Abstract
Magnetometry based on nitrogen-vacancy (NV) centers in diamond is a novel technique capable of measuring magnetic fields with high sensitivity and high spatial resolution. With the further advancements of these sensors, they may open up novel approaches for the 2D imaging of neural signals in vitro. In the present study, we investigate the feasibility of NV-based imaging by numerically simulating the magnetic signal from the auditory pathway of a rodent brainstem slice (ventral cochlear nucleus, VCN, to the medial trapezoid body, MNTB) as stimulated by both electric and optic stimulation. The resulting signal from these two stimulation methods are evaluated and compared. A realistic pathway model was created based on published data of the neural morphologies and channel dynamics of the globular bushy cells in the VCN and their axonal projections to the principal cells in the MNTB. The pathway dynamics in response to optic and electric stimulation and the emitted magnetic fields were estimated using the cable equation. For simulating the optic stimulation, the light distribution in brain tissue was numerically estimated and used to model the optogenetic neural excitation based on a four state channelrhodopsin-2 (ChR2) model. The corresponding heating was also estimated, using the bio-heat equation and was found to be low (<2°C) even at excessively strong optic signals. A peak magnetic field strength of ∼0.5 and ∼0.1 nT was calculated from the auditory brainstem pathway after electrical and optical stimulation, respectively. By increasing the stimulating light intensity four-fold (far exceeding commonly used intensities) the peak magnetic signal strength only increased to 0.2 nT. Thus, while optogenetic stimulation would be favorable to avoid artefacts in the recordings, electric stimulation achieves higher peak fields. The present simulation study predicts that high-resolution magnetic imaging of the action potentials traveling along the auditory brainstem pathway will only be possible for next generation NV sensors. However, the existing sensors already have sufficient sensitivity to support the magnetic sensing of cumulated neural signals sampled from larger parts of the pathway, which might be a promising intermediate step toward further maturing this novel technology.
Highlights
Charged nitrogen-vacancy (NV) color centers in diamond are a novel tool to sense weak magnetic fields with nano- to millimeter spatial resolution, reaching sensitivity levels of pT/Hz1/2 and below even at low frequency ranges and under ambient temperatures (Taylor et al, 2008; Fescenko et al, 2020; Zhang et al, 2021)
Our results serve as a detailed characterization of the electric and magnetic neural signals emitted from the auditory brainstem pathway of a mouse brain in response to optic or electric stimulation
Our results suggest that the action potential (AP) traveling along the axonal pathway from GB cells to medial nucleus of the trapezoid body (MNTB) principal cells will dominate the imaged neural magnetic fields, while magnetic signals from the calyces of Held or the MNTB principal cells will be far weaker
Summary
Charged nitrogen-vacancy (NV) color centers in diamond are a novel tool to sense weak magnetic fields with nano- to millimeter spatial resolution, reaching sensitivity levels of pT/Hz1/2 and below even at low frequency ranges and under ambient temperatures (Taylor et al, 2008; Fescenko et al, 2020; Zhang et al, 2021). We analyze the electromagnetic fields generated by synchronous neural activity of the auditory pathway from the ventral cochlear nucleus (VCN) to the medial nucleus of the trapezoid body (MNTB) in mouse brainstem slices (Figure 1) This pathway provides a fruitful test case as (i) it is constituted by a dense bundle of hundreds of large-diameter axons which arise from the globular bushy cells (GBCs) in the VCN (Ford et al, 2015), (ii) the pathway can accommodate high frequency (∼200–400 Hz) firing (Taschenberger and von Gersdorff, 2000; Lorteije et al, 2009), allowing more averaging trials to increase signal to noise ratio (SNR), and (iii) the pathway is well suited to image action potentials in isolation without strong contributions from other neural signals. By using PV as a promoter for Cre-dependent expression of light sensitive cation channel channelrhodopsin-2 in these neuronal populations, it should be possible to stimulate VCN and MNTB neurons with light
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