Development of Na inactivation in motor and sensory myelinated nerve fibres of Rana esculenta.

Development of Na permeability inactivation was investigated in myelinated motor (N = 12) and sensory (N = 12) nerve fibres of Rana esculenta at 20 degrees C. The K currents were blocked by 10 mM tetraethylammonium chloride, added to the superfusing solution. Additionally, in 4 fibres of each group internal CsCl was applied by diffusion from the cut internodes. Development of Na inactivation was approximated by the sum of two exponentials. The time constants of the fast and slow inactivation phase (tau h1 and tau h2) were dependent on membrane potential (E) with similar values in both fibre types for a given E. In contrast, significant differences were found in the contribution of both phases. In motor nerve fibres the amplitude of the fast phase was g = 0.70 (mean value of 12 fibres) throughout the potential range investigated (-30 mV less than or equal to E less than or equal to 40 mV). In sensory fibres g was potential dependent, increasing from 0.76 (mean value of 12 fibres) at E = -30 mV to unity at E greater than 30 mV. This difference in Na permeability inactivation is a further distinguishing property between motor and sensory nerve fibres.