Dynamical Systems in Neuroscience:

One-Dimensional Systems 79F(V)10 mV0.5 msExcited0Attractorruinsmembrane potential (mV)ExcitedV-40 mVslow transitionAttractorruinsFigure 3.28: Slow transition through the ghost of the resting state attractor in a corticalpyramidal neuron with I = 30 pA (the same neuron as in Fig. 3.15). Even thoughthe resting state has already disappeared, the function F (V ), and hence the rate ofchange, ˙V , is still small when V ≈ −46 mV.3.3.5 Slow transitionAll physical, chemical, and biological systems near saddle-node bifurcations possesscertain universal features that do not depend on particulars of the systems. Consequently,all neural systems near such a bifurcation share common neuro-computationalproperties, which we will discuss in detail in Chapter 7. Here we take a look at onesuch property — slow transition through the ruins (or ghost) of the rest state attractor,which is relevant to the dynamics of many neocortical neurons.In Fig. 3.28 we show the function F (V ) of the system (3.5) with I = 30 pA,which is greater than the bifurcation value 16 pA, and the corresponding behaviorof a cortical neuron; compare with Fig. 3.15. The system has only one attractor— the excited state, and any solution starting from an arbitrary initial conditionshould quickly approach this attractor. However, the solutions starting from the initialconditions around −50 mV do not seem to hurry. Instead, they slow down near −46mV and spend a considerable amount of time in the voltage range corresponding to theresting state, as if the state were still present. The closer is I to the bifurcation value,the more time the membrane potential spends in the neighborhood of the resting state.Obviously, such a slow transition cannot be explained by a slow activation of the inwardNa + current, since Na + activation in a cortical neuron is practically instantaneous.The slow transition occurs because the neuron or the system (3.5) in Fig. 3.28 isnear a saddle-node bifurcation. Even though I is greater than the bifurcation value,and the rest state attractor is already annihilated, the function F (V ) is barely abovethe V -axis at the “annihilation site”. In other words, the rest state attractor hasalready been ruined, but its “ruins” (or its “ghost”) can still be felt because˙V = F (V ) ≈ 0(at attractor ruins, V ≈ −46 mV),as one can see in Fig. 3.28. In Chapter 7 we will show how this property explains the