tha  = 20	 % (mV)		: v 1/2 for inf
qa   = 9
Ra   = 0.02      %	(/ms)		: max act rate
Rb   = 0.002    %	Rb   = 0.002	(/ms)		: max deact rate	

v = -90:0.1:60;
o = ones(size(v))
x = v-tha;
a = Ra  * (x ./ (1 - exp(-x/qa)));
b = -Rb * (v - tha) ./ (1 - exp((v - tha)/qa));
plot(v,a,'r')
hold on
plot(v,b)