Compound or tetanic contractions of muscle

A second method of grading the response of a muscle as a whole is by altering the frequency at which the separate stimuli reach the muscle. When a frog's gastrocnemius muscle is stimulated with a series of rapidly repeated stimuli (twenty or more per second) it remains in a state of more or less maintained contraction as long as the stimuli are continued, or until fatigue develops to terminate the response. This maintained contraction is known as physiological tetanus. When the stimuli follow each other so rapidly as to prevent even partial relaxation, the response is termed complete tetanus. If, on the other hand, the stimuli are less frequent and permit the muscle to partly relax, but not completely, before the following stimulus initiates a new response and the myogram shows a condition of incomplete maintenance of the contractile phase, it is then known as incomplete tetanus. Various grades of incomplete tetanus are possible. Complete tetanus probably involves all of the muscle fibers of which a muscle is composed.

Tetanic contractions present three fundamentally important characteristic, namely: first, maintenance or increased duration over that of the simple twitch, second, the more or less fusion of the separate responses initiated within the muscle by the separate stimuli, and third, the phenomenon of summation of the single responses so that the extent of the contraction in tetanus is much greater than that presented by a simple twitch. It would seem that in a single twitch acting against a load, the contractile stress of the muscle passes off before the load is lifted to the maximum height possible. Summation is probably due to the added mechanical effect of the load since in the second response the lever stands at a higher abscissa. The single twitch is inefficient because it is too brief except for handling very light loads against very low resistances. This principle of summation was inferred in the discussion of fatigue given in a previous paragraph. It depends upon the application of a second stimulus at some time during the contraction or relaxation phase of the response initiated by a previous stimulus.

The greatest summation occurs when the second stimulus is so timed as to reach the muscle just as it reaches the state of greatest tension, and diminishes as it appears earlier or later in the contraction phase. Any conditions which will prolong the contraction and especially the relaxation phase, such as fatigue, diminished temperature, and others, will diminish the number of stimuli necessary to induce a state of tetanus and, therefore, will bring about an exaggeration of the normal response.