Muscle contraction has often been studied and demonstrated using the gastrocnemius (calf) muscle of a frog, which can easily be isolated from the leg along with its connected sciatic nerve (see insight 11.3). This nerve-muscle preparation can be attached to stimulating electrodes and to a recording device that produces a myogram, a chart of the timing and strength of the muscle's contraction.
A sufficiently weak electrical stimulus to a muscle causes no contraction. By gradually increasing the voltage and stimulating the muscle again, we can determine the threshold, or minimum voltage necessary to generate an action potential in the muscle fiber and produce a contraction. The action potential triggers the release of a pulse of Ca2+ into the cytoplasm and activates the sliding filament mechanism. At threshold or higher, a stimulus thus causes a quick cycle of contraction and relaxation called a twitch (fig. 11.13).
There is a delay, or latent period, of about 2 milliseconds (msec) between the onset of the stimulus and the onset of the twitch. This is the time required for excitation, excitation-contraction coupling, and tensing of the series-elastic components of the muscle. The force generated during this time is called internal tension. It is not visible on the myogram because it causes no shortening of the muscle.
Once the series-elastic components are taut, the muscle begins to produce external tension and move a resisting object, or load. This is called the contraction phase of the twitch. In the frog gastrocnemius preparation, the load is the sensor of the recording apparatus; in the body, it is usually a bone. By analogy, imagine lifting a weight from a table with a rubber band. At first, internal tension would
Latent — period
Latent — period
Figure 11.13 A Muscle Twitch.
What role does ATP play during the relaxation phase?
Saladin: Anatomy & I 11. Muscular Tissue I Text I © The McGraw-Hill
Physiology: The Unity of Companies, 2003 Form and Function, Third Edition
424 Part Two Support and Movement stretch the rubber band. Then as the rubber band became taut, external tension would lift the weight.
The contraction phase is short-lived, because the sacroplasmic reticulum quickly pumps Ca2+ back into itself before the muscle develops maximal force. As the Ca2+ level in the cytoplasm falls, myosin releases the thin filaments and muscle tension declines. This is seen in the myogram as the relaxation phase. The entire twitch lasts from about 7 to 100 msec.
Insight 11.3 Medical History
Galvani, Volta, and Animal Electricity
The invention of modern dry cells can be traced to studies of frog muscle by Italian anatomist Luigi Galvani (1737-98). He suspended isolated frog legs from a copper hook and noticed that they twitched when touched with an iron scalpel. He attributed this to "animal electricity" in the legs. The physicist Alessandro Volta (1745-1827) investigated Galvani's discovery further. He concluded that when two different metals (such as the copper hook and iron scalpel) are separated by an electrolyte solution (a frog's tissue fluids), a chemical reaction occurs that produces an electrical current. This current had stimulated the muscle in the legs of Galvani's frogs and caused the twitch. Based on this principle, Volta invented the first simple voltaic cell, the forerunner of today's dry cells.
Proportion of nerve fibers excited
Figure 11.14 The Relationship Between Stimulus Intensity (voltage) and Muscle Tension. Weak stimuli (1-2) fail to stimulate any nerve fibers and therefore produce no muscle contraction. When stimuli reach or exceed threshold (3-7), they excite more and more nerve fibers and motor units and produce stronger and stronger contractions. This is multiple motor unit summation (recruitment). Once all of the nerve fibers are stimulated (7-9), further increases in stimulus strength produce no further increase in muscle tension.
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