The effects of temperature on muscular contraction Since the dynamics of muscular contraction involve the intrinsic chemical and physical changes which occur during the contractile process, the influence which a rise or fall of temperature will have on muscular activity may be anticipated from the effects such changes would have on these separate processes. We know that for each rise of ten degrees in temperature chemical reactions are only increased in rate from two to three times while physical reactions are only increased once. The rate of development of an isometric twitch has a temperature coefficient of about 2.5 for each ten degrees Centigrade, while the same coefficient for its subsidence is about 3.6. The development of energy in contracting muscle is associated with a chemical change, the production of acid. The immediate neutralization of this acid is likewise a chemical reaction as is also the recovery phase of the muscle. The mechanical changes undergone by the contracting muscle involve physical adjustments. Heat, then, within certain limits accelerates all phases of the muscular contraction; the latent period is shortened, the rapidity and effectiveness of the contraction are increased while the relaxation phase is especially accelerated. Cold has opposite effects. These again are most pronounced on the relaxation phase which becomes progressively slower and more prolonged as the temperature is lowered until a critical minimum temperature is reached. At this temperature the properties of irritability and contractility are reversibly suspended. The most favorable temperature for muscular activity is about one degree above the normal body temperature. At a certain critical high temperature the muscle substance is irreversibly destroyed. For frog's muscle this critical temperature is between 34° and 37° C. For mammalian muscle this value lies in the neighborhood of 45° C. The "all-or-nothing" phenomenon of muscle contraction We are all familiar with the fact that our voluntary and reflex muscle contractions may at times be weak and at other times strong, in short the contraction may be graded in strength at will. There are two possible explanations for this fact. In the first place, the contractions may be weaker or stronger because each separate fiber of which the muscle is composed is able to respond in varying degrees. On the other hand, the response may be variable in strength because of the fact that at one instant a few or only a part of the separate fibers may contract and at another time more or all of the fibers respond to their utmost. This principle is commonly known as the "all-or-nothing" phenomenon, and may be stated that the response of a single muscle fiber cannot be altered by varying the stimulus,--other conditions remaining the same, the response is maximal or it does not occur at all. It should be pointed out that this applies only to the muscle fiber as a unit and not to the muscle as a whole. A graded response of the entire muscle, composed of thousands of muscle fibers, is possible within certain limits by varying the number of fibers actively contracting at one given time. When only a few fibers contract, the response will be minimal; when they all contract at once, the response will be maximal. All contractions involving intermediate numbers of muscle fibers are possible and may be spoken of as submaximal responses. Normally in the intact animal, this grading is mediated through the central nervous system where the number of nerve fibers involved. in the efferent discharge may vary from a few to all of the fibers running to the muscle in question. The "all-or-nothing" law also applies to nerve fibers and probably to all irritable tissues.		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