( ( When ( ( a migraine ) hits ) ) ( , ( it ( ( 's ( ( ( ( the result ) ( of ( ( those cells ) ( ( ( causing ( the ( blood vessels ) ) ) nearby ) ( to ( ( dilate and ) inflame ) ) ) ) ) ) , ) ( resulting ( in ( the headache ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (DT a) (NN migraine)) (VP (VBZ hits)))) (, ,) (NP (PRP it)) (VP (VBZ 's) (NP (NP (DT the) (NN result)) (PP (IN of) (NP (NP (DT those) (NNS cells)) (VP (VBG causing) (NP (DT the) (NN blood) (NNS vessels)) (ADVP (RB nearby)) (S (VP (TO to) (VP (VB dilate) (CC and) (VB inflame))))))) (, ,) (VP (VBG resulting) (PP (IN in) (NP (DT the) (NN headache)))))) (. .)))
When a migraine hits, it's the result of those cells causing the blood vessels nearby to dilate and inflame, resulting in the 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))) (. .)))