Stability of the internal environment in which neuronal elements are situated is unquestionably an important prerequisite for the effective transmission of information in the nervous system. During the past decade our knowledge on the microenvironment of nerve cells has expanded. The conception that the microenvironment of neurones comprises a fluid with a relatively simple and stable composition is no longer accepted; the microenvironment is now envisaged as a dynamic structure whose composition, shape, and volume changes, thereby significantly influencing neuronal function and the trans­ mission of information in the nervous system. The modern conception of the neuronal microenvironment is based on the results of research over the last 20 years. The extracellular space (ECS) is comprehended not only as a relatively stable microenvironment containing neurones and glial cells (Bernard 1878), but also as a channel for communica­ tion between them. The close proximity of the neuronal elements in the CNS and the narrowness of the intercellular spaces provides a basis not only for interaction between the elements themselves, but also between the elements and their microenvironment. Substances which can cross the cell membranes can easily find their way through the microenvironment to adjacent cellular elements. In this way the microenvironment can assure non-synaptic com­ munication between the relevant neurones. Signalization can be coded by modulation of the chemical composition of the ECS in the vicinity of the cell membrane and does not require classic connection by axones, dendrites, and synapses.

1 Introduction.- 2 Ion-Selective Microelectrodes.- 3 K+ Homeostasis in the ECS.- 3.1 Stability of K+ in the Extracellular Fluid.- 3.2 Sources of [K+]e Increases.- 3.3 Redistribution of Extracellularly Accumulated K+.- 4 Dynamic [K+]e Changes.- 4.1 Dynamic [K+]e Changes in the Spinal Cord...- 4.2 Dynamic [K+]e Changes in the Brain.- 4.3 Functional Significance of [K+]e Changes in the CNS.- 4.4 Dynamic K+ Changes in the Organ of Corti.- 4.5 Changes in K+ Concentration in the Retina.- 5 Dynamic Changes in Extracellular Na+, Cl-, and Ca2+ Concentration.- 5.1 Changes Induced in Resting [Ca2+]e During Stimulation of Afferent Input.- 5.2 [Ca2+]e Changes in Pathological States.- 5.3 Functional Significance of Dynamic [Ca2+]e Changes.- 6 Dynamic pHe Changes.- 6.1 Extracellular Buffering Power.- 6.2 Activity-Related Dynamic pHe Changes in Nervous Tissue.- 6.3 Mechanisms of pHe Changes in the CNS.- 6.4 Role of Glial Cells in pHe Homeostasis.- 6.5 pHe Changes in the Retina.- 6.6 pHe Changes During Anoxia, Ischaemia, Epilepsy, and SD.- 6.7 Functional Significance of pHe Changes.- 7 Dynamic Changes in Size of the ECS.- 7.1 Measurement of Changes in Size of the ECS by Means of K+-ISMs.- 7.2 Changes Induced in Size of the ECS by Electrical Stimulation.- 7.3 Changes Induced in Size of the ECS by Adequate Stimulation.- 7.4 Mechanisms of Dynamic Changes in Size of the ECS.- 7.5 Functional Significance of Dynamic Volume Changes in the Microenvironment of Nerve Cells.- 8 Conclusion.- References.