Variation of the input resistance and membrane potential of a neuron in trace formation.

Trace changes in electrical activity, input resistance (Rinp), and membrane potential (MP) of brain neurons are studied in the mollusk Limnaea stagnalis for intracellular stimulation with a sinusoidal threshold current with a frequency of 0.1 Hz during 20 min. Some neurons are shown to exhibit an effect of facilitation, a rise in the level of activity being attending by an increase in Rinp and depolarization. Other neurons displayed lowered activity with a decrease of Rinp and hyperpolarization. The selectivity of the Rinp variations relative to the parameters of the stimuli (maximum changes at the frequency of the current used) suggests that it is precisely trace changes of Rinp which lie at the basis of the neuronal plasticity in "learning." Some neurons in this series of experiments did not alter their electrical response, Rinp, or MP for stimulation. The possible reason for the non-uniform reaction of different neurons to identical stimulation is discussed.