Antidromic discharges in dorsal roots of decerebrate cats. II: studies during treadmill locomotion.

In a previous companion paper [Brain Res. 846 (1999) 87-105] we have shown that the dorsal root activity of a decerebrate cat is composed of both orthodromic and antidromic discharges as determined by spike triggered averaging (STA). Furthermore we have shown that, during fictive locomotion in decerebrate and paralyzed cats, antidromic discharges peak in different parts of locomotion cycle but mainly in the flexion phase. In the present study, we have recorded unit potentials from dorsal rootlets during treadmill locomotion in order to understand better the role of movement-related feedback in the generation of antidromic potentials. The unitary activity of 92 antidromically discharging units was recorded in proximal stumps of cut dorsal roots, and that of 20 such units was recorded in uncut roots using two bipolar Ag/AgCl electrodes in both cases. The activity of 80% (74/92) units in cut filaments and of 70% (14/20) units in uncut ones was phasewise related to stepping movements. The peaks of activity of different units occurred during different phases of the step cycle both in cut and uncut filaments. In most cases, the peak of activity was superimposed upon a background of sustained discharge. After blocking the orthodromic flow in a filament (local anesthesia or distal section), the antidromic discharges continued to peak during the same phase but the rate of the discharges increased. We conclude that movement-related afferent feedback significantly modulates the antidromic discharges in dorsal roots during treadmill locomotion. We suggest that these antidromic discharges have a role in controlling afferent feedback during movement.