J. exp. Biol. 112, 129-145 (1984) 129 J*rinted in Great Britain © The Company of Biologists Limited 1984 RELATIONSHIPS BETWEEN NEURONAL STRUCTURE AND FUNCTION BY JOHN P. MILLER AND GWEN A. JACOBS Department of Zoology, University of California, Berkeley and Department of Biology, State University of New York, Albany, U.SA. SUMMARY The geometry and electrical properties of a neurone determine how synaptic inputs and endogenously generated currents are integrated and transformed into the signals it transmits to other cells. The dependence of neuronal integration upon dendritic geometry has been studied extensively over the last three decades, both by experimentalists and by theoreticians. We review some of the general principles that have emerged from this work, and summarize recent studies that serve to illustrate these principles. The discussion is organized around the analysis of neuronal structure at three different levels. At the 'macroscopic' level, we show how the dendritic branching structure of an identified interneurone in the cricket cereal af- ferent system determines the directional sensitivity within its receptive field. At the 'microscopic' level, we illustrate the dependence of synaptic efficacy upon dendritic length, and demonstrate a very surprising result: that the extension (or 'growth') of a dendrite out beyond the point of a synaptic contact can increase the efficacy of that synapse. At the 'ultra- structural' level, we show how the structural and electrical properties of dendritic spines might have profound effects upon synaptic integration. INTRODUCTION It is a fundamental principle of neurobiology that the function of a neurone is dependent upon its structure. The structure determines, to a large extent, how the synaptic inputs and endogenously generated currents of neurones are 'integrated' and transformed into a 'meaningful' output to other neurones, muscles or effector organs. This functional dependence upon neuronal structure is manifest at three levels within a nerve cell: the 'macroscopic', 'microscopic' and 'ultrastructural'. By 'macroscopic' structure, we mean the overall shape of a neurone, how many dendritic and axonal branches it has, and where in the nervous system those branches are located. By 'microscopic' structure, we mean the shape, length, diameter and branching points of the individual branches. By 'ultrastructure', we mean the shapes of the neurites near the points where synaptic contacts are located, the 'local' contours, dendritic spines and varicosities. The dependence of neuronal integration upon dendritic geometry at each of these three levels has been studied extensively over the last three decades, both analytically Key words: Synaptic efficacy, neuronal modelling, laser microsurgery.