The Journal of Neurophysiology Vol. 77 No. 3 March 1997, pp. 1470-1486
Copyright ©1997 The American Physiological Society

Contribution of Voltage-Dependent Potassium Channels to the Somatic Shunt in Neck Motoneurons of the Cat

D. M. Campbell and P. K. Rose

Medical Research Council Group in Sensory-Motor Neuroscience, Department of Physiology, Queen's University, Kingston, Ontario K7L 3N6, Canada

Campbell, D. M. and P. K. Rose. Contribution of voltage-dependent potassium channels to the somatic shunt in neck motoneurons of the cat. J. Neurophysiol. 77: 1470-1486, 1997. The specific membrane resistivity of motoneurons at or near the soma (R_ms) is much lower than the specific membrane resistivity of the dendritic tree (R_md). The goal of the present experiments was to investigate the contribution of tonically active voltage-dependent potassium channels at or near the soma to the low R_ms. These channels were blocked with the use of intracellular injections of cesium. Input resistance (R_N), R_ms/R_md, a conductance representing voltage-dependent potassium channels on the soma, G_K, and a conductance attributed to damage caused by electrode impalement, G_Da, were estimated from voltage responses to a step of current. The effect of intracellular injections of cesium on electrotonic structure was determined with the use of two strategies: 1) a population analysis that compared data from two groups of motoneurons, those recorded with potassium acetate electrodes (control group) and those recorded with cesium acetate electrodes after the motoneurons were loaded with cesium; and 2) an analysis of changes in electrotonic structure that occurred over the course of multiple injections of cesium or during similar periods of time in control cells. R_N of control and cesium-loaded motoneurons was similar. Over the course of the recordings, R_N of control cells usually increased and this increase was associated with a hyperpolarization. In contrast, increases in R_N after successive injections of cesium were closely linked to a depolarization. At corresponding membrane potentials, R_ms/R_md of cesium-loaded motoneurons was greater than R_ms/R_md of control motoneurons. Over the course of cesium injections, R_ms/R_md increased and the membrane potential depolarized. In contrast, increases in R_ms/R_md observed during the course of recordings from control cells were associated with a hyperpolarization. Compared with control cells at corresponding membrane potentials, G_K was less in cesium-loaded cells. Increases in G_K that occurred over the course of recordings in control cells were closely coupled to a depolarization. In cesium-loaded cells, G_K usually decreased as the cell depolarized during the injections. In control cells, increases in G_Da that occurred during the recording period were closely coupled to a depolarization. In contrast, changes in G_Da that occurred during cesium injections were not related to the change in membrane potential and did not explain the corresponding changes in R_ms/R_md and membrane potential. The results of this study indicate that voltage-dependent potassium channels contribute to the somatic shunt (low R_ms) in neck motoneurons and provide a voltage-dependent mechanism for the dynamic regulation of motoneuron electrotonic properties.

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