Neural repetitive firing: a comparative study of membrane properties of crustacean walking leg axons.

1. Repetitive activity and membrane conductance parameters of crab walking leg axons have been studied in the double sucrose gap. 2. The responses to constant current stimulus could be classified into three catagories; highly repetitive with wide firing frequency range, type I; highly repetitive with narrow frequency range, type II; and nonrepetitive or repetitive to only a limited degree, type III. The minimum firing frequency for type I axons was much greater than for other recording techniques. 3. Voltage-clamp currents in type III axons were qualitatively similar to those of squid or lobster axon. 4. The outward membrane currents of type I and II axons showed a transient phase in addition to the usual delayed current. The magnitude of this transient was a function of both the holding and test voltages. 5. The direction of the transient current reversed in potassium-rich saline. 6. The type I repetitive response in the walking leg axons appears to be generated by the same types of conductance changes that have been demonstrated in molluscan central neurons.