Sodium activation opening rate

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The rate α_m(V) controls the opening contribution α_m(1−m) in the m gate equation. Its apparent 0/0 at V=−40 mV has a finite limit of 1 per millisecond; the code explicitly supplies that limiting value near the singular point. This avoids a removable numerical singularity.

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Original context

    // State variables
    float V = -65.0f;        // mV - Membrane potential
    float m = 0.05f;         // Na+ activation
    float h = 0.6f;          // Na+ inactivation
    float n = 0.32f;         // K+ activation

    // Rate functions for gating variables
    // α_m(V) = 0.1(V+40)/(1-exp(-(V+40)/10))
    float alpha_m(float V_) const {
        float x = V_ + 40.0f;
        if (std::abs(x) < 1e-6f) return 1.0f;  // L'Hôpital limit
        return 0.1f * x / (1.0f - std::exp(-x / 10.0f));
    }

    // β_m(V) = 4*exp(-(V+65)/18)
    float beta_m(float V_) const {
        return 4.0f * std::exp(-(V_ + 65.0f) / 18.0f);
    }

α_m(V) = 0.1(V+40)/(1-exp(-(V+40)/10))

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