CONSTANT AND VARIABLE ASPECTS OF AXONAL PHENOTYPE IN CEREBRAL-CORTEX

In order to determine to what extent the terminal arbors of phylogenet ically and functionally distant axons are constructed according to com mon 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 a rbor formation. CCC and MTC axons are indistinguishable for total arbo r length and number of branches, although these parameters can vary ac ross individual axons of each group. MTC axons have longer and bouton- richer end-branches (the 'transmission compartment') while, in CCC axo ns, proximal, boutonless branches (the 'conduction compartment') predo minate. Therefore, the two classes of axons appear to he specialized f or performing different types of operations, in agreement with the ava ilable electrophysiological data and computer simulations. Differences in the length of branches were also observed between MTC axons of nor mal and 'barrelless' mice, suggesting that this parameter can be regul ated by conditions at the terminal sites.