Models like the Hodgkin-Huxley and Morris-Lecar include a current term: applied current.
From what I understand, experimentally, this is the injected current, applied by the experimenter. But what is it in nature? This term can't be 0, or it must be replaced by a non zero term for the neuron model to remain excitatory or oscillatory (excitable vs. pacemaker cells, for example).
This is highlighted when we couple neurons together:
Bifurcations in a synaptically coupled Morris-Lecar neuron model
Rajesh G Kavasseri
Where the applied current is the bifurcation parameter (Tsumoto treats it the same in Bifurcations of the Morris Lecar neuron model
But in a large network of say, 100 or so neurons, where we must have a significant current term applied to each neuron, it seems kind of silly to motivate from the point of view the experimenter injecting the current into each neuron.
So what, in nature, provides these currents? In the paper by Kavasseri, the synaptic coupling term is applied in addition to the applied current (as it should, the applied current is a steady state here, the coupling term is more of an impulse, a perturbation) so it's not from the coupling according to this treatment.
Are intrinsic, passive or global currents involved? What replaces "I applied" (aka "I external") in the system in nature? I'm also not counting perturbations caused by a stimulus. These currents must be more-or-less steady-state in comparison to the propagation of action potentials and significantly above 0 for the mathematical model to do what it's supposed to.