Sentence:  Furthermore , conditioned reactions are fundamentally altered when the hypothalamic sympathetic reactivity is augmented beyond a <WSD>critical</WSD> level , and several types of behavioral changes probably related to the degree of central autonomic `` tuning '' are observed . 
Meaning of the word marked with <WSD> tags:  at or of a point at which a property or phenomenon suffers an abrupt change especially having enough mass to sustain a chain reaction