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A major function of glial cells in the central nervous system is to buffer the extracellular potassium concentration, [K+]o. A local rise in [K+]o causes potassium ions to enter glial cells, which have membranes that are highly permeable to K+; potassium then leaves the glial cells at other locations where [K+]o has not risen. We report here the first study of the individual ion channels mediating potassium buffering by glial cells. The patch-clamp technique was employed to record single channel currents in Müller cells, the radial glia of the vertebrate retina. Those cells have 94% of their potassium conductance in an endfoot apposed to the vitreous humour, causing K+ released from active retinal neurones to be buffered preferentially to the vitreous. Recordings from patches of endfoot and cell body membrane show that a single type of inward-rectifying K+ channel mediates potassium buffering at both cell locations. The non-uniform density of K+ conductance is due to a non-uniform distribution of one type of K+ channel, rather than to the cell expressing high conductance channels at the endfoot and low conductance channels elsewhere on the cell.

BOOK REVIEWS a mathematically unsophisticated audience, and so mathematical arguments for the most part proceed formally or are kept, on an intuitive level. On occasion, however, the unavoidable amount of imprecision which such an approach entails is compounded through carelessness. For example, the adjective "continuous" is used in a number of objectionable ways, not the least of which is the definition of a "continuous signal" as "any function of a continuous variable." In a somewhat different category is the incorrect assertion made in a footnote on p. 48 to the effect that, a necessary condition for a differential equation to uniquely specify a system is that the system lie nonanticipative. Despite these critical comments, a reader armed with the appropriate mathematical caveats will find much of value in this book, which is likely to be successful as an introductory text on modern system theory.

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