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OBJECTIVES

Objectives

After studying this chapter, the student should be able to:

  • Define neuroplasticity and describe Hebbian plasticity.

  • Distinguish between short-term and long-term plasticity and describe the general features of each.

  • Diagram the main neuronal populations and synaptic connections in the hippocampus.

  • Identify the phases and parts of long-term potentiation (LTP) and long-term depression (LTD) and describe the features of each.

  • Describe the biochemical and molecular mechanisms underlying early and late LTP.

  • Define a silent synapse and how it becomes unsilenced.

  • Diagram the synaptic morphologic changes during LTP and LTD.

  • Outline the mechanisms of cerebellar and hippocampal LTD.

  • Identify other regions where plasticity has been documented, and distinguish the type of memory proposed for each.

OVERVIEW OF NEUROPLASTICITY

Neuronal circuits are considered the primary mediators of the brain’s diverse and varied functional abilities. Defined as groups of interconnected neurons or networks of interconnected brain regions, neuronal circuits execute specific brain functions and behavior and are responsible for integrating information and performing complicated cognitive tasks. During development and throughout life, neuronal circuits form and reorganize by a process called neuroplasticity. Neuroplasticity is proposed to be the process by which permanent learning and memory take place in the brain and also enables the brain to recover from injury and disorders. In prenatal and postnatal developmental periods, neuroplasticity involves the differentiation of new neurons from progenitor cells, axonal and dendritic outgrowth, and the formation, pruning, and reorganization of synapses. In the adolescent and adult brain, neuroplasticity involves the ability of the brain to form and reorganize synaptic connections (called synaptic plasticity) and modulate excitability, especially in response to experience or following injury. It is now clear that modifications to neural circuits are ongoing, as existing circuits are structurally and functionally remodeled in response to experience throughout the life span of an organism.

Three main categories of synaptic plasticity mechanisms are considered candidate processes that mediate or contribute to changes in neuronal circuits during development and in the adult brain. One well-studied category, referred to as Hebbian or associative plasticity, is an activity-dependent and synapse-specific change in the synaptic response; furthermore, it includes both short- and long-term mechanisms of plasticity. Another more recently characterized category is homeostatic plasticity, a type of synaptic plasticity that is not synapse specific but involves more global and compensatory changes in neuronal excitability and synaptic responses that act to stabilize neuronal and circuit activity. A third category was defined by the discovery that synaptic plasticity is itself modifiable; that is, synaptic plasticity is plastic. Referred to as metaplasticity, synaptic plasticity can be governed by and adapt as a function of the previous activity of the postsynaptic neuron or neural network. This chapter will focus mainly on Hebbian plasticity because research over 4 decades has provided a wealth of information about its physiologic features, functions, and underlying molecular mechanisms. Understanding how all 3 categories of synaptic plasticity cooperate to produce the important changes ...

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