Strength-duration properties of a rapidly adapting insect sensory neuron

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1. A technique was developed to measure the strength-duration relationship for action potential initiation in the rapidly adapting sensory neuron of the femoral tactile spine in the cockroach,Periplaneta americana. Extracellular current was applied via a microelectrode adjacent to the axon where it leaves the soma. A computer-controlled successive approximation algorithm reliably determined threshold levels for rectangular current pulses of varying duration. 2. Experiments were conducted under resting conditions and also with constant depolarizing and hyperpolarizing base currents. Strength-duration data were well fitted by theoretical relationships derived for these conditions and assuming a constant charge threshold for the axon. The fitted equations gave values of the rheobasic current (threshold current for an infinitely long pulse) and the membrane time constant. The time constant was in good agreement with previous estimates obtained from this preparation using different techniques. 3. Both the rheobasic current and the time constant were functions of the constant base current. The rheobasic current increased strongly with depolarization so that it was always more positive than the base current. This gives a quantitative explanation of the observation that no depolarizing stimulus can sustain continuous action potential firing in this neuron. 4. With mild hyperpolarizing currents the rheobasic current decreased but with strong hyperpolarizations it increased and the time constant was reduced. This suggests that additional ionic conductance limits threshold behavior with strong hyperolarizations. 5. Experiments with blocking agents and ionic substitution suggested that a potassium A-current is involved in the threshold behavior with strong hyperpolarizations. Some evidence for increased chloride conductance under these conditions was also obtained. 6. No evidence was found that potassium or chloride currents are involved in the threshold behavior or rapid adaptation under normal or depolarized conditions.