If a series of stimuli be applied in succession, at intervals less than the duration of a single contraction, a summation of con tractions occurs, which results in the accumulation of effect until the muscle has shortened to about one-half of the length it attains during a single contraction, or about one-fourth the normal length of the relaxed muscle; it then remains contracted to the same extent for some time, and does not shorten further, though the stimulus be increased in rate or strength. As long as the stimuli are continued, the various single contractions caused by the individual shocks are fused together (Fig. 191); but if the intervals between the stimuli be nearly as long as the time occupied by a single contraction, the line drawn by the lever will show notches indicating the apices of the fused single contractions (Figs. 192 and 193).
Fig. 191. Curve of tetanus resulting from 30 stimulations per second, drawn by a frog's muscle on a drum, the surface of which moves 1.5 centimetres per second. The stimulation commences at " 30," and ceases just before the lever begins to fall. No trace of the individual contractions of which the tetanus is composed can be recognized.
Fig. 192. Curve of tetanus composed of imperfectly fused contractions resulting from 12 stimulations per second. The serrations on the left of the curve indicate the individual contractions.
This condition of summation of contractions is called tetanus, and is said, by some, to be the manner in which muscular motion is produced by the action of the nerves in obedience to the will.
With from fifteen a second to upwards of many hundreds of induced shocks one can produce tetanus in a frog's muscle. The lowest rate of electric stimulation at which human muscle passes into complete tetanus is about 25 per sec. The number of stimuli required varies with the rate of contraction of the muscle employed, the quick-contracting bird's muscle requiring 70 per second, while the exceptionally slow-moving tortoise muscle only requires 3 per second. According to some, there is a limit to the number of stimuli which will cause tetanus - 360 per second is named as the maximum for a certain strength of stimulus; with stronger stimuli, even when more frequent, tetanus occurs. It has been shown that many thousand stimuli per second can cause tetanus even with very weak currents. If tetanus be kept up for some seconds, and the stimulation be then suddenly stopped, the lever falls rapidly for a certain distance, but the muscle does not quite return to its normal length for some few seconds. This residual contraction is easily overcome by any substantial load. If kept in a state of tetanus by weak stimulation, after some time the muscle commences to relax from fatigue, at first rapidly, then more slowly. This falling off of the tetanic contraction may be prevented by increasing the stimulus.
Fig.193. Tetanus produced by 8 stimulations per second. The more perfect fusion of the single contractions shown toward the end of the curve depends on the altered condition of the muscle.