Discontinuous nature of tetanic contractions and refractory period

Although the mechanical response in complete tetanus is maintained and continuous, that is to say, no sign of relaxation is apparent, the true character of the response to the rapidly repeated stimuli can be shown to be discontinuous. The most striking proof of this fact is the electrical change. Within certain limits, the number of electrical changes corresponds with the rate of stimulation. A further proof of the discontinuous nature of tetanic contractions lies in the sounds produced by contracting muscles. This correspondence between the internal response of muscle and the rate of stimulation will extend from a single stimulus up to about one thousand stimuli per second for the muscles of man and other warmblooded animals. When the rate of stimuli exceeds this higher figure, the muscle fails to respond to each stimulus but responds only to every other one.

This result is due to the fact that immediately following a response to a single stimulus there is a period, as it were, of forced rest or period of adjustment during which the muscle is non-irritable and fails to respond. This brief period is known as the refractory period and is of about 0.005 second duration in skeletal muscle. It serves as a means of further protection to the muscle against a demand which is beyond its capacity. This discontinuous character of the response is to be explained on the basis of the discontinuous nature of the chemical changes occurring in response to the rapidly repeated stimuli. Likewise the refractory period and limits to which these separate changes can take place must find their explanation in the factors controlling the biochemical reactions involved. The refractory period of muscle is so brief, however, that it probably never functions to limit the frequency of the response in man.

The importance of tetanic contractions lies in the fact that this type of contraction is practically the only one to be found in the human body during muscular exercise or activity. This fact, in the intact human body, is due to the manner in which the muscles receive their stimuli and are controlled through the central nervous system. When a muscle or group of muscles is called into action, a shower of impulses is sent from the nerve centers over the motor nerve fibers which innervate these muscles. A few, many or all of the muscle fibers of the muscles involved may be stimulated, the response being graded in proportion.