Neuromodulators may alter the output of a physiological system by acting on the associated inputs (for instance, central pattern generators).  However, modeling work suggests that this alone is insufficient, because the neuromuscular transformation from neural input to muscular output may be tuned for particular ranges of input. Stern et al. (2007) suggest that neuromodulators must act not only on the input system but must change the transformation itself to produce the proper  contractions of muscles as output.