Neocortical efferent neurons with very slowly conducting axons: strategies for reliable antidromic identification
Introduction
Neocortical efferent neurons may be identified physiologically by their antidromic activation following electrical stimulation of cortical target structures. This technique has enjoyed considerable success and has yielded insight into the nature of the information flow to neocortical target structures (e.g. Asanuma et al., 1968, Palmer and Rosenquist, 1974, Harvey, 1980, Bullier et al., 1988, Swadlow, 1988, Swadlow, 1989). When multiple efferent systems are studied within the same region, comparisons may be made, under comparable conditions of parallel streams of information that are leaving the cortex. Although conceptually simple, reliable antidromic identification of efferent populations poses numerous technical challenges and is subject to a host of sampling biases. The thesis of this review is that these biases converge to predominantly impede the identification of neurons with slowly conducting axons. This problem is particularly acute in studies of neocortex, where many efferent systems have large sub-populations with very slowly conducting, nonmyelinated axons. For example, the corpus callosum of rat, rabbit, monkey and human all contain a considerable subpopulation of nonmyelinated axons, median callosal axon diameters in each species are <1 μm (see Tomasch, 1954, Seggie and Berry, 1972, Waxman and Swadlow, 1978Swadlow,Waxman and Geschwind, 1980Lamantia and Rakic, 1990, Ringo et al., 1994for review). Similarly, the layer-6 corticothalamic projection contains many very slowly conducting axons, as do ipsilateral corticocortical projections (Tsumoto and Suda, 1980, Harvey, 1980, Ferster and Lindstrom, 1983, Bullier et al., 1988, Swadlow, 1988, Swadlow, 1989, Swadlow, 1990, Swadlow, 1994). In this regard it is interesting to note that the great majority of antidromic studies of neocortical efferent neurons have focused on layer-5 neurons with relatively fast-conducting axons. For example, two extensively studied populations have been corticospinal neurons of sensory/motor cortices (e.g. Brooks et al., 1961, Towe et al., 1964, Asanuma et al., 1968, Asanuma et al., 1976, Wiesendanger, 1973, Steriade et al., 1974, Jankowska et al., 1975, Shinoda et al., 1976, Lemon, 1980, Sapienza et al., 1981) and corticotectal neurons of the visual cortex (e.g. Palmer and Rosenquist, 1974, Finlay et al., 1976, Lemmon and Pearlman, 1981, Weyand et al., 1986a, Weyand et al., 1986b). Far less is known about efferent neurons with fine-diameter, slowly conducting axons. Antidromic latencies of 30–40 ms and more are not uncommon in these neurons and special procedures are required to insure their adequate sampling. The purpose of this study is to review the problems that make identification of such neurons difficult and to suggest strategies for maximizing yield and minimizing sampling biases.
Much of the data presented here were obtained from a series of recent studies of the efferent systems emerging from several sensory and motor areas of rabbit neocortex (Swadlow, 1988, Swadlow, 1989, Swadlow, 1990, Swadlow, 1991, Swadlow, 1994). In each of these areas four corresponding efferent populations were identified and studied in the awake state. These included callosal (CC) neurons, ipsilateral corticocortical (C-IC) neurons and descending corticofugal neurons of layers 5 (CF-5) and layer 6 (CF-6).
Section snippets
Five necessary steps to antidromic identification
At least five steps must be successfully completed before an efferent neuron can be identified by antidromic activation. Although four of these steps (2–5) are necessarily sequential, steps 1 and 4 sometimes occur simultaneously. Each step is subject to problems and sampling biases, which are summarized in Table 1.
Identifying the site of axonal stimulation
In some experiments it is important to precisely localize the axonal stimulation site that generated the antidromic impulse (e.g. Shinoda et al., 1976, Zarzecki, 1989, Nowak and Bullier, 1996). In general, although current threshold-distance relations will vary with different axonal types, the confidence with which one can localize an axonal stimulation site is inversely related to the stimulus intensity that generated the antidromic action potential (e.g. Ranck, 1975). Thus, the greater the
Identifying the axonal termination site of efferent neurons
Because axons are readily activated in passage, as well as at their termination sites even precise localization of the site of axonal stimulation does not necessarily yield information concerning the axonal termination site. In some cases, morphological information can provide constraints on the locus of possible termination sites. For example, retrograde tracing studies (e.g. Jones (1984)for review) show that the descending corticofugal projection from layer-6 terminate in thalamic nuclei, but
Conclusions
Although many efferent systems of the neocortex have large subpopulations with very slowly conducting axons, such neurons have remained relatively unstudied and their functions are obscure. The present review analyzes the steps that must necessarily precede antidromic identification of any cortical efferent neuron. Each of these steps is associated with pitfalls and potential sampling biases, most of which selectively impede the identification of neurons with slowly conducting axons. Most
Acknowledgements
This work was supported by grants from the National Institute of Neurological Disorders and Stroke (NS-32021) and the National Science Foundation (IBN-9723967)
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