic spikes to include spatiality and waveform combinations.