Voltage-modulated membrane resistance in coupled leech neurons

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Abstract
1. Resistive interactions have been studied between two pairs of large identifiable neurons in ganglion 6 of the leech CNS, called the lateral and rostral cells. Both are motor neurons causing penile eversion. 2. Lateral and rostral neurons have different membrane resistance properties. Input resistances of lateral neurons are virtually constant. By contrast, membrane resistances of rostral neurons are highly voltage dependent. When depolarized from resting potential to firing level, a rostral neuron's input resistance can increase 10-fold, from 30 to 300 Momega. 3. Voltage-dependent membrane characteristics of rostral neurons cause resistive interactions with lateral neurons to be nonlinear. DC potentials evoked in lateral cells are transmitted to rostral cells with an efficiency varying over a 10-fold range. Hyperpolarizing coupling is weak, with coupling factors of about 0.03. Depolarizing coupling factors increase progressively with increasing lateral neuron depolarization, reaching values of up to 0.3. 4. Membrane resistance changes in rostral neurons accompany lateral to rostral cell interactions. Input resistances increase during depolarizing and decrease during hyperpolarizing coupling potentials. The lateral to rostral cell junctional resistance is high and invariant, as evidenced by uniformly weak coupling in the reverse direction, from rostral to lateral neurons. 5. In conclusion, asymmetries in lateral to rostral cell interaction are based on postsynaptic rather than junctional resistance changes. The impact of the lateral onto the rostral cell's excitability contains a nonlinear component besides the usual linear additive one. As in conventional resistive coupling, depolarizing coupling potentials raise the rostral neuron closer to its voltage threshold. But more significantly, depolarizing coupling potentials lower the rostral neuron's current threshold because increases in resistance proportionately reduce the amount of excitatory current needed to reach firing level. Thus, the resistance change acts to amplify the input signal efficiency. In addition to the static changes in current threshold, the reostral neuron also changes dynamically. Membrane resistance increases lead to increases in space constant shrinking the neuron's electrical lenght. 6. Other properties of the network have been analyzed. The pair of lateral neurons is strongly coupled, whereas the pair of rostral neurons is weakly coupled, the coupling factors are 0.3 and 0.05, respectively. Hyperpolarizing membrane time constants for the lateral and rostral neurons are estimated to be between 100 and 200 ms. Time constants of depolarized rostral neurons are significantly larger.