  We analyze the response of the Morris-Lecar model to a periodic train of
short current pulses in the period-amplitude plane. For a wide parameter range
encompassing both class 2 and class 3 behavior in Hodgkin's classification
there is a multimodal transition between the set of odd modes and the set of
all modes. It is located between the 2:1 and 3:1 locked-in regions. It is the
same dynamic instability as the one discovered earlier in the Hodgkin-Huxley
model and observed experimentally in squid giant axons. It appears
simultaneously with the bistability of the states 2:1 and 3:1 in the
perithreshold regime. These results imply that the multimodal transition may be
a universal property of resonant neurons.
