Beyond the primary sensory areas, axon properties are more difficult to characterize both physiologically and structurally. On the basis of structural evidence, interareal corticocortical connections have been subdivided into two broad categories: feedforward and feedback (Fig. 8). The terms reflect an assumption of serial organization, whereby feedforward connections progress from primary sensory through higher order areas. They are reciprocated in the "reverse" directions by feedback connections. Feedforward axons originate from neurons mainly in layer 3, terminate mainly in layer 4, and have one to four spatially separated arbors, each about 0.2 mm in diameter. Serial section reconstruction of axons labeled with the newer ante-rograde tracers demonstrates that individual arbors carry 50-400 terminal specializations, for a total number per axon of 400-1000 boutons. Feedback axons originate from neurons mainly in layer 6, terminate heavily in layer 1, and tend to have a single, rod-like axon at least 1.0 mm long, also with 400-1000 terminal specializations. On the basis of these data, if each bouton corresponds to one synapse, and if no more than one synapse is made with any postsynaptic neuron, a divergence factor of 1:400-1000 is suggested. No estimates are available for convergence in these pathways, and there are only sparse data available concerning postsynaptic populations. In primates, feedback inputs to layer 1 are commonly thought to target predominantly apical dendrites since these are a major constituent of layer 1, but the actual mix and identity of the postsynaptic targets are unknown. Similarly, the actual postsynaptic targets of feedforward connections, while probably including apical dendrites of deeper neurons that pass through layer 4 and the basal dendrites of overlying neurons in layer 3, are not known. Confocal microscopy may be an improved means of approaching these questions because it offers the necessary resolution of EM—to verify that boutons close to a structure are actually contacting that structure—without impractically slow EM processing and analysis.
In contrast to thalamocortical connections to primary sensory cortices, the functional properties of feedforward and feedback axons are not known. A convenient assumption is that feedforward axons are relays from early to higher order cortical areas and are
involved in progressive elaborations, whereas feedback axons exert a return, "modulatory" influence. This is clearly an oversimplification, however, in several respects. First, neither of these systems operate in isolation but, rather, are part of a broader network of thalamocortical, callosal, and intrinsic connections, among others. Second, the efficacy of feedback connections, despite their location on distal dendrites in layer 1, may actually be considerable since recent work has identified mechanisms for active boosting of synaptic inputs at dendritic locations distant from the axon hillock. Third, both feedforward and feedback axons, rather than operating as complementary pairs, likely comprise many functionally significant subdivisions.
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