ON THE ACTIONS THAT ONE NERVE-CELL CAN HAVE ON ANOTHER - DISTINGUISHING DRIVERS FROM MODULATORS

When one nerve cell acts on another, its postsynaptic effect can vary greatly. In sensory systems, inputs from ''drivers'' can be differenti ated from those of ''modulators.'' The driver can be identified as the transmitter of receptive field properties; the modulator can be ident ified as altering the probability of certain aspects of that transmiss ion. Where receptive fields are not available, the distinction is more difficult and currently is undefined. We use the visual pathways, par ticularly the thalamic geniculate relay for which much relevant eviden ce is available, to explore ways in which drivers can be distinguished from modulators. The extent to which the distinction may apply first to other parts of the thalamus and then, possibly, to other parts of t he brain is considered. We suggest the following distinctions: Cross-c orrelograms from driver inputs have sharper peaks than those from modu lators; there are likely to be few drivers but many modulators for any one cell; and drivers are likely to act only through ionotropic recep tors having a fast postsynaptic effect whereas modulators also are lik ely to activate metabotropic receptors having a slow and prolonged pos tsynaptic effect.