( ( Further study ) ( ( is ( necessary ( to ( determine ( ( wether IGF-1 ) ( is ( having ( ( an effect ) ( ( ( ( only ( on ( muscle fibers ) ) ) or ) ( on ( nervous tissues ) ) ) ( as well ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Further) (NN study)) (VP (VBZ is) (ADJP (JJ necessary) (S (VP (TO to) (VP (VB determine) (SBAR (S (NP (NN wether) (NN IGF-1)) (VP (VBZ is) (VP (VBG having) (NP (NP (DT an) (NN effect)) (PP (PP (ADVP (RB only)) (IN on) (NP (NN muscle) (NNS fibers))) (CC or) (PP (IN on) (NP (JJ nervous) (NNS tissues))) (ADVP (RB as) (RB well))))))))))))) (. .)))
Further study is necessary to determine wether IGF-1 is having  an effect only on muscle fibers or on nervous tissues as well.
(ROOT (S (NP (JJ Only) (NNS animals)) (VP (VBP have) (NP (NP (NN muscle) (NN tissue)) (CC and) (NP (JJ nervous) (NN tissue)))) (. .)))
Only animals have muscle tissue and nervous tissue.