Systematic variations in the conduction velocity of slowly conducting axons in the rabbit corpus callosum

Extracellular spikes were recorded from the cell bodies of antidromically activated callosal axons in rabbit visual cortex. Callosal axons were stimulated 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 antidromic activation. A decrease in antidromic latency always occurred when an antidromic volley followed either a spontaneous spike or a preceding antidromically 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.