Constant and variable aspects of axonal phenotype in cerebral cortex

Abstract

In order to determine to what extent the terminal arbors of phylogenetically and functionally distant axons are constructed according to common rules, we have compared visual callosal axons in cats (CCC axons) with thalamocortical axons to the whisker representation in mice (MTC axons). Both similarities and differences were found. Maximal order of branching, branching angles, topological distribution of branches and boutons are similar for all axons, indicating strong constraints in arbor formation. CCC and MTC axons are indistinguishable for total arbor length and number of branches, although these parameters can vary across individual axons of each group. MTC axons have longer and bouton-richer end-branches (the 'transmission compartment') while, in CCC axons, proximal, boutonless branches (the 'conduction compartment') predominate. Therefore, the two classes of axons appear to be specialized for performing different types of operations, in agreement with the available electrophysiological data and computer simulations. Differences in the length of branches were also observed between MTC axons of normal and 'barrelless' mice, suggesting that this parameter can be regulated by conditions at the terminal sites.

The Crossed Projection to the Striatum in Two Species of Monkey and in Humans: Behavioral and Evolutionary Significance

“…They distributed a rather simple terminal arbor, but, along their striatal trajectory they projected a number of small branches and carried several swellings, some of which may be passing boutons. This suggests that the projection has a modulatory rather than a powerful driving action on striatal neurons in line with previously proposed concepts (Tettoni et al 1998). …”

“…The axons show a very circumscribed terminal arbor and distribute both terminal boutons and varicosities which may represent passing synapses over long intrastriatal trajectories. This suggests that the projection has a spatially distributed, modulatory rather than powerfully driving action on striatal neurons in line with previously proposed concepts (Tettoni et al 1998).…”

Axon Branching and Synaptic Bouton Phenotypes in GABAergic Nonpyramidal Cell Subtypes

“…The frequency histogram of interbouton intervals was characteristically skewed in all cells with a long tail at long intervals and a low incidence at short intervals (Fig. 6Ca,Cb) (Hellwig et al, 1994;Tettoni et al, 1998). It could be well fitted by the gamma distribution for each cell (peak location, 3.1 Ϯ 0.8 m) ( Table 2).…”

“…Reconstructed neurons were quantitatively analyzed with NeuroExplorer (MicroBrightField) and the programs that we wrote to obtain the bouton density and branching angles. The algorithm for measuring curvilinear distances between boutons (interbouton interval) was kindly provided by Dr. L. Tettoni (Lausanne University, Lausanne, Switzerland) (Tettoni et al, 1996(Tettoni et al, , 1998. Curve fitting of the distribution histogram was performed by IGOR Pro.…”

“…Frequency histograms of interbouton intervals were extremely skewed, with a peak on the shorter interval and a long tail (Hellwig et al, 1994;Kincaid et al, 1998). Therefore, the interbouton interval is represented by the median for each cell (Amir et al, 1993;Tettoni et al, 1998;Anderson et al, 2002). All these parameters of angles, intervals, and distances were measured in three dimensions.…”