The major advantages and disadvantages of the autoradiographic method for tracing pathways in the central nervous system have become clear in the decade since its introduction. Attention is focused here on two major problems associated with the interpretation of autoradiographic experiments. First, it is often difficult to determine the effective size of an injection site, which may be different for different projections in the same experiment. Quantitative evidence presented here also shows that the apparent size of injection sites in the hippocampus decreases considerably in diameter 1 to 2 weeks after the injections are made. And second, the morphology of labeled pathways must be inferred from a pattern of silver grains lying over the tissue section. Several examples are used to illustrate the point that the autoradiographic method should be used in conjunction with retrograde transport, immunohistochemical, and electron microscopic techniques to investigate the origin, course, and synaptic relationships of individual pathways in the brain and spinal cord.
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