78 The Conduction of Impulses by Nerves /4 : 4 



However, certain invertebrate axons are easier to work with because of 

 their greater diameter. The maximum diameter of the vertebrate axon 

 is about 20 microns. By contrast, squids have axons larger than 200 /x 

 in diameter; these are visible to the unaided eye. Insects, too, sometimes 

 have large axons; the cockroach has some as large as 50 \x. The extreme 

 size of the squid axon has made it of especial interest and importance in 

 all studies of the conduction of impulses by nervous tissues. The 

 synapses of the large invertebrate neurons have also proved convenient 

 to study. The material in this chapter and in Chapter 24 is based in 

 part on studies of invertebrate axons. 



4. The Spike Potential 



An electrode placed on the crushed end of a nerve bundle is in contact 

 with the interior of the axons of this nerve. The potential is negative 

 relative to the medium surrounding the axons. This potential, in normal 

 axons, ranges from 50 to 100 mv. It is comparable in size to the contact 



potentials and polarization poten- 

 Outside Medium tials which can occur at electrodes. 



•+■ 4- J- J. J. J. _i_ ■ 



However, when these artifacts are 

 Axophsm (or Cytoplasm ) reduced to the microvolt range, the 



true resting potential of the nerve 

 axon remains. Diagrammatically, 

 the axon may be represented, as in 



Figure 4. Diagrammatic representation Figure 4, by an insulator shaped as a 



of resting axon. This figure is an alter- cylindrical shell. The inner and 



nate way of expressing the information outer faces of ^ sheU are charged> 



in Figure 1(c). The hollow §hdl j g fiUed wkh Qne 



conducting medium (cytoplasm) and 

 immersed in another (intercellular fluid). This picture applies to all 

 axons except the thickly myelinated ones, which will be discussed further 

 later in this section. 



The existence of these potentials across the extremely thin axon 

 membrane indicates the ability of these membranes to withstand very 

 high electrical field strengths. Dry air breaks down at 3 x 10 6 v/m. 

 Many insulators (including corrosion on spark plugs) raise the field 

 strength necessary for breakdown of air as high as 10 8 v/m. At the 

 surfaces of many biological cells, including neurons, it appears that 

 high field strengths of about 10 8 v/m occur. These are in such small 

 regions that numbers for air prove misleading. The cell membranes 

 are more nearly analogous to the junctions between two dissimilar 

 metals. At the latter, field strengths as high as 5 x 10 9 v/m are known 



