def stimulus(t):
    """
    External Current

    |  :param t: time
    |  :return: step up to 10 uA/cm^2 at t>100
    |           step down to 0 uA/cm^2 at t>200
    |           step up to 35 uA/cm^2 at t>300
    |           step down to 0 uA/cm^2 at t>400
    """
    return 10 * (t > 100) - 10 * (t > 200) + 35 * (t > 300) - 35 * (t > 400)