PROLONGED FACILITATION OF SYNAPTIC EXCITATION IN SYMPATHETIC GANGLIA

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Abstract
Following the conduction of one or more volleys of impulses along the pre-synaptic and postsynaptic neurones of a sympathetic ganglion, there are long-lasting changes which facilitate subsequent excitation of the ganglion cells by impulses in the presynaptic fibers. By increasing the frequency and duration of the conditioning train of impulses, the number of ganglion cells responding to a given preganglionic volley and the duration of this facilitation are increased. Following a single conditioning volley, a small degree of facilitation is detectable for a few secs. After many volleys, the effect may last for more than 5 mins. Antidromic stimulation does not facilitate the response of a ganglion cell to synaptic excitation; on the contrary, it lowers the irritability of the cell. Also, the excitation of a ganglion cell by one set of presynaptic endings does not increase the irritability of the cell for subsequent excitation by other presynaptic endings. Furthermore the response of a cell to acetylcholine is reduced by previous synaptic or antidromic excitation. These facts argue against the possibility that prolonged facilitation is due to previous activity of the ganglion cell itself. The duration and magnitude of the prolonged facilitation is unaffected by modifying the circulation through a ganglion. This and other evidence indicate that the effects are not due to the persistence of some chemical agent in the synaptic region. There remains the possibility that prolonged facilitation is due to a long-continued alteration in the presynaptic endings, which causes the nerve impulses arriving at the synapse to have a more effective excitatory action. The relation of these phenomena to recent observation of prolonged after-discharge of rhythmically recurring impulses is discussed.