( ( Many scientists ) ( ( believe ( that ( ( a ( migraine attack ) ) ( is ( caused ( by ( ( a sequence ) ( of ( events ( that ( ( causes ( blood vessels ) ) ( in ( the ( brain ( to ( ( ( ( ( tighten , ) then ) dilate ) , ) ( resulting ( in ( ( the ( throbbing pain ) ) ( of ( a ( migraine headache ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Many) (NNS scientists)) (VP (VBP believe) (SBAR (IN that) (S (NP (DT a) (JJ migraine) (NN attack)) (VP (VBZ is) (VP (VBN caused) (PP (IN by) (NP (NP (DT a) (NN sequence)) (PP (IN of) (NP (NP (NNS events)) (SBAR (WHNP (WDT that)) (S (VP (VBZ causes) (NP (NN blood) (NNS vessels)) (PP (IN in) (NP (DT the) (NN brain) (S (VP (TO to) (VP (VB tighten) (, ,) (ADVP (RB then)) (VB dilate) (, ,) (S (VP (VBG resulting) (PP (IN in) (NP (NP (DT the) (VBG throbbing) (NN pain)) (PP (IN of) (NP (DT a) (JJ migraine) (NN headache)))))))))))))))))))))))) (. .)))
Many scientists believe that a migraine attack is caused by a sequence of events that causes blood vessels in the brain to tighten, then dilate, resulting in the throbbing pain of a migraine headache.
(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))) (. .)))