Determination of the chronaxie and rheobase of denervated limb muscles in conscious rabbits

Measurements of the rheobase and chronaxie can be used to define the excitability of nerves and muscles. The aim of this study was to obtain a record over many weeks of changes in the rheobase and chronaxie of denervated rabbit tibialis anterior muscle (TA). A custom-built electronic stimulator was implanted into the peritoneal cavity of New Zealand White rabbits. Large stainless steel electrodes were placed on the denervated TA muscle. Rheobase and chronaxie were measured noninvasively at weekly intervals by means of a laptop PC, which communicated with the stimulator via a radio-frequency link. At each setting the denervated TA was palpated manually to detect the response of the muscle. During the first few days after denervation the rheobase increased transiently to 0.8 +/- 0.13 mA, approximately twice the value for normal innervated muscle, then decreased to normal for the remainder of the experimental period. Chronaxie underwent a significant 3-fold increase from 4.5 +/- 1.1 ms to 14.1 +/- 1.1 ms during the first two weeks of denervation and remained elevated throughout. The custom-built implantable electronic stimulator allowed changes in muscle excitability to be studied over a long period of denervation within individual animals, providing an accurate assessment of the time course of denervation-induced changes in muscle excitability.