STRENGTH-DURATION CURVE OF CONDUCTIVE SPINAL-CORD EVOKED-POTENTIALS IN CATS

Strength-duration curves of the ascending and descending conductive sp inal cord potentials (SCEPs) in cats were obtained using constant curr ent stimuli. For the formulation of numeric indices of excitability, t he rheobase is defined as the minimal current strength below which res ponse cannot occur even if the current continues, and the chronaxie is defined as the minimal duration of a current required to evoke the po tential at twice the rheobase strength. The chronaxies and rheobases w ere calculated from the constructed strength-duration curves. The purp ose of this study is to produce strength-duration curves and to evalua te the utility of chronaxies and rheobases for SCEPs, This study showe d the following results: (1) there was a hyperbolic relationship betwe en stimulus strength and stimulus duration at threshold values, simila r to that seen in peripheral nerves; (2) the ascending and descending tracts of SCEP were mediated through the same pathway (based on the si milar chronaxies and rheobases); (3) following spinal cord compression the chronaxie and rheobase increased significantly (P < 0.05), which is similar to peripheral nerve disturbance. However, the rheobase decr eased significantly following slight spinal cord compression (P < 0.05 ) and systemic cooling (P < 0.01), and the strength-duration curve shi fted showing a tendency towards decrease of the galvanic threshold, th erefore, amplitude augmentation with slight compression and with decre ase in temperature seems to contribute to the reduction of the thresho ld. The strength-duration curve, the chronaxie and the rheobase may be useful in assessing spinal cord function.