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.