( ( Unlike ( ( skeletal ( muscle cells ) ) ( which ( can ( ( contract continuously ) ( in tetanus ) ) ) ) ) ) ( , ( ( cardiac ( muscle cells ) ) ( ( must ( repolarize ( ( in order ) ( to ( contract again ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN Unlike) (NP (NP (JJ skeletal) (NN muscle) (NNS cells)) (SBAR (WHNP (WDT which)) (S (VP (MD can) (VP (VB contract) (ADVP (RB continuously)) (PP (IN in) (NP (NN tetanus))))))))) (, ,) (NP (JJ cardiac) (NN muscle) (NNS cells)) (VP (MD must) (VP (VB repolarize) (SBAR (IN in) (NN order) (S (VP (TO to) (VP (VB contract) (ADVP (RB again)))))))) (. .)))
Unlike skeletal muscle cells which can contract continuously in tetanus, cardiac muscle cells must repolarize in order to contract again.
(ROOT (S (NP (JJ Skeletal) (NNS muscles)) (VP (MD must) (VP (VB work) (PP (IN in) (NP (NNS pairs))) (S (VP (TO to) (VP (VB move) (NP (NNS bones)) (ADVP (RB back) (CC and) (RB forth)) (PP (IN at) (NP (NP (DT a) (NN joint)) (SBAR (IN because) (S (NP (PRP they)) (VP (MD can) (ADVP (RB only)) (VP (VB contract)))))))))))) (. .)))