Neurons evaluate both the amplitude and the meaning of signals.

The neuronal threshold, which can be determined by the level of depolarization immediately prior to spike generation, is different for responses to conditioned and discriminated stimuli after conditioning. However, it is impossible to determine excitability within a response to stimuli that failed to generate a spike. In the present study we examined the role of the AP threshold of two related Helix defensive neurons in the initiation of an AP during elaboration of the neuronal analog of a classical conditional reflex. These neurons fire in a synchronous manner though spikes sometimes are not generated simultaneously. We compared the change in spike threshold within the responses with other parameters of intracellular activity that also affect the neuronal response, but which can be measured without an analysis of the spike waveform (spike latency, slope of excitatory postsynaptic potentials, etc.). We found that the change in slope is not the reason for the change in the threshold during pairing. The thresholds within conditioned and discriminated responses affected spike generation and their values correlated with the spike presence or absence in related neurons. The excitability changed transiently within the responses, since these alterations were selective for the conditioned and discriminated stimuli and applied primarily to the first spike of the response. The firing threshold seems to be a dynamic property of excitable membrane. Neurons appear to evaluate the significance of the input signal, transiently change their own excitability and only after that compare the magnitude of this signal and threshold.