a=0.04; b=0.003; c=0.0015; i0=0; 
modell={v'[t]==v[t] (a-v[t]) (v[t]-1) - w[t] + i0,
        w'[t]==b v[t] - c w[t],
        v[0]==v0, w[0]==w0};
v0=0.1; w0=0; tmax=500;
loesung=NDSolve[modell,{v,w},{t,0,tmax},MaxSteps->1000];
bild1=Plot[Evaluate[v[t]/.loesung],{t,0,tmax},PlotRange->All,
ImageSize->{400,300}];
bild2=Plot[Evaluate[w[t]/.loesung],{t,0,tmax},PlotRange->All,
PlotStyle->{Dashing[{0.01}]},ImageSize->{400,300}];
bild=Show[bild1,bild2,AxesLabel->{"t","v,w"},ImageSize->{400,300}];