Synaptic plasticity

The most fascinating and important property of the mam-malian brain is its remarkable plasticity, which can be thought of as the ability of experience to modify neural circuitry and thereby to modify future thought, behavior, and feeling. Thinking simplistically, neural activity can modify the behavior of neural circuits by one of three mech-anisms: (a) by modifying the strength or efficacy of synaptic transmission at preexisting synapses, (b) by eliciting the growth of new synaptic connections or the pruning away of existing ones, or (c) by modulating the excitability prop-erties of individual neurons. Synaptic plasticity refers to the first of these mechanisms, and for almost 100 years, activity-dependent changes in the efficacy of synaptic communica-tion have been proposed to play an important role in the remarkable capacity of the brain to translate transient expe-riences into seemingly infinite numbers of memories that can last for decades. Because of its fundamental importance, there has been an enormous amount of work describing the many forms of synaptic plasticity and their underlying mechanisms. Synaptic transmission can either be enhanced or de-pressed by activity, and these alterations span temporal do-mains ranging from milliseconds to enduring modifications that may persist for days or weeks and perhaps even longer. Transient forms of synaptic plasticity have been associated with short-term adaptations to sensory inputs, transient changes in behavioral states, and short-lasting forms of memory. More lasting changes are thought to play impor-tant roles in the construction of neural circuits during devel-opment and with long-term forms of memory in the mature nervous system. Given these diverse functions, it is not sur-prising that many forms andmechanisms of synaptic plastic-ity have been described. In this chapter, I provide a brief overview of some of the forms of synaptic plasticity found at excitatory synapses in the mammalian brain, focusing on long-term potentiation (LTP) and long-term depressio