Brain Research, 59 (1973) 389-394 389 © Elsevier ScientificPublishing Company, Amsterdam - Printed in The Netherlands Equivalence of synaptic and injected: current in determining the membrane potential trajectory during moto,neuron rhythmic firing PETER C. SCHWINDT AND WILLIAM H. CALVIN* Departments of Physiology and Biophysics, and Neurological Surgery, University of Washington School of Medicine, Seattle, Wash. 98195 (U.S.A.) (Accepted June 7th, 1973) Rhythmic firing in cat spinal motoneurons may be induced either by sustained synaptic input 1 or by a constant current injected through the recording micro- electrodO~,lL Differences in rhythmic firing behavior under sustained synaptic input as compared to injected current would suggest that the mechanisms underlying rhythmic firing are sensitive to the magnitude and perhaps the spatial distribution of the conductance changes accompanying sustained synaptic input. In many cases, sustained synaptic input and injected current seem to be equivalent in determining the overall input-output relation of the ceil: added synaptic input may shift the plot of firing frequency versus injected current (the f-I curve) along the current axis as if the synaptic current simply added to (or subtracted from) the injected current s,9. On the other hand, synaptic input from supraspinal stimulation sometimes alters the slope of the f-I curve la,1s,19, and a recent theoretical investigation 1~ indicated that an f-I slope alteration should be seen when certain types of synaptic input are added. The membrane potential between spikes (the 'trajectory') provides a more direct indication of the events underlying rhythmic firing than the f-I curve. Our recent studies 16,17 have shown that 3 types of trajectory change, each corresponding to a segment11, ~2 of the f-I curve, may be seen when firing rate is altered by injected current alone. Even if it did not change the f-I curve slope, added synaptic input might result in different trajectory behavior, indicating a dependence of the mechanisms under- lying rhythmic firing on the mode of driving current application or on the associated synaptic conductance changes. To test this possibility, the behavior of the membrane potential between spikes was examined during rhythmic firing with and without added synaptic input. Experimental and data processing methods have previously been described 3,5-7, 16 Cats were anesthetized with pentobarbital (35-40 mg/kg) and paralyzed with gallamine triethiodide. Rhythmic firing was produced by injecting constant current * Reprint requests to Dr. Calvin, Department of Neurological Surgery RI-20, University of Washing- ton, Seattle, Wash. 98195, U.S.A.