The ease with which a membrane can be stimulated depends on two factors: the strength of a stimulus and the duration for which the stimulus is applied. As the strength of stimulus increases, the time required to excite the membrane decreases and vice versa. Any index of membrane excitability must therefore take into account both the above factors. Three such indices of membrane excitability are rheobase, utilization time and chronaxie.
Rheobase is the theoretical minimum current of electrical stimulus which when applied for an infinite length of time will excite the membrane. Utilization time is the time taken by a stimulus of rheobase strength to excite the membrane. Chronaxie is the time required to stimulate the membrane using double the rheobase strength of stimulus.
Chronaxie is a practical index of membrane excitability. Its value ranges from 0.02 ms in the largest peripheral neurons (Aa) to a maximum of 1.5 ms in the smallest C-fibers. A low chronaxie means greater membrane excitability. Serial measurements of chronaxie provide useful information regarding the progress of nerve healing following injury and surgical repair.