IonicModelFitzHughNagumo.h

// SPDX-FileCopyrightText: Copyright (c) Stanford University, The Regents of the
// University of California, and others. SPDX-License-Identifier: BSD-3-Clause
 
#ifndef IONIC_FITZHUGH_NAGUMO_H
#define IONIC_FITZHUGH_NAGUMO_H
 
#include "IonicModel.h"
 
#include "Vector.h"
#include "utils.h"
 
/**
 * @brief FitzHugh-Nagumo ionic model.
 *
 * The reaction terms and default parameters follow equations 14--15 and
 * Figure 3 of [3]. This parameterization is self-oscillatory, with an unstable
 * equilibrium at zero.
 *
 * **References**:
 * 1. [FitzHugh (1961)](https://doi.org/10.1016/S0006-3495(61)86902-6)
 * 2. [Nagumo, Arimoto, Yoshizawa (1962)](https://doi.org/10.1109/JRPROC.1962.288235)
 * 3. [Goktepe, Kuhl (2009)](https://doi.org/10.1002/nme.2571)
 */
class FitzHughNagumo : public IonicModel {
public:
  /// Model label.
  static inline const std::string label = "FN";
 
  /// State variables.
  static inline const InitialStates initial_X = {{"V", 1.0e-3}, {"w", 1.0e-3}};
 
  /// Gating variables.
  static inline const InitialStates initial_Xg = {};
 
  /// Index of the recovery variable (w), used as calcium proxy for
  /// electromechanical coupling.
  static constexpr unsigned int calcium_index = 1;
 
  /// Model parameters class.
  class Parameters : public IonicModelParameters {
  public:
    Parameters() : IonicModelParameters(label, initial_X, initial_Xg) {
      constexpr bool required = true;
 
      add_parameter("alpha", -0.50, required);
      add_parameter("a", 0.0, required);
      add_parameter("b", -0.60, required);
      add_parameter("c", 50.0, required);
    }
  };
 
  /// Constructor.
  FitzHughNagumo()
      : IonicModel(initial_X, initial_Xg,
                   /* Vrest_ = */ 0.0, /* Vscale_ = */ 1.0,
                   /* Tscale_ = */ 1.0, /* Voffset_ = */ 0.0) {}
 
  /// Construct an instance of model parameters.
  virtual std::unique_ptr<IonicModelParameters>
  get_parameters() const override {
    return std::make_unique<Parameters>();
  }
 
  /// Read model parameters from a parameter object.
  virtual void read_parameters(const IonicModelParameters &params) override;
 
  /// Distribute model parameters to all parallel processes.
  virtual void distribute_parameters(const CmMod &cm_mod,
                                     const cmType &cm) override;
 
  /// Get the calcium proxy index.
  virtual unsigned int get_calcium_index() const override {
    return calcium_index;
  }
 
protected:
  /// @name Model parameters
  /// @{
 
  double alpha = -0.50; ///< [1].
  double a = 0.0;       ///< [1].
  double b = -0.60;     ///< [1].
  double c = 50.0;      ///< [1].
 
  /// @}
 
  /// Update variable with analytical solution. This model has none, so this
  /// method does nothing.
  virtual void update_g(const unsigned int zone_id, const double dt,
                        const Vector<double> &X,
                        Vector<double> &Xg) const override {}
 
  /// Model right-hand side.
  virtual Vector<double> getf(const unsigned int zone_id,
                              const Vector<double> &X, const Vector<double> &Xg,
                              const double I_stim,
                              const double I_sac) const override;
 
  /// Model jacobian.
  virtual Array<double> getj(const unsigned int zone_id,
                             const Vector<double> &X, const Vector<double> &Xg,
                             const double Ksac) const override;
};
 
#endif