  An important tool in the investigation of the early stages of
neurodegenerative disease is the study of dissected living tissue from the
brain of an animal model. Such investigations allow the physical structure of
individual neurons and neural circuits to be probed alongside neuronal
electrical activity, disruption of which can shed light on the mechanisms of
emergence of disease. Existing techniques for recording activity rely on
potentially damaging direct interaction with the sample, either mechanically as
point electrical probes or via intense focused laser light combined with highly
specific genetic modification and/or potentially toxic fluorescent dyes. In
this work, we instead perform passive, microscopic-scale recording of
electrical activity using a biocompatible quantum sensor based on colour
centres in diamond. We record biomagnetic field induced by ionic currents in
mouse corpus callosum axons without direct sample interaction, accurately
recovering signals corresponding to action potential propagation while
demonstrating in situ pharmacology during biomagnetic recording through
tetrodotoxin inhibition of voltage gated sodium channels. Our results open a
promising new avenue for the microscopic recording of neuronal signals,
offering the prospect of high resolution imaging of electrical circuits in the
living mammalian brain.
