MEASUREMENT OF IMPOSED VOLTAGE GRADIENT ADEQUATE TO MODULATE NEURONAL FIRING

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Abstract
The voltage in the extracellular field across a single nerve cell was measured with microelectrodes during polarization with imposed current just sufficient to modify the frequency of firing of an already active cell, using a preparation from the nonadapting stretch receptor of the crayfish abdomen and another from the cardiac ganglion of the lobster. It is argued that, unless the current density can be determined across the strategic portion of the cell membrane, this is the most suitable measure of the intensity of imposed currents which can exert an effect on the probability of firing of a neuron. The transmembrane potential does not provide such measure. In the most effective axis of polarization, a voltage gradient in the neighborhood of 0.1 mv/100u markedly influenced active cells. Currents of more than 20 times this value are required to fire a silent cell, even if it has been poised, i.e., the adapting stretch. The actual value of the critical voltage drop across the essential structure can only be smaller than this, and it may be much smaller. The transmembrane potential change with polarization may be from 0 to nearly 0.5 this value and is thus, at most, several orders of magnitude smaller than the changes in membrane potential for threshold electrical stimulation or for several cases cited from the literature associated with alteration in activity or with inhibition or excitation. The findings may mean that exceedingly critical firing levels in voltage across the membrane exist which would have to be confined to a certain portion of the cell near one of the poles relative to the effective axis of polarization. Alternatively, it is supposed that the imposed current acts by creating or increasing a gradient between different portions of the cell membrane, not across it but between one point on the surface and another. These possibilities are not mutually exclusive, and they suggest 2 new or little-considered parameters of neuronal state critically significant in determining activity over and above other conditions. The great sensitivity of neurons to small voltage differences supports the view that electric field actions can play a role in the determination of probability of firing of units in ganglionic masses, in response to physiologically available currents. If true, this conclusion emphasizes the significance of morphology of the cell and of architectural arrangement of groups of neurons and their dendritic ramifications.