EXPERIMENTAL NEUROLOGY 43, 445-451 (19%) Systematic Variations in the Conduction Velocity of Slowly Conducting Axons in the Rabbit Corpus Callosum HARVEY A. SWADLOW Centerfor Braiik Rrscarrh, University of Rochester, Rochester, New York 14642 a.+kd Department of Psychology, CJRiversity of Midmi, Coral Gables, Florida 33124 Received December l&1973; revision received January l&l974 Extracellular spikes were recorded from the cell bodies of antidromically activated callosal axons in rabbit visual cortex. Callosal axons were stim- ulated near their terminals in the contralateral cortex. The primary method for differentiating antidromic from synaptic activation was the test for collision of impulses. Additional tests provided further confirmation of anti- dromic activation. A decrease in antidromic latency always occurred when an antidromic volley followed either a spontaneous spike or a preceding anti- dromically elicited spike at appropriate intervals. The time course and magnitude of the latency decrease coincided with that of a threshold decrease at the site of electrical stimulation. The antidromic latency decrease was primarily due to an increase in axon conduction velocity. These systematic variations in conduction velocity and stimulus threshold strongly suggest that an afterdepolarization follows the activation of callosal axons. While such afterpotentials are known to occur in unmyelinated C fibers, the present evidence suggests that they also occur in the smallest of myelinated axons. INTRODUCTION In the preceding experiment (S), extracellular spikes were recorded from the cell bodies of antidromically activated callosal axons in the conscious rabbit. The primary criterion for differentiating antidromic from synaptic activation was the test for collision of impulses (1). Further confirmation of antidromic activation was provided by tests for post-collision recovery (8) and by examination of refractory periods and waveform changes which resulted from double volley stimulation. Units which sent an axon across the corpus callosum (callosal neurons) were thereby distinguished from units which responded to callosal input. The estimated conduction velocity of callosal axons studied was quite low. The median conduction velocity was less than 2 meters per second 445 Copyright @ 1974 by Academic Press, Inc. All rights of reproduction in any form reserved.