( ( ( ( ( ( The heart ) ( is ( a muscle ) ) ) , ) but ) ( it ( ( is not ) ( controlled ( by ( motor neurons ) ) ) ) ) ) . )
(ROOT (S (S (NP (DT The) (NN heart)) (VP (VBZ is) (NP (DT a) (NN muscle)))) (, ,) (CC but) (S (NP (PRP it)) (VP (VBZ is) (RB not) (VP (VBN controlled) (PP (IN by) (NP (NN motor) (NNS neurons)))))) (. .)))
The heart is a muscle, but it is not controlled by motor neurons.
(ROOT (S (NP (JJ Muscle) (NNS groups)) (VP (VBP are) (VP (VBN controlled) (PP (IN by) (NP (DT the) (NN motor) (NN cortex))))) (. .)))