( ( During ( ( the ( headache phase ) ) ( of ( a ( migraine attack ) ) ) ) ) ( , ( there ( ( is ( dilation ( of ( ( the ( blood vessels ) ) ( surrounding ( the ( cerebral ( nerve endings ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN During) (NP (NP (DT the) (NN headache) (NN phase)) (PP (IN of) (NP (DT a) (JJ migraine) (NN attack))))) (, ,) (NP (EX there)) (VP (VBZ is) (NP (NP (NN dilation)) (PP (IN of) (NP (NP (DT the) (NN blood) (NNS vessels)) (VP (VBG surrounding) (NP (DT the) (JJ cerebral) (NN nerve) (NNS endings))))))) (. .)))
During the headache phase of a migraine attack, there is dilation of the blood vessels surrounding the cerebral nerve endings.
(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))) (. .)))