Chronaxie

Electrophysiology metric

Chronaxie is the minimum time required for an electric current double the strength of the rheobase to stimulate a muscle or a neuron. Rheobase is the lowest intensity with indefinite pulse duration which just stimulated muscles or nerves. Chronaxie is dependent on the density of voltage gated sodium channels in the cell, which affect that cell's excitability. Chronaxie varies across different types of tissue: fast twitch muscles have a lower chronaxie, slow twitch muscles have a higher one. Chronaxie is the tissue excitability parameter that
permits choice of the optimum stimulus pulse duration for stimulation of any excitable tissue. Chronaxie (c) is the Lapicque descriptor of the stimulus pulse duration for a current
of twice rheobasic (b) strength, which is the threshold current
for an infinitely long duration stimulus pulse. Lapicque showed
that these two quantities (c,b) define the strength duration curve
for current: I = b(1+c/d), where d is the pulse duration. However, there are two other electrical parameters used to
describe a stimulus: energy and charge. The minimum energy
occurs with a pulse duration equal to chronaxie. Minimum
charge (bc) occurs with an infinitely short duration pulse. Choice of a pulse duration equal to 10c requires a current of
only 10% above rheobase (b). Choice of a pulse duration of
0.1c requires a charge of 10% above the minimum charge (bc).

An electrode is inserted into the muscle of interest, which is then stimulated using surface current. Chronaxie values increase resulting from hyperventilation can be ascribed to a change in skin impedance, the physiological factors responsible for this change being under the influence of the autonomic nervous system. This example of the preponderating influence which the condition of the skin and the underlying tissues may exert compels caution in judging the results of chronaxie measurements by percutaneous stimulation. A fresh and normal sartorius placed straight in a Ringer solution
and stimulated through the solution without any direct contact with the electrodes is subject to give two very distinct strength duration curves, one of them being spread over several hundredths of a second.

The main value of chronaxie is comparing excitability across different experiments and measurements using the same standard, thus making data comparisons easier. Electrical stimulation based on chronaxie could regulate myoD gene expression in denervated muscle fibers. 20 muscle contractions, induced by electrical stimulation using surface electrodes and applied on alternate days based on muscle excitability, similar to protocols used in human clinical rehabilitation, were able to reduce the accumulation of mRNA in the myoD and atrogin 1 of denervated muscles, these
expressions being related to muscle growth and atrophy, respectively. The increase in myoD levels after denervation is possibly related not only to activation and proliferation
of the satellite cells but also to regulation of the cell cycle. Several studies have suggested that the function of denervation induced myoD may be to prevent the muscle atrophy induced by denervation.

Chronaxie and excitability values' medical application is electromyography, a technique for evaluating and recording the electrical activity produced by skeletal muscle. Rheobase may not necessarily be the electric current of choice. Electromyography is used to diagnose neuropathies, myopathies, and neuromuscular junction diseases.

Acute intoxication of rats with aldrin decreases chronaxie, whereas chronic exposure to this chlorinated hydrocarbon insecticide has the reverse effect. Chronic exposure of rats to the closely related epoxide, dieldrin, has been suggested to reduce their muscular efficiency in performing a work exercise. Dieldrin is a chlorinated hydrocarbon insecticide once widely used in crop protection and preservation. Among the diverse symptoms resulting from intoxication are muscular twitching, increasing in severity to epileptiform convulsions with loss of consciousness. Strychnine, which has a spinal locus of activity, causes tonic hind limb extension in mice, which is thought to be due to a removal of the effect of inhibitory interneurons on the nervous pathway to extensor muscles. Leptazol, on the other hand, produces a similar tonic extension by an excitatory action predominantly on cerebral structures. Diphenylhydantoin selectively elevated the threshold convulsive dose of leptazol but not that of strychnine hydrochloride, indicating an anticonvulsant activity on the nervous pathway between the predominant locus of activity of leptazol and the hind limbs.