Sentence:  A state of parasympathetic `` tuning '' of the hypothalamus induced experimentally causes not only an increase in the parasympathetic reactivity this structure to direct and reflexly induced stimuli , but leads also to an autonomic reversal : a <WSD>stimulus</WSD> acting sympathetically under control conditions elicits in this state of tuning a parasympathetic response ! 
Meaning of the word marked with <WSD> tags:  any stimulating information or event; acts to arouse action