80 The Conduction of Impulses by Nerves /4 : 4 



where a transmitted spike potential is generated. The spike potential 

 then travels along the axon at a characteristic velocity. 



The spike potential is an all-or-none response. Either there is a 

 transmitted spike or there is not. If the spike is present, its height and 

 shape are independent of the stimulus strength. The neuron acts in a 

 similar manner to a flip-flop electronic circuit such as used in counters 

 and in digital computers. That is to say, the neuron is either in the 

 conducting or nonconducting state; nothing is transmitted in between. 

 This analogy seems so strong that it is hard to avoid describing the 

 computer in anthropomorphic terms and the nervous system in terms of 

 a digital computer. 



Positive Negative 

 After After 



Resting Potential Potential Spike Before 



+ + + + +++++ + + + + - + + + + + 



+ + + + +++ + +++ + + - + + + + + 



Figure 6. Space distribution of charges along an axon con- 

 ducting a spike potential. Arrow shows direction in which 

 spike is moving. 



If an axon is cut, and the two pieces insulated electrically, no impulse 

 travels from one part to the other. However, if the two are connected 

 by a metallic conductor, or a salt bridge, the spike potential crosses 

 readily from one part of the axon to the other. This emphasizes the 

 essentially electrical nature of the action potential. These spike poten- 

 tials occur in tissues, which are fluid-like media. Currents in fluids are 

 carried by ions, therefore it is appropriate to consider the resting potential 

 as well as the spike potential as due to ionic distributions. 



While the spike potential is present at the axon, another one cannot 

 be started. By contrast, several subthreshold stimuli may be summed to 

 give a response if they come close enough together in time. During the 

 positive after-potential, the threshold is increased. The lengths of 

 time for these potentials and the rate of conduction of the spike potentials 

 led to the classification of vertebrate axons presented in the table on page 

 81. 



Particular attention should be called to the giant squid axon. From 



