Effects of dorsolateral spinal lesions on stretch reflex threshold and stiffness in awake cats

Measurements of threshold angle and incremental dynamic stiffness (IDS) wer e derived from triceps surae stretch reflexes, elicited by ramp and hold fl exion at the ankle joint of four cats that were tested while awake. Stretch reflexes were assessed from trials that began from different ankle joint s tart positions or were matched using a post-hoc analysis for initial backgr ound force during testing sessions before and following unilateral lesions of the dorsolateral funiculus at levels ranging from T13 to L3. Unilateral lesions of the dorsolateral funiculus (DLF) produced significant ipsilatera l decreases in stretch reflex threshold and increases in reflex gain, measu red as incremental dynamic stiffness (IDS). ANCOVA testing indicated that t he reduction in threshold, but not the increase in IDS, was dependent upon the level of background force. Reflex testing from different start angles d emonstrated that DLF lesions diminished the correlation between threshold a nd IDS. Intravenous infusion of ketamine dose-dependently reduced IDS, comp ared with testing in the unanaesthetized state. Postoperative reflex testin g during infusion of ketamine at 22.2 mg/kg per h, when electromyographic r esponses were reduced to 24% of control levels, abolished differences in ID S between the ipsilateral and contralateral hindlimbs. These and related ob servations suggest that the postoperative increase in IDS in awake animals was not due to an increase in passive stiffness.