Activity-dependent and age-dependent recruitment and regulation of synapses in identified Crustacean Neurones1

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Abstract
Adaptation of neural systems to altered activity and age often involves recruitment, inactivation or modification of synapses. Crustacean motor systems are amenable to experimental investigation of these processes. They possess large identifiable neurones that can be observed over long periods during development, adulthood, regeneration and degeneration. Numerous small individual synapses are present on the transmitting terminals of the motor neurones; their ultrastructural features are non-uniform, indicating different degrees of functional potency. Ultrastructural studies show many more individual synapses than required for maximal quantal output; probably some are ineffective, but can be brought into a transmitting state in a short time by neural activity. During development, progressive reorganization and relocation of synapses take place. As the size of a postsynaptic target changes, synapses are added, and functionally adaptive alterations in quantal content and quantal effectiveness occur. Sectioning an axon results in slow degeneration of distal processes, but transmission is sustained for months. Short-term adjustments in number of effective synapses occur in response to altered activity. If activity of a neurone is chronically increased or decreased, characteristic semi-permanent adaptations in physiology and ultrastructure are seen. Synaptic transmission at low frequencies is down-regulated, while resistance to synaptic depression increases. These effects require protein synthesis, and at least two different changes - one related to down-regulation of synapses, the other related to fatigue resistance - can be selectively demonstrated through critically timed interruption of axoplasmic transport or imposition of different patterns of neural activity. In older animals, the ability to adapt to activity is reduced in some neurones, but may be restored during regeneration of neural processes. Selective changes in activity in one of several neurones innervating a common postsynaptic target lead to adaptive changes in synaptic transmission of non-active neurones, indicating activity-mediated interaction and adjustment. Mechanisms of adaptation similar to those outlined here probably occur in nervous systems of other species.