The neuronal basis of the anesthetic state: a comparative physiological approach. II. The influence of anesthetics on various reactions in flies.

The sensitivity of specific neuronal pathways to Halothane and N2O has been investigated in flies. The effects were tested by monitoring the responses of photoreceptors and their second order neurons, as well as two behavioral responses--a leg reflex induced by light flashes and head movements induced by moving optical patterns--chosen because their neuronal substrates are fairly well known. Sensitivity to both agents rises with the length of dendrites and the number of input synapses of the neurons involved. The findings confirms the hypothesis, formulated in Part I of this paper, that neurons with long dendrites and/or axonal endings and large numbers of input synapses are the elements in the central nervous system with the highest sensitivity to anesthetic action. Under physiological conditions this kind of neuron is capable of "gain-control": the relationship between input and output is modified according to functional requirements. Possible molecular mechanisms leading to functional impairment under anesthesia are discussed.