NEOCORTICAL EFFERENT NEURONS WITH VERY SLOWLY CONDUCTING AXONS - STRATEGIES FOR RELIABLE ANTIDROMIC IDENTIFICATION

Although simple in concept, reliable antidromic identification of effe rent populations poses numerous technical challenges and is subject to a host of sampling biases, most of which select against the detection of the neurons with slowly conducting axons. This problem is particul arly acute in studies of the neocortex. Many neocortical efferent syst ems have large sub-populations with very slowly conducting, nonmyelina ted axons and these elements have been relatively neglected in antidro mic studies of neocortical neurons. The present review attempts to red ress this problem by analyzing the steps that must necessarily precede antidromic identification and the sampling biases associated with eac h of these steps. These steps include (1) initial recognition that the microelectrode is near a neuron; (2) activation of the efferent axon via the stimulating electrode; (3) conduction of the antidromic impuls e from stimulation site to soma; (4) detection of the antidromic spike in the extracellular record and (5) discriminating antidromic from sy naptic activation. Experimental strategies are suggested for minimizin g the sampling biases associated with each of these steps; most of whi ch can be reduced or eliminated by appropriate experimental procedures . Careful attention to such procedures will make it possible to better understand the nature and function of the information flow along the very slowly conducting axonal systems of the neocortex. (C) 1998 Elsev ier Science B.V. All rights reserved.