( Stimulus ( ( ( ( causes ( ( an ( instantaneous reversal ) ) ( of ( this polarity ) ) ) ) , ) ( ( ( causing ( ( the muscle ) ( to contract ) ) ) ( -LRB- ( ( the ( mechanical characteristic ) ) -RRB- ) ) ) ( producing ( a ( ( twitch or ) movement ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Stimulus)) (VP (VBZ causes) (NP (NP (DT an) (JJ instantaneous) (NN reversal)) (PP (IN of) (NP (DT this) (NN polarity)))) (, ,) (S (VP (VBG causing) (NP (NP (DT the) (NN muscle)) (PP (TO to) (NP (NN contract)))) (PRN (-LRB- -LRB-) (NP (DT the) (JJ mechanical) (NN characteristic)) (-RRB- -RRB-)) (S (VP (VBG producing) (NP (DT a) (NN twitch) (CC or) (NN movement))))))) (. .)))
Stimulus causes an instantaneous reversal of this polarity, causing the muscle to contract (the mechanical characteristic) producing a twitch or movement.
(ROOT (S (NP (NNS Muscles)) (VP (VBP need) (NP (NN stimulus)) (PP (TO to) (NP (NN contract)))) (. .)))