Introduction

Imagine 1015 neurons - all wired and interconnected through synapses into one motherboard - the brain, which in turn determines all that we do throughout our lives. Imagine also all those synaptic connections in different areas of this organ, being concurrently active, either transmitting or receiving information in a highly ordered manner. Also imagine the immaculate orchestration that would be required to connect this organ appropriately and the dire consequences if this wiring were to

'Departments of Anatomy and Physiology, Health Sciences Center, 3330 Hospital Dr. N.W., Calgary, AB, Canada T2N 4N1; [email protected]

go haywire. Considering all this, it is then not so difficult to envisage challenges confronting our resolve to break the connectivity code of mammalian nervous system - let alone understand its functionality. The sheer numbers of mammalian neurons, their smaller sizes, and the rate at which the synaptic connectivity proceeds, all render it rather impossible to study synapse formation between defined sets of pre- and postsynaptic neurons. Invertebrates, on the other hand, and particularly mollusks, are endowed with relatively simple nervous systems, consisting of some 20,000-30,000 neurons. The molluscan neurons are often behaviorally well defined and have large somata that are readily identifiable on the basis of their size (50 p.m-1 mm), position, and coloration. Moreover, injured adult molluscan neurons have the innate propensity to recapitulate their developmental patterns of connectivity with remarkable accuracy 1. Although the snail models leg behind their fly and worm counterparts regarding the genetic know-how of synaptic connectivity, they are nevertheless amenable to direct electrophysio-logical analysis at the level of single pre- and postsynaptic neurons.

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