Fig. 1. Examples of plasticity in intrinsic neuronal excitability. (A) Diagram illustrating the concept of potentiation of intrinsic excitability. The circles represent two different neurons connected by an excitatory synapse (triangle). In the case of potentiation of excitability, one form of plasticity in intrinsic properties of neurons, activity in the pre-synaptic neuron potentiates the connection with the post-synaptic neuron. This potentiation is the result of an increase in the overall excitability of the post-synaptic neuron independent of changes in synaptic strength. One possible mechanism for this change in excitability is a decrease in action potential threshold. The result of this increased excitability is an increase in the input-output function of the postsynaptic cell: more spikes are elicited post-synaptically for the same amount of pre-synaptic input. This results in a de facto increase in synaptic strength mediated by changes in the intrinsic excitability of the post-synaptic neuron. Examples of this kind of plasticity have recently been published (Cudmore and Turrigiano, 2004; Li et al., 2004; Xu et al., 2005). (B) Diagram illustrating the concept of compensatory changes in excitability leading to conserved neuronal output. In this case, the post-synaptic neuron maintains a constant level of spiking activity as a result of compensatory decreases in excitability triggered by increased pre-synaptic activity. The outcome is a constantly maintained level of spiking activity in the post-synaptic cell, which could be the result of an increased action potential threshold, for example. For examples of this kind of plasticity, see recent publications (Aptowicz et al., 2004; Brickley et al., 2001; van Welie et al., 2004).