Effect of membrane polarization and synaptic activity on the timing of antidromic invasion

The latency from the stimulus (S) to the IS and SD components of the antidromic spike was measured in motoneurones and spinocerebellar tract cells following displacement of the membrane potential either by current pulses or by synaptic potentials. Changes in the latency to the SD spike (S-SD delay) were mainly caused by changes in the IS-SD delay and varied from 10 to 100 musec per mV change in membrane potential, depending on the initial value of the IS-SD delay. Changes in the S-IS delay were also observed and these changes could, especially in spinocerebellar cells, give a significant contribution to the change in the total delay. EPSPs shortened the S-SD delay as efficiently as current-evoked depolarizations of similar magnitude while IPSPs were often more effective in prolonging the delay than current-evoked hyperpolarizations. This difference was related to the larger conductance increase during IPSPs than during IPSPs and to the longer IS-SD delays at hyperpolarized potentials. The presented data contribute to the understanding of the method which uses extracellular recording of antidromic latency changes as an indirect measure of intracellular membrane potential changes. Our results show that the recording of antidromic latency changes is a particularly sensitive method for detecting inhibition of neurones.