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Abstract
During the past two decades several revisions of the concepts underlying interneuronal
communication in the central nervous system have been advanced. We propose here to
classify communicational phenomena between cells of the central neural tissue under
two general frames: "wiring" and "volume" transmission. "Wiring" transmission is defined
as intercellular communication occurring through a well-defined connecting structure.
Thus, wiring transmission is characterized by the presence of physically identifiable
communication channels within the neuronal and/or glial cell network. It includes
synaptic transmission but also other types of intercellular communication through
a connecting structure (e.g., gap junctions). "Volume" transmission is characterized
by signal diffusion in a three-dimensional fashion within the brain extracellular
fluid. Thus, multiple, structurally often not well characterized extracellular pathways
connect intercommunicating cells. Volume transmission includes short- (but larger
than synaptic cleft, i.e. about 20 nm) and long-distance diffusion of signals through
the extracellular and cerebrospinal fluid. It must be underlined that the definitions
of wiring and volume transmission focus on the modality of transmission and are neutral
with respect to the source and target of the transmission, as well as type of informational
substance transmitted. Therefore, any cell present in the neural tissue (neurons,
astroglia, microglia, ependyma, tanycytes, etc.) can be a source or a target of wiring
and volume transmission. In this paper we discuss the basic definitions and some distinctive
characteristics of the two types of transmission. In addition, we review the evidence
for different types of intercellular communication besides synaptic transmission in
the central nervous system during phylogeny, and in vertebrates in physiological and
pathological conditions.