Graphical representation of biological networks. A. The arcs associated with the functions a, b, c and d. We will use dashed arcs, connecting to arcs of the type a and b to highlight that the associated function is nonlinearly dependent on a species of the network: in the example above, a31 = a31(x2). B. The graph associated with equations (2); external inputs are represented as orange nodes. C. Examples of sigmoidal functions. D. Examples of complementary sigmoidal functions. In our general model (4), functions d(·) and c(·) are naturally associated with Hill function terms.