Hebb and homeostasis in neuronal plasticity

The positive-feedback nature of Hebbian plasticity can destabilize the prop erties of neuronal networks. Recent work has demonstrated that this destabi lizing influence is counteracted by a number of homeostatic plasticity mech anisms that stabilize neuronal activity. Such mechanisms include global cha nges in synaptic strengths, changes in neuronal excitability, and the regul ation of synapse number. These recent studies suggest that Hebbian and home ostatic plasticity often target the same molecular substrates, and have opp osing effects on synaptic or neuronal properties. These advances significan tly broaden our framework for understanding the effects of activity on syna ptic function and neuronal excitability.