PRINCIPLES OF PHYSIOLOGY JQS 



latter process, which can occur without utihzing oxygen, provides energy 

 for the resynthesis of adenosine triphosphosphate and phosphocreatine. 



Capturing food or evading enemies may call for prolonged bursts of 

 muscular activity. Although both the rate of breathing and the rate of 

 the heart beat may increase markedly during prolonged exertion, these 

 changes could not supply the muscles with enough oxygen to enable 

 them to contract repeatedly if the contraction process itself required 

 oxygen. That muscle contraction, and part of the recovery process, occur 

 without the utilization of oxygen is clearly important for survival. Dur- 

 ing violent exercise glycogen is converted to lactic acid faster than the 

 lactic acid can be oxidized. Lactic acid accumulates and the muscle is said 

 to have incurred an "oxygen debt," which is repaid after the period of 

 exertion by continued rapid breathing. This supplies enough extra 

 oxygen to oxidize part of the accumulated lactic acid. Some of the energy 

 released by the oxidation of the lactic acid in the Krebs cycle and the 

 electron transmitter system (Fig. 4.2) is used to resynthesize glycogen 

 from the remainder of the lactic acid and to restore the energy-rich com- 

 pounds, ATP and phosphocreatine, to their normal condition. A muscle 

 that has contracted many times, has depleted its stores of energy-rich 

 phosphates and glycogen and has accumulated a lot of lactic acid, is 

 unable to contract again and is said to be fatigued. 



One theory of muscle contraction states that the energy for contrac- 

 tion is transferred from the ATP to the actomyosin at the moment of 

 contraction, and that after this energy has been used in the physical short- 

 ening of the muscle fiber, the muscle fiber simply relaxes passively. The 

 second view, which is more widely held at present, states that contraction 

 is analogous to the releasing of a stretched spring and that energy must 

 be put into the system to bring about the relaxation— the stretching— 

 of the muscle fiber. The stimulation of the muscle by a nerve impulse, in 

 this theory, is like the releasing of a trigger which has been holding the 

 stretched spring. 



It was noted in Figure 5.10 that an action potential was associated 

 with muscle contraction. Muscles in general are arranged with their 

 fibers in parallel, so that the voltage difference in a large muscle is no 

 greater than that of a single fiber. In the electric organ of the electric 

 eel, however, the electric plates are modified muscle cells (motor end 

 plates) arranged in series. Although each plate has a potential difference 

 of about 0.1 volt, the discharge of the entire organ, made of several 

 thousand plates, amounts to several hundred volts. 



32. Irritability and Response 



The muscles just described, together with cilia, glands, nematocysts 

 and so on, are eflFectors-they do things. To ensure that these effectors 

 do the right things at the right time, animals are equipped with re- 

 ceptors—a variety of sense organs-and with nervous and endocrine sys- 

 tems to coordinate the activity of the effectors. 



Irritability or excitability is a fundamental property of all proto- 

 plasm. AVaves of excitation are conducted, although very slowly, by the 



