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A new cellular mechanism for coupling inputs arriving at different cortical layers

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Abstract

Pyramidal neurons in layer 5 of the neocortex of the brain extend their axons and dendrites into all layers. They are also unusual in having both an axonal and a dendritic zone for the initiation of action potentials1,2,3,4,5,6. Distal dendritic inputs, which normally appear greatly attenuated at the axon, must cross a high threshold at the dendritic initiation zone to evoke calcium action potentials1,7 but can then generate bursts of axonal action potentials. Here we show that a single back-propagating sodium action potential generated in the axon8 facilitates the initiation of these calcium action potentials when it coincides with distal dendritic input within a time window of several milliseconds. Inhibitory dendritic input can selectively block the initiation of dendritic calcium action potentials, preventing bursts of axonal action potentials. Thus, excitatory and inhibitory postsynaptic potentials arising in the distal dendrites can exert significantly greater control over action potential initiation in the axon than would be expected from their electrotonically isolated locations. The coincidence of a single back-propagating action potential with a subthreshold distal excitatory postsynaptic potential to evoke a burst of axonal action potentials represents a new mechanism by which the main cortical output neurons can associate inputs arriving at different cortical layers.

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Figure 1: Coupling of a back-propagating action potential (AP) with distal subthreshold current injection.
Figure 2: Precision of timing required.
Figure 3: Extracellularly evoked BAC-firing.
Figure 4: Inhibition of BAC firing.

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Acknowledgements

We thank B. Katz, A. Korngreen, G. Borst and A. Silver for their helpful comments. M.E.L. was supported by an Alexander von Humboldt scholarship and J.J.Z. was supported by a Max-Planck Gesellschaft fellowship.

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  1. Abt. Zellphysiologie, Max-Planck-Institut für Medizinische Forschung, Jahnstrasse, D-69120 29, Heidelberg, Germany

    Matthew E. Larkum, J. Julius Zhu & Bert Sakmann

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  1. Matthew E. Larkum
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  2. J. Julius Zhu
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  3. Bert Sakmann
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Larkum, M., Zhu, J. & Sakmann, B. A new cellular mechanism for coupling inputs arriving at different cortical layers. Nature 398, 338–341 (1999). https://doi.org/10.1038/18686

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