Muscle: Chemistry Of Contraction (1969)

[Music] [Music] muscles contract and there is movement and fundamental to movement is the single muscle fiber the basic unit of contraction when a fiber of a voluntary muscle is examined under the light microscope a definite pattern of light and dark stripes or striations is observed during contraction there is an obvious shortening of the entire fiber on more detailed examination however this contraction is seen to be the result of of the much finer shortening of the striations themselves the substructures responsible for this movement could not be adequately observed until the development of the electron microscope then the complex internal arrangement of the muscle fiber was revealed it was found that the fiber is made up of small substructures fibral and the fibral in turn are composed of still smaller units which were named filaments two kinds of filaments could be distinguished relatively thin ones which form the light stripes and relatively thick ones which form the dark stripes this accounts for the striated appearance of the fibral examination of a fibral in cross-section showed that the component filaments form a definite pattern this pattern is due to the regular arrangement of the thick and thin filaments the thick filaments on chemical analysis were found to be composed of the protein myosin while the thin filaments are composed of the protein actin based on the Visual Evidence of electron microscope studies a working model of how the filaments behave during contraction was constructed it suggests that during contraction the filaments slide over and past one another this is the sliding filament theory of muscular contraction the chemical interactions of the thick and thin filaments during contraction has been intensively studied by Dr George marishal of the University of Lou Belgium the Visual Evidence for the sliding filament theory is quite strong but in order to accept it completely we must have additional evidence that these filaments can contract if they cannot then the theory is without any value whatsoever in order to test this Theory we are making two preparations of the same voluntary muscle from the first preparation we are extracting all the chemical compounds of the muscle except the filaments the Mel Jews contains a number of chemicals including the compound adenosine Ty phosphate from the sign preparation we extract only the filament materials from the thick filament we get the protein myosin from the thin filament we get the actin in the process of extraction the myazin and actin combines to form a substance called actomyosin here is a solution of actomyosin now I'll scir the solution into the water and I get a beautiful thre now the question is to know whether this threat can contract as we know that a muscle can be quite easily stimulated by electrical shocks let us try first this procedure no response try again no contraction the threat cannot be excited with electrical stimul but it can perhaps be contracted with a chemical stimulus extracted from the muscle itself let us try it contacts beautifully so this EXP experiment show us two things first that the actomyosin is really the contac proteins of the muscle and second that the nerve impulse somehow causes the release of some chemical factors which uh when it is brought into contact with the acto myazin causes its contration the question now is to identify this Factor here I have attached to this Rod a muscle from a habit which has been dipped into glycerol in order to remove all the component of the muscle cells except the filaments let us take one of these filament of the muscle well this one is a bit too small there's a beautiful one now here I have several substances which have been isolated from the muscle and we'll try to make the fat contact by adding them one at a time let us try first uh chotin phosphate does not contact so let's try lactic acid nothing phosphate you find many phosphate in the musle nothing no contraction let us try another substance calcium for for instance nothing the cells are known to contain a lot of ATP Ado enti phosphate it contracts beautifully so it appears that ATP is the key substance which is necessary for the muscle contraction let us try another experiment on a fresh fibers in a fresh solution and now let us add the ATP there is no contraction so it appears that in muscle the situation is not quite as simple as we thought besides ATP some other component is also needed let us try again to add the other substan one at a time 1 let us try now with lactic acid nothing pal there there is beautiful contraction so this experiment show that ATP and calcium both are needed together in order to trigger the contraction for a long time we wondered what happened inside the muscle but now we think that we can uh have a clear description of the chain of events that is occurring let me show you on a simple model what happens ah now here we have the thick filament of Marin and the thin filament of actin now I need a muscle cell it can be represented by this paper in the muscle cell there are the thick filament of myazin and the thin filament of actin the calcium in the hesting muscle is not in contact with the filament but it is kept inside some membranous structure that I can represent with this bottle now we have another membranous structure which are represented by these magnets all along the filaments and in the resting muscle we have also the adenosine Ty phosphate with its three phosphates this adenos in triphosphate is in contact with the filament here is the nerve which inates the muscle cells if a nerve impulse comes along this nerve it will trigger an electrical disturbance which will travel along the membrane of the muscle this electrical disturbance triggers the secretion of calcium ions which will reach all the filaments this initiates the contraction the energy for the contraction is furnished by the ATP the last phosphate of this ATP is broken down and the energy released is used to bring about the contraction then at the end of the contraction the calcium is removed and because the calcium is gone the contraction stops the the ATP is then resynthesized a new terminal phosphate is added and the muscle has returned to its resting conditions it is then ready for a new contraction but of course things are not so simple the chemical reactions are vastly more complex but in a simple way I have demonstrated the basic events which occur during the contraction and how how the energy of ATP is used to produce the muscular contraction [Music]