Is behavior or morphology a more sensitive indicator of central nervous system toxicity?

Both behavior and morphology can be altered by exposure of the CNS to toxic substances. The brain is an organ with considerable structural redundancy and this presumably accounts for some of the ability of the CNS to maintain normal function in the presence of some structural damage. Compensation for damage may also occur through a form of "learning" due to the biochemical and morphological plasticity of the CNS. Examples of these kinds of compensation are enzyme induction and axonal sprouting. Compensatory changes such as these are likely to require days or weeks to develop. On the other hand, short-term, reversible effects of substances such as drugs are not likely to cause morphological changes at doses which affect behavior. The importance of appropriate quantitative data on both morphology and behavior in evaluation of the CNS toxicity of substances is evident.