Non-Associative Learning and Memory

Abstract

This chapter discusses the biochemical reaction underlying learning and memory by highlighting specific, well-documented examples from studies of sensitization in Aplysia californica. The types of biochemical reactions underlying information storage fall into three general classes: short-term changes, intermediate- and long-term changes, and lifelong changes. Short-term changes are mediated by the presence of extracellular or intracellular messenger molecules and are subject to fairly rapid removal due to specific breakdown or clearance mechanisms. The prototype example is the acute action of a neurotransmitter on cellular biophysical or synaptic properties. In this phase the memory trace resides in the continued presence of the stimulus. The duration of the memory is dependent on ongoing production of the signal, for example, the continued release of neurotransmitter into the synaptic cleft. The generation of the persisting species may be produced by direct covalent modification of a preexisting molecule, the triggering of an enzymatic modification of a preexisting molecule, increased synthesis of an active enzyme, or altered gene expression resulting in enzyme activation or synthesis. The mnemogenic reaction could be triggered by the transient signal of Category 1, the persisting signal of Category 2, or by a distinct and parallel mechanism. The mnemogenic reaction, being self-perpetuating, does not have a half-life in the normal sense, but potentially can be reversed by a specific triggered mechanism. The activated mnemogenic species maintains the memory trace and results in the expression of memory through impinging on some biophysical, metabolic, or structural neuronal component.