AXONS, BUT NOT CELL-BODIES, ARE ACTIVATED BY ELECTRICAL-STIMULATION IN CORTICAL GRAY-MATTER I - EVIDENCE FROM CHRONAXIE MEASUREMENTS

Extracellular electrical stimulation of the gray matter is often used to determine the function of a given cortical area or pathway. However , when it is used to elicit postsynaptic effects, the presynaptic neur onal elements activated by electrical stimulation have never been clea rly identified: it could be the excitable dendrites, the cell body, th e axon initial segment, or the axonal branches. To identify these elem ents, we performed two series of experiments on slices of rat visual c ortex maintained in vitro. The first series of experiments, reported i n this paper, was aimed at determining the chronaxie, a temporal param eter related to the membrane properties of the neuronal elements. In o rder to identify the presynaptic elements that were activated by extra cellular electrical stimulation, chronaxies corresponding to postsynap tic responses were measured and compared with those corresponding to t he activation of axons (antidromic activation) and those corresponding to the activation of cell bodies (intracellular current injection in intracellularly recorded neurons). The chronaxie for orthodromic activ ation was similar to that for axonal activation, but was 40 times smal ler than the chronaxie for direct cell body activation. This suggests that, whenever a postsynaptic response is elicited after electrical st imulation of the cortical gray matter, axons (either axonal branches o r axon initial segments), but not cell bodies, are the neuronal elemen ts activated.