Variability, compensation and homeostasis in neuron and network function

Eve Marder and Jean-Marc Goaillard

Hebbian learning can be appropriately balanced by stability mechanisms that allow neurons and synaptic connections to be maintained in appropriate operating ranges (by Turrigiano and Nelson, various mechanisms including synaptic scaling and changes in individual ionic currents).

omeostatic tuning rules that maintain a constant activity pattern could, in principle, operate to tune conductances so that an individual neuron remains within a given region of parameter space, although its values for one or more conductances may be substantially
altered.

Variability in channel densities
How can we reconcile the apparent sensitivity of many neurons to rapid pharmacological treatments with new data indicating that individual neurons within a class can differ by as much as two- to fourfold in the densities of many of their currents?
Computational models show that a number of different compensating combinations of conductances can result in similar activity patterns38,51.

In contrast to pharmacological manipulations, slow mechanisms that function during development and over days and weeks can result in a set of compensating conductances that give rise to a target activity pattern.

Figure 2 | Neurons with similar intrinsic properties have different ratios of conductances.

Figure 3 | Comparison of short-term pharmacological manipulations and long-term genetic deletions.

Slow developmental and homeostatic mechanisms can ‘find’ multiple solutions of correlated
and compensating values of membrane conductances consistent with a given activity pattern, even while rapid pharmacological treatments that vary the value of one current at a time result in altered activity57.