W. Otto Friesen

Identification of Leech Swim Neurons Using a Resonance Technique

We have developed a technique to identify neuronal oscillators and to predict the period in the oscillations. The technique involves injecting subthreshold sinusoidal current over a range of frequencies into the neuron of interest and recording the resulting membrane potential oscillations. The analysis requires the decomposition of these oscillations into their harmonic components and the determination of the amplitude and phase of the first harmonic for each frequency. We have shown that neuronal oscillators exhibit a resonance peak in the membrane potential amplitude as well as positive and negative phase angles. In addition, the frequency at which the resonance peak occurs predicts the normal oscillating frequency. In applying this technique to the mathematical model of the squid axon (Hodgkin and Huxley 1952) we predicted accurately the range of firing frequencies for this model. Our preliminary results suggest that this technique can be applied to neuronal circuit oscillators as well, for we found a resonance peak at approximately 1 Hz in the membrane potential of a leech motor neuron (part of the leech swim circuit), which agrees well with the normal swimming frequency of the leech (0.5 to 2.0 Hz).