COORDINATION 



413 



ity the impulse travels at exactly the same 

 speed. Furthermore, once the impulse 

 is started it continues with equal vigor 

 throughout its course, unlike electricity 

 which as it travels along a wire gradually 

 diminishes in intensity the farther it goes. 



The fact that the nervous impulse con- 

 tinues throughout its course with equal 

 vigor indicates that something is added to 

 it as it travels. It might be compared to a 

 path of inflammable material where each 

 portion ignites the succeeding part so that 

 the entire trail burns with equal intensity. 

 A minimum amount of heat must be sup- 

 plied to initiate ignition, but once ignited 

 the flame burns with equal vigor from that 

 point forward. Furthermore, any excess 

 heat beyond the minimum necessary to ig- 

 nite the material will not change the situa- 

 tion. It is likewise with a neuron, once 

 stimulated, the impulse travels with equal 

 intensity throughout the course of the cell. 

 The neuron, like the muscle cell, obeys the 

 "all or none" principle. In other words, if 

 a given stimulus elicits an impulse, the im- 

 pulse starts and continues throughout its 

 course with full vigor. No matter how the 

 minimum stimulus is altered, the impulse 

 travels with its full force or it does not 

 travel at all. 



Once an impulse passes over a nerve fiber 

 there is a short period when the nerve is in- 

 capable of transmitting a second impulse, 

 that is, it refuses to accept any further stim- 

 ulus. This is known as the refractory period 

 and is very short in most nerve fibers, last- 

 ing 0.001 to 0.005 of a second. It means that 

 some reorganization is essential before the 

 nerve fiber can once again be stimulated. 

 This, in turn, is due to the physical make-up 

 of the nerve fiber itself. The outside mem- 

 brane of the nerve fiber is positively charged 

 while the inside is negatively charged ( Fig. 

 16-14), and is therefore said to be polarized. 

 This condition is maintained untfl an im- 

 pulse passes along which brings about a 

 chemical change, resulting in the mixing of 

 the charged ions through the outer perme- 



siimulus 



qolvanometer 



muscle 



B 



Fig. 16-13. Electrical changes occur in a nerve when an 

 impulse passes over it {A to E) as indicated by the 

 deflection of the galvanometer needle. Similar changes 

 occur in a contracting muscle. 



