Helmut Schmidt,
Gerald Hahn,
Gustavo Deco,
Thomas Knösche
Poster presentation link: https://meet.google.com/xsy-yfrh-gsnThe poster will be presented by Helmut Schmidt.Axonal connections are widely regarded as faithful transmitters of neuronal signals with fixed delays. The reasoning behind this is that local field potentials caused by spikes travelling along axons are too small to have an effect on other axons. We demonstrate that, although the local field potentials generated by single spikes are of the order of microvolts, the collective local field potential generated by spike volleys can reach several millivolts. As a consequence, the resulting depolarisation of the axonal membranes (i.e. ephaptic coupling) increases the velocity of spikes, and therefore reduces axonal transmission delays between brain areas.
We first compute the local field potential (LFP) using the line approximation [1,2] for a spike in a single axon. We find that it generates an LFP with about 20 microvolts amplitude, which is too weak to have a significant effect on neighbouring axons (Figure A). Next, we extend this formalism to fibre bundles to compute the LFP generated by spike volleys, with different levels of synchrony. Such spike volleys can generate LFPs with amplitudes of several millivolts (Figure B), and the amplitude of the LFP depends strongly on the level of synchrony of the spike volley. Finally, we devise a spike propagation model in which the LFPs generated by spikes modulate their propagation velocity. This model reveals that with increasing number of spikes in a spike volley, the axonal transmission delays decrease (Figure C).
To the best of our knowledge, this study is the first that investigates the effect of LFPs on axonal signal transmission in macroscopic fibre bundles. The main result is that axonal transmission delays decrease if spike volleys are sufficiently large and synchronous. This is in contrast to studies investigating ephaptic coupling between spikes at the microscopic level (e.g. [3]), which have used a different model setup that resulted in increasing axonal transmission delays. Our results are a possible explanation for the decreasing stimulus latency with increasing stimulus intensity observed in many psychological experiments (e.g. [4]). We speculate that the modulation of axonal transmission delays contributes to the flexible synchronisation of high frequency oscillations (e.g. gamma oscillations). AcknowledgementsThis work has been support by the German Research Foundation (DFG), SPP2041. References1. Holt GR, Koch C. Electrical interactions via the extracellular potential near cell bodies. J Comp Neurosc 1999, 6:169-184.2. McColgan T, Liu J, Kuokkanen PT, Carr CE, Wagner H, Kempter R. Dipolar extracellular potentials generated by axonal projections. eLife 2017, 6:e25106. 3\. Binczak S, Eilbeck JC, Scott AC. Ephaptic coupling of myelinated nerve fibres. Physica D 2001, 148:159-1744. Ulrich R, Rinkenauer G, Miller J. Effects of stimulus duration and intensity on simple reaction time and response force. J Exp Psychol 1998, 24:915-928.
