ABSTRACT
The complex functions of the nervous system depend upon both the specificity and plasticity of the synaptic connections that constitute the communication links in neural circuitry. How synaptic connections are specified during assembly of the nervous system and how they are later modified to bring about learning and memory are among the most fascinating problems in neurobiology. Specificity during the early stages of neural circuit assembly, including path finding by growth cones and axon fasciculation, appears to depend upon guidance cues encoded in cell surface proteins that differentially label axon pathways. It is possible that synaptic specificity arises solely from selective fasciculation by the guidance of neurons to the appropriate targets and that further positional or identification cues are unnecessary. Yet, different subsets of similar neurons that follow a common pathway and innervate a common target may still demonstrate specific differences in their synapses.
