FitzHugh–Nagumo ODEs

<MorpheusModel version="4">
    <Description>
        <Title>FitzHugh–Nagumo ODEs</Title>
        <Details>Full title:		FitzHugh–Nagumo ODEs
Authors:		L. Edelstein-Keshet
Contributors:	Y. Xiao
Date:		10.05.2022
Software:		Morpheus (open-source). Download from https://morpheus.gitlab.io
Model ID:		https://identifiers.org/morpheus/M2005
Reference:		L. Edelstein-Keshet: Mathematical Models in Cell Biology
Comment:		ODEs for the FN well-mixed system. The FN equations are commonly used as a toy model for neuronal excitation. u = "voltage", v = "recovery variables". Everything is well-mixed and two ODEs are solved for two variables, u and v.</Details>
    </Description>
    <Global>
        <Variable symbol="u" name="Voltage" value="1"/>
        <Variable symbol="v" name="Recovery Variable" value="1"/>
        <System time-step="0.1" solver="Runge-Kutta [fixed, O(4)]">
            <Constant symbol="a" name="middle value" value="0.25"/>
            <Constant symbol="b" name="v decay rate" value="1.0"/>
            <Constant symbol="epsilon" name="slow relative timescale" value="0.05"/>
            <Constant symbol="I_0" name="Stimulus" value="0.25"/>
            <DiffEqn symbol-ref="u">
                <Expression> u*(1-u)*(u-a)-v + I_0  </Expression>
            </DiffEqn>
            <DiffEqn symbol-ref="v">
                <Expression> epsilon*(u-b*v)</Expression>
            </DiffEqn>
        </System>
    </Global>
    <Space>
        <Lattice class="linear">
            <Size symbol="size" value="1, 0, 0"/>
            <Neighborhood>
                <Order>1</Order>
            </Neighborhood>
        </Lattice>
        <SpaceSymbol symbol="space"/>
    </Space>
    <Time>
        <StartTime value="0"/>
        <StopTime symbol="stoptime" value="200"/>
        <TimeSymbol symbol="time"/>
    </Time>
    <Analysis>
        <Logger time-step="1">
            <Input>
                <Symbol symbol-ref="u"/>
                <Symbol symbol-ref="v"/>
            </Input>
            <Plots>
                <Plot time-step="-1">
                    <Style line-width="2.0" style="lines"/>
                    <Terminal terminal="png"/>
                    <X-axis>
                        <Symbol symbol-ref="time"/>
                    </X-axis>
                    <Y-axis>
                        <Symbol symbol-ref="u"/>
                        <Symbol symbol-ref="v"/>
                    </Y-axis>
                </Plot>
            </Plots>
            <Output>
                <TextOutput/>
            </Output>
        </Logger>
        <ModelGraph format="svg" reduced="false" include-tags="#untagged"/>
    </Analysis>
</MorpheusModel>