For an optical technology to be feasible as a substitution of electronics one, it is required the waveguides be capable of confining and directing light signals in much smaller dimensions than the operating wavelength of the light, i.e. subwavelength optical components must be realized. One approach to miniaturize the size of the photonic components is to benefit the hybrid nature of Surface Plasmon Polariton which addresses the problem with the diffraction limit of regular photonic components. Here, the authors present a two-dimensional numerical simulation of a passive photonic element based on a saturable absorber material as an analogy to the biological neurons activation function. They demonstrate that at telecom wavelengths a highly confined SPP mode can be modulated in a nonlinear fashion by considering the carrier dynamics of graphene/PMMA polymeric composite as a two-level system. The sigmoid type nonlinear activation function derived from this study, further characterized and the parameters which appeared to be effective on the performance of the structure, such as saturation intensity, modulation depth, and thickness of the synaptic part as a Fabry-Perot structure, has been studied.