Slow Action of Ca on Myelinated Nerve Fibres of Xenopus laevis

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Abstract
Single myelinated nerve fibers were isolated and the nodal currents were recorded with potential clamp technique. Rapid solution changes were performed by use of a recording chamber so shaped that it had a minimal dead space. A volume 10 times larger than the dead space flowed past the node within about 20 ms. The half-time of the change in Na+ current (INa) amplitude associated with changes between solutions of different [Na] [Na+ concentration] was 100-150 ms, whereas the half-time of the change in INa associated with changes between solutions of different [Ca2+] was 200-300 ms. The external [Ca2+] affects INa through its action on the Na+ permeability properties of the nodal membrane. The difference in rate of effect between changes in [Na+] and [Ca2+] was also noticeable when simultaneous changes were performed. These findings were discussed with regard to the properties of the nodal gap.