The panglial syncytium maintains ionic conditions required for normal neuronal electrical activity in the central nervous system (CNS). Vital among these homeostatic functions is "potassium siphoning," a process originally proposed to explain astrocytic sequestration and long-distance disposal of K(+) released from unmyelinated axons during each action potential. Fundamentally different, more efficient processes are required in myelinated axons, where axonal K(+) efflux occurs exclusively beneath and enclosed within the myelin sheath, precluding direct sequestration of K(+) by nearby astrocytes. Molecular mechanisms for entry of excess K(+) and obligatorily-associated osmotic water from axons into innermost myelin are not well characterized, whereas at the output end, axonally-derived K(+) and associated osmotic water are known to be expelled by Kir4.1 and aquaporin-4 channels concentrated in astrocyte endfeet that surround capillaries and that form the glia limitans. Between myelin (input end) and astrocyte endfeet (output end) is a vast network of astrocyte "intermediaries" that are strongly inter-linked, including with myelin, by abundant gap junctions that disperse excess K(+) and water throughout the panglial syncytium, thereby greatly reducing K(+)-induced osmotic swelling of myelin. Here, I review original reports that established the concept of potassium siphoning in unmyelinated CNS axons, summarize recent revolutions in our understanding of K(+) efflux during axonal saltatory conduction, then describe additional components required by myelinated axons for a newly-described process of voltage-augmented "dynamic" potassium siphoning. If any of several molecular components of the panglial syncytium are compromised, K(+) siphoning is blocked, myelin is destroyed, and axonal saltatory conduction ceases. Thus, a common thread linking several CNS demyelinating diseases is the disruption of potassium siphoning/water transport within the panglial syncytium. Continued progress in molecular identification and subcellular mapping of glial ion and water channels will lead to a better understanding of demyelinating diseases of the CNS and to development of improved treatment regimens.
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