Observations on impulse conduction along central axons.

This note calls attention to the facts that (i) the conduction velocities of central axons may not be predicted on the basis of diameter alone, (ii) that such structure-function relations as do exist may not be invariant. Data are presented which indicate that conduction velocities of rabbit callosal axons vary with the history of impulse conduction along the fiber. Increases and decreases of conduction velocity occur. Constant latency does not, therefore, constitute a necessary condition for identification of antidromically activated neurons, and variable latency does not constitute a sufficient condition for identification of synaptically activated neurons. The results are further discussed in terms of temporal coding of information in the central nervous system.