Preface

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Abstract
In the thirty years since identified neurons became a subject of serious attention in neuroscience its accomplishments have become too diverse to be reviewed in any meaningful way in a single volume. The goal of this volume is to be a milestone, which both indicates the distance we have traveled and points out the long road ahead, without attempting to provide the detail of a road map or travel guide; John Fentress’s classic volume in 1976, ‘Simpler Networks and Behavior’ was perhaps the last real overview of the whole field. The four authors here (Theodore H. Bullock, James L. Larimer, Janet L. Leonard, and George J. Mpitsos) present individual views of the role of identifiable neurons in the organization and control of behavior. Since 1967, when Felix Strumwasser contributed a chapter entitled ‘Types of information stored in individual neurons’ to C.A.G. Wiersma’s classic volume, ‘Invertebrate Nervous Systems’ and identified eight types of information, it has become very clear that identifiable neurons (and probably all neurons) are much more flexible and multifunctional than we once assumed and/or hoped. As Ted Bullock’s overview points out, the last twenty years have seen a more or less geometric increase in the number of known neurotransmitters, neuromodulators, ion channels, second messengers, etc. As the arsenal of neurobiological techniques increases, so do the opportunities to identify neurons individually. Jim Larimer’s review of the classic system of ‘command neurons’, the abdominal positioning interneurons in the crayfish abdomen, shows that essentially all of the neurons involved are potentially identifiable and that understanding the neuronal control of behavior in this system requires consideration of large numbers of neurons and a statistical approach. This approach involves a novel application of techniques and statistics to neuroscience, such as mark-recapture studies and the Lincoln index, which are standard techniques in population biology but are used here to estimate the size of populations and functional groups of neurons involved in behavior. These techniques should have a wide application in neurobiology and will help identify the boundaries between ‘simple’ and more complex systems. In fact, one of the striking new developments over the last 20 years is a blurring of the lines between studies of ‘simple’ systems of identifiable neurons on the one hand and more complex neuronal networks involving large numbers of more or less anonymous neurons. Ted Bullock uses the term ‘addressable’ to describe both individually identifiable neurons and identifiable sets of neurons, and one wonders whether the study of identifiable neurons per se will remain a distinct subdiscipline. Just as Linnaeus’s theory of the ‘fixity’ of species was a necessary precursor to Darwin’s theory of the mutability of species, so the concept of the invariant neuron and the fixity of circuits, along with the existence of identifiable neurons, has made it possible to document the intrinsic variability of individual neurons and the dynamic interactions that form circuits.

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