The excitability of different tissues varies widely. Some tissues are highly excitable whereas others are dull.
While considering the excitability of a tissue two factors are to be considered:
(a) The strength of the current used, and
(b) The duration of the current flowing through the tissue.
These two factors bear an inverse relation. The stronger the current the lesser will be the duration required. But there is a limit. Two standards have been established to measure the excitability of various tissues. [Instruments used-Lapicque’s liquid potentiometer or Keith Lucas’s spring rheotome] The strength standard (intensity threshold) is called rheobase. The duration standard (time characteristic) is called by Lapicque as Chronaxie.
Rheobase is defined as the minimal galvanic current which when allowed to flow indefinitely will excite a tissue. Chronaxie is defined as the shortest duration of stimulus required to excite a tissue by a current strength equal to twice of Rheobase voltage. Chronaxie of a tissue is a definite measure of its excitability.
A less excitable tissue has a longer chronaxie, whereas a more excitable tissue has a shorter one. The speed of activity of tissue and its chronaxie bear a direct relation. A tissue, having shorter chronaxie, has shorter period of action, shorter refractory period and a quicker rate of propagation.
Sluggish tissues, such as some form of plain muscle, have longer chronaxies. Owing to this, they fail to be excited by Faradic stimulation (induction shock), because the duration of the current-here is very short.
Chronaxies of the various tissues are as follows:
i. The tissues of the new-born child possess chronaxies which are ten times longer than those of adults. This explains the slower movements of infants.
ii. Tissue of cold-blooded animals generally possesses longer chronaxies (i.e., they are more sluggish) than those of warm-blooded types.
iii. Muscles:
a. The chronaxies of the skeletal muscles are shorter than those of plain and cardiac. Flexor muscles have shorter chronaxies than extensors. The pale muscle fibres (more active) have shorter chronaxies than the red muscles (sluggish).
Fatigue lengthens and adrenaline shortens the chronaxie of skeletal muscles. Similarly, cold lengthens and warm shortens it. A degenerating muscle has longer chronaxie; that is why it fails to be excited by Faradic stimulation in the early stage of reaction of degeneration.
b. Chronaxie of the plain muscle is longer than that of the skeletal muscle. Vagal stimulation shortens and sympathetic stimulation lengthens the chronaxie of the plain muscles of the stomach. Stretching diminishes their chronaxies and increases excitability.
c. Cardiac Muscle- The chronaxies of ventricular and atrial muscles are same but that of the junctional tissues is three times longer. Stimulation of vagus, stretching, cold, and inhibitory drugs like acetylcholine shortens the chronaxie. Warmth, sympathetic stimulation and accelerator drugs like atropine, adrenaline, etc., lengthen chronaxie.
iv. Nerves:
The chronaxie of the autonomic nerves is more than that of the somatic nerves. Nerve fibres of larger diameter (which conduct more rapidly), have shorter chronaxies than thinner ones (having less conductivity).The chronaxies of the corresponding sensory and motor nerves are about the same.
In a degenerating nerve the chronaxie becomes longer. [The chronaxies of certain frog’s tissues are as follows – skeletal muscles—0.3-0.9 millisecond, ventricles—3.5 milli seconds, stomach-30-100 milliseconds, pigments cells of the skin-11,000-15,000 milliseconds.]