Intraneuronal mutability: implications for memory mechanisms.

Existing theories of learning and memory focus on the generation of functional associations among neuronal aggregates, while largely ignoring molecular plasticity within single neurons. However, recent work indicates that individual neurons are profoundly plastic, altering neurotransmitter function with altered environmental stimuli. Consequently, concepts of network plasticity must now be complemented by concepts of neuronal plasticity. It is formally proposed that many transmitter regulatory molecules function as symbols, encoding environmental information by transducing brief extracellular events into long-term alteration of function. The molecular mechanisms underlying this memorial process are described in detail. The model defines an intraneuronal, molecular representational system that interconnects several levels of function, from environmental event to neuronal alteration to physiologic response and behavior. Symbol manipulation in this system is semantically as well as syntactically functional.