The last of the factors mentioned above—the effect of fatigue on the irritability of the nerve-muscle combination, or on the muscle alone—can be tested by determining variations in the least stimulus capable of causing the slightest contraction, the so-called “threshold stimulus.” As the irritability lessens, the threshold stimulus must necessarily be higher. The height of the threshold is therefore a measure of irritability. How does fatigue affect the irritability of nerve-muscle and muscle? How is the irritability of fatigued structures affected by rest? How is it influenced by adrenin or by adrenal secretion? Answers to these questions were sought in researches carried on by C. M. Gruber[1] in 1913.
The Method of Determining the Threshold Stimulus
The neuro-muscular arrangements used in these researches were in many respects similar to those already described in the account of experiments by Nice and myself. To avoid the influence of an anesthetic some of the animals were decerebrated under ether and then used as in the experiments in which urethane was the anesthetic. The nerve (the peroneus communis) supplying the tibialis anticus muscle was bared and severed; and near the cut end shielded platinum electrodes were applied. These electrodes were used in fatiguing the muscle. Between these electrodes and the muscle other platinum electrodes could be quickly applied to determine the threshold stimulus and the tissue resistance. These second electrodes were removed except when in use, and when replaced were set always in the same position. Care was taken, before replacing them, to wipe off moisture on the nerve or on the platinum points.
For determining the threshold stimulus of the muscle the skin and other overlying tissues were cut away from the tibialis anticus in two places about 5 centimeters apart. Through these openings platinum needle electrodes could be thrust into the muscle whenever readings were to be taken. Local polarization was avoided by reinserting the needles into fresh points on the exposed areas whenever new readings were to be taken.
The tendon of the tibialis anticus was attached, as in the previous experiments, by a strong thread passing about pulleys to a lever which when lifted stretched a spring. During the determination of the threshold the spring was detached from the lever, so that only the pull of the lever itself (about 15 grams) was exerted on the muscle.
The method of measuring the stimulating value of the electric current which was used in testing the threshold was that devised by E. G. Martin[*] of the Harvard Laboratory—a method by which the strength of an induced electric shock is calculable in definite units. If the tissue resistance enters into the calculation these are called β units. When the threshold of the nerve-muscle was taken, the apparatus for the determination was connected with the nerve through the electrodes nearer the muscle. They were separated from the fatiguing electrodes by more than 3 centimeters, and arranged so that the kathode was next the muscle. When the threshold of the muscle was taken directly the apparatus was connected with the muscle through platinum needle electrodes thrust into it. The position of the secondary coil of the inductorium, in every case, was read by moving it away from the primary coil until the very smallest possible contraction of the muscle was obtained. Four of these readings were made, one with tissue resistance, the others with 10,000, 20,000, and 30,000 ohms additional resistance in the secondary circuit. Only break shocks were employed—the make shocks were short-circuited. Immediately after the determination of the position of the secondary coil, and before the electrodes were removed or disconnected, three readings of the tissue resistance were made. From these data four values for β were calculated.
[*] For a full account of Dr. Martin’s method of calculating the strength of electric stimuli, see Martin: The Measurement of Induction Shocks, New York, 1912.
The strength of the primary current for determining the threshold of the nerve-muscle was usually .01 ampere, but in a few cases .05 ampere was used. For normal muscle it was .05 ampere and for denervated muscle 1.0 ampere. The inductorium, which was used throughout, had a secondary resistance of 1400 ohms. This was added to the average tissue resistance in making corrections—corrections were made also for core magnetization.
The Lessening of Neuro-muscular Irritability by Fatigue
The threshold for the peroneus communis nerve in decerebrate animals varied from 0.319 to 2.96 units, with an average in sixteen experiments of 1.179.[*] This average is the same as that found by E. L. Porter[2] for the radial nerve in the spinal cat. For animals under urethane anesthesia a higher average was obtained. In these it varied from .644 to 7.05, or an average in ten experiments of 3.081.