THE CONDUCTION OF THE NERVOUS IMPULSE 837 



ment. Thus excitation is best considered in connection with the recep- 

 tors and a consideration of it may be postponed until we are ready to 

 take up the aspect of central nervous activity which leads to sensation 

 and the phenomenon of consciousness. Response must be studied in mus- 

 cular tissue and its treatment may be put off until we have seen what 

 the nervous mechanism initiating muscular activity consists of. Con- 

 ductivity is readily studied in nerve, and since this process forms the 

 base of all nervous activity, it must be treated before the study of the 

 activity of the nervous system as a whole can be undertaken. 



Conduction in the Nerve Fiber 



When a nerve fiber supplying a muscle is stimulated, the response of 

 the muscle is so prompt that for many years physiologists despaired of 

 determining the rate at which the nerve impulse traveled along the 

 nerve. Helmholtz succeeded, however, in devising a simple method by 

 which the velocity of the impulse could be measured and found that 

 in the sciatic nerve of the frog the rate of propagation was about 30 

 meters a second. Later determinations have shown that in the nerves 

 of man the velocity of the impulses is three or four times as great, a dif- 

 ference which may be attributed to the higher temperature of the human 

 body. It is seen from this that very little time is lost in the transmission 

 of impulses along nerve trunks, and rapid responses to stimulation are 

 thus insured. Although nerve fibers are closely bound together within a 

 nerve trunk, impulses cannot spread in a lateral direction from one fiber 

 to its neighbor. Consequently if a branch of the lumbar plexus is stim- 

 ulated, contraction occurs in a more limited group of muscles than if 

 the nerve trunk is stimulated below the junction of the various parts 

 of the plexus. The response of muscles to the artificial stimulation of 

 a nerve trunk also differs in its distribution from responses induced 

 reflexly or volitionally. These facts show that the constituent fibers 

 of a nerve trunk are completely isolated from one another. 

 . Within the single nerve fiber nervous impulses may travel in either 

 direction along its length irrespective of the direction in which con- 

 duction normally takes place. Thus if a collateral of a motor neuron 

 is stimulated close to its termination in a muscle, the impulse will travel 

 up the collateral to its point of junction with a second collateral, and 

 down the latter to cause a contraction of the muscles which this in- 

 nervates. The polarity which is such an important feature of the activ- 

 ity of the reflex arc as a whole is not exhibited by the conduction within 

 a single neuron, which thus preserves one of the fundamental proper- 

 ties of the primitive nerve net, 



The All or None Nature of Conduction. Activity cannot occur in the 

 body without the expenditure of energy. It is pertinent to inquire what 



