Changes in the axonal conduction velocity of pyramidal tract neurons in the aged cat

The present study was undertaken to determine whether age-dependent changes in axonal conduction velocity occur in pyramidal tract neurons. A total of 260 and 254 pyramidal tract neurons were recorded extracellularly in the m otor cortex of adult control and aged cats, respectively. These cells were activated antidromically by electrical stimulation of the medullary pyramid al tract. Fast- and slow-conducting neurons were identified according to th eir axonal conduction velocity in both control and aged cats. While 51% of pyramidal tract neurons recorded in the control cats were fast conducting ( conduction velocity greater than 20 mis), only 26% of pyramidal tract neuro ns in the aged cats were fast conducting. There was a 43% decrease in the m edian conduction velocity for the entire population of pyramidal tract neur ons in aged cats when compared with that of pyramidal tract neurons in the control cats (P < 0.001, Mann-Whitney U-test). A linear relationship betwee n the spike duration of pyramidal tract neurons and their antidromic latenc y was present in both control and aged cats. However, the regression slope was significantly reduced in aged cats. This reduction was due to the appea rance of a group of pyramidal tract neurons with relatively shorter spike d urations but slower axonal conduction velocities in the aged cat. Sample in tracellular data confirmed the above results. These observations form the b asis for the following conclusions: (i) there is a decrease in median condu ction velocity of pyramidal tract neurons in aged cats; (ii) the reduction in the axonal conduction velocity of pyramidal tract neurons in aged cats i s due, in part, to fibers that previously belonged to the fast-conducting g roup and now conduct at slower velocity. (C) 1999 IBRO. Published by Elsevi er Science Ltd.