HETEROGENEITY OF MOTOR-NEURON DRIVER POTENTIAL PROPERTIES ALONG THE ANTERIOR-POSTERIOR AXIS OF THE LOBSTER CARDIAC GANGLION

Endogenous long-duration burst-organizing potentials (driver potentials, DPs) generated by neurons in the lobster cardiac ganglion play a critical role in the ability of the system to generate rhythmic bursts of nerve impulses. The DPs are normally terminated by a voltage-dependent potassium current, but when this is suppressed by tetraethylammonium ion (TEA), the five motorneurons in the system show heterogeneity in properties. Perfusion with TEA increases DP amplitude in all cells in a similar fashion, but it increases DP duration disproportionately in the most anterior motorneurons. Substitution of barium ions for calcium also prolongs DPs in all of the motorneurons, but the effect of this treatment is not different along the anterior-posterior axis of the ganglion. The results suggest that the different behavior of the neurons reflects a difference in either the extent of calcium inactivation of the calcium current responsible for the DP, or in the kinetics or magnitude of a calcium-activated potassium conductance which contributes to membrane repolarizaton. In contrast to previously reported results, reduction in extracellular sodium ion concentration decreases driver potential amplitude and/or duration. This effect is not differentially expressed in different motorneurons.