Figure 1123

Summation of isometric contractions produced by shortening the time between stimuli S2 and S3.

possible for a second action potential to be initiated during the period of mechanical activity. Figure 11-23 illustrates the tension generated during isometric contractions of a muscle fiber in response to three successive stimuli. In Figure 11-23a, the isometric twitch following the first stimulus Sj lasts 150 ms. The second stimulus S2, applied to the muscle fiber 200 ms after Sj when the fiber has completely relaxed, causes a second identical twitch, and a third stimulus S3, equally timed, produces a third identical twitch. In Figure 11-23b, the interval between Sj and S2 remains 200 ms, but a third stimulus is applied 60 ms after S2, when the mechanical response resulting from S2 is beginning to decrease but has not yet ended. Stimulus S3 induces a contractile response whose peak tension is greater than that produced by S2. In Figure 11-23c, the interval between S2 and S3 is further reduced to 10 ms, and the resulting peak tension is even greater. Indeed, the mechanical response to S3 is a smooth continuation of the mechanical response already induced by S2.

The increase in muscle tension from successive action potentials occurring during the phase of mechanical activity is known as summation. A maintained contraction in response to repetitive stimulation is known as a tetanus (tetanic contraction). At low stimulation frequencies, the tension may oscillate as the muscle fiber partially relaxes between stimuli, producing an unfused tetanus. A fused tetanus, with no oscillations, is produced at higher stimulation frequencies (Figure 11-24).

As the frequency of action potentials increases, the level of tension increases by summation until a maximal fused tetanic tension is reached, beyond which tension no longer increases with further increases in stimulation frequency. This maximal tetanic tension is about three to five times greater than the isometric twitch tension. Since different muscle fibers have different contraction times, the stimulus frequency that will produce a maximal tetanic tension differs from fiber to fiber.

Summation can be explained by events occurring in the muscle fiber. The explanation requires one new piece of information: A muscle contains passive elastic elements (portions of the thick and thin filaments and tendons) that are in series with the contractile (force-generating) elements. These series elastic elements act like springs through which the active force generated by the cross bridges must pass to be applied to the load. Therefore, the time course of the rise in tension during an isometric contraction includes the time required to stretch the series elastic elements.

The tension produced by a muscle fiber at any instant depends upon (1) the number of cross bridges bound to actin and undergoing step 2 of the cross-bridge cycle in each sarcomere, (2) the force produced by each cross bridge and, (3) the amount of time the

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Muscle CHAPTER ELEVEN

Muscle CHAPTER ELEVEN

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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