The strength-duration time constant of both cutaneous and motor afferents decreases with age, and this corresponds to an increase in rheobase. Two possible reasons for this age-related decrease in the strength-duration time constant have been proposed. First, nerve geometry might change with age because of axonal loss and neural fibrosis. Secondly, the persistent Na+ conductance might decrease maturation. Significant decreases in threshold for sensory and motor fibers have been observed during ischemia. These decreases in threshold were furthermore associated with significant increases in the strength-duration time constant, appreciably indicating a significant decrease in rheobase current. These changes are thought to be the result of non-inactivating, voltage-dependent Na+ channels, which are active at resting potential.