A single identified neuron was repeatedly isolated by axotomy from the central nervous system of the nudibranch mollusk Hermissenda crassicornis. An early voltage-dependent outward K+ current of this neuron was reduced and more rapidly inactivated for animals previously trained with paired but not randomized light and rotation. Since this current change can affect interneuron and motorneuron output via known synaptic pathways, it helps explain a long-lasting behavioral change that shows the defining features of vertebrate associative learning.

No abstract

The effect of associative training on an identified putative motoneuron (MN1) in an identified visual input-output neural pathway was studied in Hermissenda crassicornis. The increased impulse frequency of the MN1 cell in response to a light step (MN1-LR) was found to be smaller up to 54 hr after associative training with paired light and rotation stimuli. No change was found in animals which received these training stimuli randomly. The MN1 activity in darkness, namely, baseline impulse frequency, spike amplitude, input resistance, and resting potential, did not change after training. The MN1-LR was positively correlated on retention days with the behavioral latency of the animal to enter the illuminated area of a light intensity gradient. However, the input resistance of the type B photoreceptor was inversely correlated with this behavioral latency as well as with MN1-LR. These findings suggest that the reduction of MN1-LR may be caused by the biophysical changes which have been found in the type B photoreceptor membrane. Furthermore, these findings support several other studies which indicate that specific biophysical changes in the type B photoreceptor membrane play a causal role in the observed behavioral modifications after associative training in Hermissenda.