observed with a number of model systems, such as development, LTP, environmental enrichment, NMDA administration, and adult reactive synaptogenesis. Modifications in synaptic structure as outlined in this chapter would markedly alter synaptic physiology, allowing, as Hebb suggested, a more efficacious future transmission across the synapse. This process of synaptic morphological plasticity is presumed to underlie processes such as learning and memory.
As pointed out earlier, the storage of information likely involves interactions within a complex network of synaptic contacts, including a probable balance between different synaptic types, such as excitatory and inhibitory synapses. Therefore, the process outlined in this chapter is, at best, a description of events occurring only in certain synaptic populations within a larger complex system. Such a system could contain synapses undergoing different or opposing processes to those described.
It is also important to realize that synaptic activation may not necessarily proceed through all three phases outlined here, to the final production of complex perforated synapses or new synapses and dendritic material. It would seem reasonable that the extent of the morphological changes would be relative to the degree of synaptic activation. It might be possible, for example, for certain levels of stimulation to induce changes only of Phase I; because these events involve only transient shape changes, the record, either electrophysiologically or behaviorally, would be short-lived. Greater levels of activation may be necessary to induce the changes in Phase II, or ultimately Phase III, which would translate into progressively larger and more permanent morphological changes, and therefore more permanent electrophysiological or behavioral records of the events.
It is tempting to correlate the stages of synaptic efficacy change with the temporal components of memory. For example, immediate or short- term memory might involve the earlier phases of synaptic modification, whereas long-term memory storage would require the more permanent later phases of synaptic structural changes. Alterations in postsynaptic shape could be rapidly produced and short-lived, and as such could form an excellent system for the short-term storage of information. The formation of the very large perforated synapses, new synapses, dendritic spines, and dendritic length would represent a more substantial, less rapidly reversed, investment of cellular resources, forming an ideal basis for the longer term storage of memories.
Adinolfi A. M. ( 1972a). "Morphogenesis of synaptic junctions in layers I and II of the somatic sensory cortex". Experimental Neurology, 34, 372-383.
Adinolfi A. M. ( 1972b). "The organization of paramembranous densities during postnatal maturation of synaptic junctions in the cerebral cortex". Experimental Neurology, 34, 383-393.