Fig. 7: Neuromorphic ion sensing.

a Measured (symbols) and modeled (lines) spiking current \({I}_{{{{{{\rm{spike}}}}}}}\) as a function of time. The OAN is exposed to various Na⁺ concentrations. When the ion concentration is below the minimum physiological level, no spiking is observed. When the ion concentration is above the excitation threshold, OAN spiking is obtained and its spiking frequency is modulated with \({c}_{{{{{{\rm{N}}}}}}{{{{{{\rm{a}}}}}}}^{+}}\). b Spiking frequency \({f}_{{{{{{\rm{spike}}}}}}}\) as a function of Na⁺ concentration. Symbols are the numerical simulations and the full line is calculated with the analytical model in Supplementary Eq. (26). c \({f}_{{{{{{\rm{spike}}}}}}}\) as a function of K⁺ concentration. d \({f}_{{{{{{\rm{spike}}}}}}}\) as a function of Ca²⁺ concentration. In b–d the vertical dashed line shows the minimum physiological concentration of each ion. OAN spiking is triggered by the specific ion concentration. e Spiking frequency as a function of ion concentration without and with noise. f Ion-selective spiking^: spiking frequency as a function of selected K⁺ concentration. The OAN is not spiking when the concentration of interfering ions (Na⁺) is varied. g OEND characteristics \({V}_{{{{{{\rm{OEND}}}}}}}\left({I}_{{{{{{\rm{OEND}}}}}}}\right)\) modulated by the ion concentration (solid lines). Load line (dashed dark line) crosses the OEND characteristics in various points inside the spiking region (orange area), depending on ion concentration. At large ion concentration, the crossing point \({{{{{\bf{U}}}}}}=({V}_{{{{{{\bf{U}}}}}}},\,{I}_{{{{{{\bf{U}}}}}}})\) is outside the spiking region and the OAN is silent. h Crossing voltage \({V}_{{{{{{\bf{U}}}}}}}\) (cross symbols) and current \({I}_{{{{{{\bf{U}}}}}}}\) (plus symbols) as a function of ionic concentration \(c\). \({V}_{{{{{{\bf{U}}}}}}}\) decreases with \(c\) while \({I}_{{{{{{\bf{U}}}}}}}\) increases. In the OAN excitability region (orange region) both \({V}_{{{{{{\bf{U}}}}}}}\) and \({I}_{{{{{{\bf{U}}}}}}}\) logarithmically increase with ion concentration. Outside the excitability region, the OAN is silent.