448 - Multicellular Animals, Especially Man 



est fibers transmit at the rate of 120 meters 

 per second, and even in the smaller fibers 

 impulses are conducted almost as fast. 



Aside from the speed of transmission, 

 nerve impulses are fundamentally similar to 

 excitations in other tissues (p. 190). Like exci- 

 tations generally, the nerve impulse is a 

 wavelike protoplasmic disturbance (p. 191) 

 that is always distinguished by well-defined 

 electrical, thermal, and chemical changes. 

 Invariably the spikelike action potential 

 keeps precisely in pace with the speeding 

 impulse; and the passage of each impulse 

 involves the liberation of a small amount of 

 heat. At least 90 percent of this heat comes 

 forth subsequent to actual transmission, 

 while the nerve is consuming extra oxygen 

 and consummating its recoi>ery. 



Compared to a working muscle, a trans- 

 mitting nerve expends very little energy, al- 

 though the total extra heat produced rep- 

 resents much more energy than is liberated 

 by the actual process nf transmission. The 

 extra consumption of oxygen and produc- 

 tion of carbon dioxide, which result from 

 nervous activity, are so small that tbev could 

 not be measured until quite recently. In fact, 

 the heat produced by a stimulated nerve is 

 equivalent merely to the energy liberated by 

 the oxidation of 0.000001 gram of glycogen 

 per gram of nerve per minute of continuous 

 stimulation. Thus if the nerve contains only 

 1 percent of carbohydrate fuel, it could be 

 stimulated continuously for a week without 

 exhaustion. It is not surprising to find that 

 nerve fibers are, in fact, practically unfatiga- 

 ble, provided an adequate oxygen supply is 

 available. 



EXCITABILITY IN NERVE FIBERS 



With proper care, a nerve excised from the 

 body of a cold-blooded animal, such as a 

 frog, will remain alive and excitable for a 

 period of more than 24 hours, and excised 

 nerves have been widely employed in study- 

 ing the phenomena of excitation. Usually 

 such nerves are attached to a muscle, and the 



contraction of the muscle indicates the effec- 

 tiveness of the stimulus, which is applied 

 directly to the nerve. Each motor nerve is 

 made up of many axon fibers (p. 453), which 

 pass to the muscle in a common sheath, and 

 these axons, though divorced from their 

 centrons (p. 453), remain alive and excitable 

 for an extensive period after the nerve has 

 been removed from the body. 



When any nerve is stimulated directly with 

 an electric current, the excitation arises at 

 the negative electrode, that is, at the point 

 where the stimulating current abolishes the 

 positive charge on the outer surface of the 

 membrane (p. 190). Such a depolarization, or 

 cancellation of the resting potential, results 

 in the discharge of an impulse from the point 

 of excitation. Then a rapid wave of depolari- 

 zation, or action potential, travels along the 

 length of the fiber, and each depolarized 

 point, as it becomes excited, contributes its 

 share of energv to the propagation of the 

 impulse. 



Immediately after transmitting an impulse, 

 each part of a nerve fiber quickly recovers its 

 capacity to be re-excited. The absolute re- 

 fractory period (p. 449), during which the 

 fiber cannot be re-excited by another stimu- 

 lus, however strong, endures for only about 

 0.002 second (Fig. 25-1); but this is followed 

 by a relative refractory period (0.012 second), 

 during which an exceptionally strong stimu- 

 lus can cause an excitation. Then, before the 

 fiber returns to its original state, there is a 

 brief supernormal period. During the super- 

 normal period the fiber is susceptible to exci- 

 tation by feebler stimuli. 



Unlike an ordinary electric current, the 

 propagated action potential does not grow 

 weaker as the transmitting circuit becomes 

 longer; this is because new energy is pro- 

 vided to the impulse as it passes each point 

 along the nerve fiber. Moreover, nerve fibers, 

 like muscle fibers, respond in an all-or-none 

 fashion; each fiber discharges to the maxi- 

 mum of its capacity at the moment of excita- 

 tion. This does not mean that all impulses 

 are of the same magnitude. In fact, the mag- 



