778 



Comparative Animal Physiology 



relatively impermeable membrane so that it is momentarily permeable to 

 ions; membrane resistance decreases during conduction. 



In conduction the membrane shows a series of some five different elec- 

 trical phases. Some types of nerve fiber show certain of these electrical 

 waves more clearly than others; a composite picture of the electrical disturb- 

 ance in a nerve impulse follows: 



(1) The membrane has electrical capacitance and resistance, and when 

 subthreshold current is passed through the nerve there is current spread ac- 

 cording to the electrical constants of the membrane. This electrotonic wave 

 declines exponentially along the fiber from the source of stimulation and 



Fig. 29 1 . Local graded oscillations with emergent spike in decalcified giant axons of 

 squid. From Arvanitaki." 



Fig. 292. Action potential recorded between the inside and outside, of giant axon of 

 squid showing initial resting potential level and overshoot of action potential. From 

 Hodgkin and Huxley.^''* 



can be detected 0.5-3 mm. away from a stimulating electrode; the electro- 

 tonic (polarization) potential is best seen with stimuli of half rheobasic 

 (threshold) strength. It varies with stimulus intensity and is normally 

 symmetrical, but in reverse polarities at cathode and anode (Fig. 290). An 

 electrotonic wave spreads ahead of a propagated impulse and can be de- 

 tected beyond a region where conduction is blocked by cold. 



(2) As the stimulus strength increases and the polarization (electrotonic) 

 potential grows, out of it appears a local response or prepotential. This ap- 



