GK(Ca)-dependent cyclic potential changes in the sensory nerve terminal of frog muscle spindle

Spontaneous cyclic hyperpolarizations along the sensory nerve terminal of frog muscle spindles were observed during the application of 1-9 nA depolarizing currents across an air-gap on which the axon was bridged. An increase in the current intensity increased the amplitude and duration of the cyclic changes. Upon subthreshold depolarization, single or repetitive hyperpolarizations could be elicited after a brief electric pulse or during stretch of the receptors, respectively. The threshold was decreased in higher Ca2+, Sr2+ or Ba2+ solutions. The cyclic changes were reversibly blocked by K+- or Ca2+-blockers and quinine. These results suggest that the changes are due to GK(Ca). The site of origin of the changes was at the branching node in the capsule, as confirmed by the following results: (1) the cyclic changes were abolished upon inactivating the node by UV-irradiation; (2) in normal Ringer's solution, the rate of afferent impulses, which reflects the membrane potential at the encoding site along the non-myelinated filaments, was unmodified by the cyclic changes and was independent of the intensity of the polarizing currents within a certain range; however, it was sensitively dependent on this intensity after treatment with K+-blockers; (3) the amplitude of the impulses reaching the branching node was attenuated during the cyclic changes, but not after GK-blockade.