4 : 4/ The Conduction of Impulses by Nerves 



79 



without sparking or breakdown of any sort. Thus, it is not too surprising 

 that the neuron membrane can withstand field strengths at which dry 

 air breaks down. 



When the axon is stimulated, its surface potential changes in a 

 characteristic fashion to an action potential or a spike potential. (The 

 latter name arose from the appearance of these impulses on the screen 

 of a cathode ray oscilloscope.) Axons may be stimulated by any of a 

 wide variety of means. Electrical pulses of various shapes, heat, cold, 

 chemical changes, and mechanical pressures all lead to the same 



V t = Inside Potential 

 V - Outside Potential 



Positive After Potential 



Refractory Period 



Figure 5. Diagrammatic representation of the time course of 

 the spike potential at a fixed point along the axon. During 

 the refractory period, another impulse cannot be started. The 

 threshold for stimulation is lowered during the negative after 

 potential and raised during the positive after potential. The 

 magnitude and duration of these effects is characteristic of the 

 particular nerve fiber. 



phenomena. The local membrane polarization disappears, reverses in 

 polarity very quickly, then returns to normal over a series of "bumps." 

 The spike potential formed in this fashion travels down the axon in both 

 directions from the point of stimulation. (Owing to the nature of the 

 synapses, only one of these directions is usually effective when an intact 

 nerve is stimulated.) The time dependence of the potential at one spot 

 is shown in Figure 5. The corresponding distributions of charges along 

 the axon cylinder at a given time are shown in Figure 6. 



In laboratory experiments, spike potentials are usually excited by 

 electrical stimuli because they are easier to control in time, space, and 

 strength than are any other type of stimuli. For very weak stimuli, a 

 local response occurs which is similar to, but smaller than, the spike 

 potential. As the stimulus is increased, a certain threshold is reached 



