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. 2020 Nov 3;11:5555. doi: 10.1038/s41467-020-19304-9

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 10 — a (top) Alternating firing and nonfiring episodes in SAN pacemaker cells. During firing cells hyperpolarise and then depolarise during nonfiring. a (bottom) Expanded view of nonfiring, slowly firing, and fast firing modes from marked sequences. b Illustration of the HCN4 channel. Black ramp indicates cAMP concentration close to the cyclic nucleotide-binding domain (CNBD) which increases from left to right during sympathetic activity. Increased binding of cAMP to the CNBD of HCN4 channels successively switches the activity mode of pacemaker cells from nonfiring (left) to slow firing (middle). Increasing activity of the VN reduces cAMP levels and favours nonfiring. CDR of HCN4 antagonises the response to VN activity and acts synergistically with sympathetic nerve activity. c Tonic entrainment process in the SAN network. Pacemaker cells are connected via gap junctions. Nonfiring cells (gray) are more hyperpolarised and draw tonic currents from neighbouring firing cells, which slightly hyperpolarises neighbouring firing cells and depolarises nonfiring cells. Nonfiring cells either remain nonfiring or begin to fire slowly. We suggest that all nonfiring cells form a functional inhibitory cell pool that can be modulated by the ANS via CDR of HCN4. For further details see Discussion.