WIRING AND VOLUME TRANSMISSION IN THE CENTRAL-NERVOUS-SYSTEM - THE CONCEPT OF CLOSED AND OPEN SYNAPSES

During the past two decades, several revisions of the concepts underly ing interneuronal communication in the central nervous system (CNS) ha ve been advanced. Our group has proposed to classify intercellular com munication in the CNS under two general frames: 'wiring' (WT) and 'vol ume' transmission (VT). WT is characterized by a single 'transmission channel' made by cellular (neuronal or glial) structures acid with a r egion of discontinuity not larger than a synaptic cleft. VT is charact erized by the diffusion from a cell source (neuronal or glial) of chem ical and electrical signals in the extracellular fluid (ECF) for a dis tance larger than the synaptic cleft. Based on morphological and funct ional characteristics, and in light of the distinction proposed, six m ain modes of intercellular communication can be recognized in the CNS: gap-junction, membrane juxtaposition, and closed synapse (which repre sent WT-type modes of communication); open synapse, paracrine transmis sion and endocrine-like transmission (which represent VT-type modes of communication). Closed and open synapses are distinguished on the bas is of the sealing of the signal within or the leakage of the signal ou tside the synapse, Intra-synaptic restriction pr extra-synaptic diffus ion of transmitters are insured by a number of anatomical arrangements (e.g. glial ensheathment of synapse, size of the synaptic cleft) and functional mechanisms (e.g. density and location of transmitter re-upt ake sites and metabolic enzymes). Some central synapses can switch fro m closed to open state and vice versa, e.g. by changing the amount of transmitter released. Finally, a synapse containing several transmitte rs can work as an open synapse for one transmitter and as a dosed syna pse for another. Copyright (C) 1996 Elsevier Science Ltd.