( ( In ( a ( migraine headache ) ) ) ( , ( there ( ( 's ( ( an ( initial ( abnormal dilation ) ) ) ( of ( ( blood vessels ) ( in ( ( ( the brain ) , ) ( ( followed ( by ( ( a spasm ) ( of ( the ( blood vessel ) ) ) ) ) ) ( with ( ( a ( transient decrease ) ) ( in ( blood flow ) ) ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN In) (NP (DT a) (JJ migraine) (NN headache))) (, ,) (NP (EX there)) (VP (VBZ 's) (NP (NP (DT an) (JJ initial) (JJ abnormal) (NN dilation)) (PP (IN of) (NP (NP (NN blood) (NNS vessels)) (PP (IN in) (NP (NP (DT the) (NN brain)) (, ,) (VP (VBN followed) (PP (IN by) (NP (NP (DT a) (NN spasm)) (PP (IN of) (NP (DT the) (NN blood) (NN vessel))))) (PP (IN with) (NP (NP (DT a) (JJ transient) (NN decrease)) (PP (IN in) (NP (NN blood) (NN flow)))))))))))) (. .)))
In a migraine headache, there's an initial abnormal dilation of blood vessels in the brain, followed by a spasm of the blood vessel with a transient decrease in blood flow.
(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))) (. .)))