Many studies have suggested that differences in the expression of threshold channels could account for the sensory-motor differences in strength-duration time constant. The differences in strength-duration time constant and rheobase of normal sensory and motor axons are thought to reflect differences in expression of a persistent Na+ conductance. Additionally, sensory axons accommodate more to long-lasting hyperpolarizing currents than do motor axons, suggesting a greater expression of the hyperpolarization-activated inward rectifier channels. Finally, the electrogenic Na+/K+-ATPase is more active in sensory nerves, which have a greater dependence on this pump to maintain resting membrane potential than do motor nerves.