Monosynaptic discharges by ventral roots were studied in rats under conditions of pronounced spinal hyperreflexia 5 days after simultaneous transection of the sciatic nerve and spinal cord. In 40% of tests with such rats, an enhanced monosynaptic discharge of a ventral root was found to be followed by a synchronized and high-amplitude discharge similar in shape and amplitude to the response of the ventral root to electrical stimulation of its fibers. The threshold amplitude for elicitation of these extra discharges was close to the amplitude of the ventral root's monosynaptic discharges at which highamplitude discharges occurred. It is concluded that when the excitability of spinal reflex arcs is excessively high, ephaptic transmission of excitation probably occurs in ventral roots from fibers involved in the enhanced reflex discharge to unexcited fibers.
Excitability of spinal cord neurons in rats has been shown to be highest on days 3-5 afternerve transection [5,7]. After cordotomy, stably enhanced excitability of spinal cord reflex arcs was reported in rats 3 days postsurgery and persisted for a long time [4,6]. We assumed, therefore, that an opera-tion combining nerve transection with cordotomy would lead 5 days later to the formation in the spinal cord of a focus of enhanced excitability exceeding that observed after separate nerve or spinal cord transection. Characteristics of the reflexes from the spinal cord after such a combined operation are of interest for studies designed to gain better insight into how a generator of pathologically enhanced excitation forms in the spinal cord [3].
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