We analyze a four-dimensional slow-fast piecewise linear system consisting of two coupled McKean caricatures of the FitzHugh-Nagumo system. Each oscillator is a continuous slow-fast piecewise linear system with three zones of linearity. The coupling is one-way, that is, one subsystem evolves independently and is forcing the other subsystem. In contrast to the original FitzHugh-Nagumo system, we consider a negative slope of the linear nullcline in both the forcing and the forced system. In the forcing system, this lets us, by just changing one parameter, pass from a system having one equilibrium and a relaxation cycle to a system with three equilibria keeping the relaxation cycle. Thus, we can easily control the changes in the oscillation frequency of the forced system. The case with three equilibria and a linear slow nullcline is a new configuration of the McKean caricature, where the existence of the relaxation cycle was not studied previously. We also consider a negative slope of the y-nullcline in the forced system that enables us to reproduce a quasi-steady state called the surge. We analyze not only the qualitative behavior of the four-dimensional system, but also quantitative aspects such as the period, frequency, and amplitude of the oscillations. The system is used to reproduce all the features endowed in a former smooth model and reproduce the secretion pattern of the hypothalamic neurohormone GnRH along the ovarian cycle in different species.
Introduction.Piecewise linear (PWL) systems are a family of nonsmooth systems well known for reproducing the dynamics of models coming from applications. The first examples of PWL systems were developed for the modeling of engineering problems (such as mechanical, electronic, and control device problems) [1]. Since then, the applicability of these systems has been fully demonstrated. They have been used to model not only dynamical processes coming from engineering, but also, for instance, social behaviors and financial or biological problems [3,12,13,25]. The analysis of PWL systems revealed that they exhibit rich dynamics as smooth systems, in particular limit cycles and periodic orbits [15,17], homoclinic and heteroclinic connections [5], and strange attractors [25].The celebrated FitzHugh-Nagumo system [14,27] models, in first approximation, the behavior of an excitable system, for example, a neuron. Basically, it is assumed that a neuron behaves as an electronic circuit. The van der Pol oscillator [2] can be considered as a special