Muscle glycogen, its source and fate During the contraction of a muscle under normal conditions a number of chemical changes take place. Those best known and probably of greatest significance are the conversion of glycogen into lactic acid and the subsequent oxidation of a portion of the lactic, acid, or its equivalent in glucose, with the formation of carbon dioxide. Muscle normally contains considerable quantities of glycogen, or so-called animal starch. Muscular tissue has the power of converting the sugar (glucose) brought to it by the blood into glycogen. It is a synthetic reaction in which glucose undergoes dehydration with the result that the remainder of its molecule combines or condenses into larger and more complex molecules of the polysaccharide glycogen. Non-oxidative chemical changes The conversion of glycogen into lactic acid is a non-oxidative reaction and occurs irrespective of the presence or absence of oxygen. It could hardly be assumed that the arrival of the nerve impulse to the muscle fiber would initiate this splitting of glycogen directly. There is no doubt that the lactic acid is formed from glycogen, but evidence now available tends to support the conclusion that intermediate chemical reactions are involved and that some intermediate compound is more mobile than the ultimate precursor glycogen. The reaction is reversible and the muscle can synthesize the hexose-monophosphate from inorganic phosphates and glycogen or glucose. The prevailing view is that the hexose-monophosphate constitutes an immediate precursor of lactic acid in the anærobic phase of the glycogen-lactic-acid cycle. The important factor of the entire series of reactions would seem to be the production of lactic acid. These reactions, which are consummated rapidly, are exothermic and liberate energy which the muscles are able to utilize in part in the contractile process. The fact that lactic acid quickly disappears from the muscles in the presence of oxygen led to the belief that this acid, formed during the contraction, was oxidized immediately following the completion of the response. A muscle will continue to contract, however, for an appreciable time in the complete absence of oxygen. This is made possible by the fact that lactic acid is quickly neutralized by certain alkaline bicarbonates, phosphates, and alkaline salts of the muscle proteins. These are known collectively as the muscle buffers. The reaction between the bicarbonate and the lactic acid liberates small amounts of carbonic acid without the interaction of oxygen. Larger amounts of carbonic acid are formed in the oxidative processes which occur during the recovery phase. Soon some of the lactic acid and most of the carbonic acid formed in the manner indicated above, diffuse out into the lymph and blood stream. The more severe the muscular activity the more pronounced is this diffusion. Like the muscles, these body fluid; contain powerful buffering systems, namely, alkaline salts and the salts of the blood and lymph proteins. These muscle and body-fluid buffers are adequate, under normal conditions, to maintain practically a constant reaction of the tissues and body fluids. According to Henderson, if the reactions of the body fluids were to change to the extent of the differene in reaction between distilled and ordinary tap water, the condition would be very harmful to the life processes of the tissues. In addition to the comparatively well-known chemical reactions described in the preceeding paragraphs, the following less well-understood chemical changes occur contemporaneously with muscular contraction.		Home Classes of levers Varieties of muscle Smooth muscle The blood supply to muscles The neurone Cardiac muscle On the nomenclature of skeletal muscles The nerve supply to muscles Structure of a skeletal muscle Central nervous system The reflex arc and reflex action Physiologic properties of muscle Mechanical Changes The effects of temperature on muscular contraction Muscle glycogen, its source and fate The reaction of muscles Oxygen consumption of muscle Determination of the oxygen consumption of muscle The viscosity factor in muscle and muscle efficiency Significance of the recovery heat Production of Electricity