Action potential adaptation in the femoral tactile spine of the cockroach, Periplaneta americana

1. Adaptation of action potential discharge was studied in the sensory neuron of a rapidly adapting insect mechanoreceptor, the cockroach, Periplaneta americana, femoral tactile spine. Direct electrical stimulation of the neuron was achieved by passing current through a micro-electrode adjacent to the axon where it leaves the soma. Action potentials were monitored extracellularly further along the axon. 2. The dynamic properties of action potential generation were observed in response to steps of current and to randomly varying currents. The step and frequency responses could both be well fitted by power-laws, which were indistinguishable from the behavior observed during mechanical stimulation of the spine. 3. Electrical stimulation of the axon further from the soma also gave a power-law response, suggesting that this dynamic behavior reflects general properties of the axon membrane, rather than a specific initiation site. 4. Variations in stimulus amplitude caused large changes in the rates of adaptation to steps or random stimuli. However, these changes were in opposite directions for the two types of stimulus. 5. Depolarization by addition of a DC current to the stimuli caused increases in the rates of adaptation to steps and random stimuli. Hyperpolarization caused the opposite effects. Raising the external potassium concentration also increased the rates of adaptation while decreasing potassium concentration caused the opposite effects. 6. The results indicate that adaptation in this receptor occurs during action potential initiation. Although the adaptation is well characterized by a power-law, the parameters vary with the type of stimulus, stimulus strength, and mean polarization level. These strong nonlinearities are difficult to explain by the distributed parameter linear models which are usually invoked to account for power-law behaviour.