Journal club: Mechanisms of oscillation in dynamic clamp constructed two-cell half-center circuits

A. A. Sharp, F. K. Skinner, E. Marder
Journal of Neurophysiology Published 1 August 1996 Vol. 76 no. 2, 867-883

Dynamic clamping is merely current injection triggered by presynaptic voltage. The amount of current is determined mainly by postsynaptic voltage. There is nothing magical.

In this paper, the authors studied how systematic alterations in intrinsic and synaptic parameters affected the network behavior by using their newly-developed "dynamic clamping" on a pair of the gastric mill motor neurons, GMs. They isolated GMs by blocking synaptic transmission with picrotoxin and created reciprocally inhibitory two-cell circuits by the dynamic clamp.

The author demonstrated in this system that there was no bursting without the hyperpolarization-activated inward current (I_H). In the presence of additional I_H, a variety of circuit dynamics, including stable half-center oscillatory activity, was produced. The increase in synaptic conductance increased the burst period, whereas the increase in IH conductance reduced it. Discussion went on about synaptic threshold, saying that changes in the synaptic threshold might play a large role in turning on and off bursting activity. However,  these discussion are not so informative to others because nonspiking synapses are rarely seen other than crustaceans.

The authors often stated that they "depolarized" or "hyperpolarized" the thoreshold for synapse. This is wrong. The threshold does not "polarize." Membrane potential does. They should have stated that the threshold was changed to more depolarized or hyperpolarized levels.

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.