Sleep and dynamic stabilization of neural circuitry: a review and synthesis

A common mechanism is advanced for the lengthy stabilization of neural circuitry encoding information of both hereditary and experimental origin. Stabilization is proposed to occur through the following means and interrelationships. Synaptic function is intrinsically plastic because of greatly restricted entry of essential, relatively short-lived molecules into synaptic terminals. Alterations that accompany synaptic transmission transiently facilitate this entry ("facilitated entry"). Synaptic efficacy is enhanced as the concentration of these molecules increases following a transmission event but subsequently declines if depletion of the molecules occurs without commensurate replacement. Accordingly, if lengthy persistence of information encoded by enhancements of synaptic efficacy is to be achieved, the enhancements must be reinforced repeatedly by synaptic transmission ("dynamic stabilization"). Synapses of circuits not in frequent functional use are thought to be dynamically stabilized by spontaneous, internally generated, "non-utilitarian" excitations occurring primarily during rest or sleep. In species with complex, highly developed brains, requirements for dynamic stabilization of infrequently used circuits apparently cannot be met during rest, a restriction that may underlie the origin of sleep. Dynamic stabilization of infrequently used motor circuits of endotherms appears to occur predominantly during REM sleep.