Intact inhibition of the stretch reflex during general tetanus

Tetanus toxin at lethal doses (2-10 x 10(3) mouse minimal lethal doses per kg body weight, mMLD/kg) was injected i.v. into 10 cats under pentobarbital anaesthesia. After the appearance of the first sign of generalized tetanus the animal was anaesthetized by a mixture of urethane and chloralose. Experiments were performed several hr thereafter when the toxin action was anticipated to be optimal. The stretch reflexes were elicited manually, by the contraction of the antagonistic muscles or by a stretch device. In toxin treated animals the spontaneous electromyographic activity was inhibited by strong stretching of the tested muscle or by that of the antagonistic muscle. The stretch reflex of the extensor muscle elicited by a contraction of the flexor muscle was inhibited by electrical stimulation of the flexor afferent fibres. The stretch reflex elicited by a stretch device as well as the electrically elicited monosynaptic reflex were inhibited by conditioning stimulation of the antagonistic nerve. The inhibition curves were almost the same as those of healthy animals. It is concluded that the spinal inhibitions, such as antagonistic group Ia, autogenic group Ib, groups II and III and the presynaptic inhibitions, were kept intact in severe general tetanus.