ACTION OF TETANUS TOXIN ON CONDITIONING OF SPINAL MOTONEURONS

Abstract

Tetanus toxin, which has previously been shown to block inhibitory synaptic transmission, has been injected into the spinal cord of unanesthetized decapitate cats. Such injection results in increase of polysynaptic reflexes; appearance of polysynaptic reflexes in some nerves from which they were previously absent; increase in duration of reflex discharge; and development of spontaneous activity. The toxin regularly depresses or abolishes recurrent facilitation and recurrent inhibition. Orthodromic polysynaptic facilitation is not depressed by the toxin, but rather is usually increased. Tetanus toxin therefore differentiates between orthodromic and recurrent facilitation; and, the fact that it abolishes the latter, strongly suggests that the pathway for recurrent facilitation contains inhibitory synapses. This finding supports the view that recurrent facilitation is not a true facilitation, but rather a disinhibition.