Direct and indirect activation of nerve cells by electrical pulses applied extracellularly.

Abstract

The mode of activation of nerve cells by extracellular stimuli was investigated while recording from a selected cell with 1 electrode and applying current pulses around this cell with another electrode. The analysis was done on motoneurons and on spinal border cells from lower lumbar segments in the cat. Directly evoked action potentials were defined by their appearance in an all-or-none fashion with stable latencies of less than 0.5 ms. The lowest thresholds for their generation were 0.15-0.20 .mu.A in the spinal border cells and 0.35-0.40 .mu.A in the motoneurons. In the main series on motoneurons a correlation was established between different positions of the extracellular stimulating electrode in relation to the cells and the thresholds for the direct excitation of these cells. The positions of the electrode were defined on the basis of an analysis of the IS [initial segment] and SD [somato-dendritic] components of the action potentials recorded extracellularly around the cell when evoked by current pulses applied through the intracellular electrode; both the amplitudes of these IS and SD components and their timing with the IS and SD spikes, which were simultaneously recorded with the intracellular electrode, were then taken into account. The lowest thresholds (< 2 .mu.A) for the direct activation of cells were found nearest the initial segment of the axon. Their values increased to about 5 .mu.A at near-soma positions and to > 10 .mu.A at near-dendrites positions about 150 .mu.m away. Transsynaptically evoked action potentials which were clearly set up by the preceding EPSPs [excitatory post synaptic potentials] appeared with latencies > 0.7 ms. When single current pulses were used, the lowest thresholds for transsynaptic spike activation were usually > 5-10 .mu.A, but they considerably decreased with repetitive stimuli. These thresholds were higher than the thresholds for the direct activation of cells within the region of the initial segment, of the same order of magnitude near the soma, and lower when the stimulating electrode was nearer the dendrites than the soma and generally at all larger distances from the cells. All the observations on direct excitation of cells by extracellular stimuli (generation of the IS spike before the SD spike, lowest thresholds near the region of the initial segment of the axon, similar rates of increase in these thresholds with distance as for fibers) led to the conclusion that the effects of the extracellular stimuli were exerted primarily via spread of current to the initial segment of the axon and its depolarization. Late extracellular negativities presumably related to dendritic activation were observed in a few cells. These negativities were synchronous with late components of the intracellularly recorded action potentials.