{ diff(x1, t) = (1*k10*k2*x2 + (-1)*k10*k3*k1*x1 + 1*k10*k1*x3)/k10,
  diff(x2, t) = ((-1)*k10*k2*x2 + 1*k10*k1*x4 + (-1)*k10*k3*k1*x2)/k10,
  diff(x3, t) = (1*k10*k3*k1*x1 + (-1)*k10*k1*x3 + 1*k10*k2*x4 + (-1)*k10*k5*k2*x3)/k10,
  diff(x4, t) = ((-1)*k10*k1*x4 + 1*k10*k3*k1*x2 + (-1)*k10*k2*x4 + 1*k10*k5*k2*x3)/k10 }