( ( During ( a migraine ) ) ( , ( ( ( blood vessels ) ( in ( the head ) ) ) ( ( go ( through ( ( a cycle ) ( of ( ( extreme constriction ) ( followed ( by ( rapid dilation ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN During) (NP (DT a) (NN migraine))) (, ,) (NP (NP (NN blood) (NNS vessels)) (PP (IN in) (NP (DT the) (NN head)))) (VP (VBP go) (PP (IN through) (NP (NP (DT a) (NN cycle)) (PP (IN of) (NP (NP (JJ extreme) (NN constriction)) (VP (VBN followed) (PP (IN by) (NP (JJ rapid) (NN dilation))))))))) (. .)))
During a migraine, blood vessels in the head go through a cycle of extreme constriction followed by rapid dilation.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NP (NN blood) (NNS vessels)) (PP (IN in) (NP (DT the) (NN head)))) (VP (VBP dilate)))) (, ,) (NP (NP (DT a) (NN migraine)) (, ,) (NP (DT a) (JJ severe) (NN headache)) (, ,)) (VP (MD can) (VP (VB occur))) (. .)))