ACTION POTENTIALS IN THE OPTIC NERVE 235 



Chronaxies may vary from 0.3 millisecond (frog sciatic) to 100 milli- 

 seconds (smooth muscle), or to such extreme values as 11,000 to 15,000 

 milliseconds (pigment cells, skin of frog). An interesting analysis has 

 been made by Lewy [1935] in which he used the chronaxie in classifying 

 825 lead workers as to their over- and underexcitability resulting from 

 certain pathological conditions. 



We may conclude that the coefficient of diminution (k) of the excita- 

 tory process is characteristic of the tissue, that the rheobase (B) and the 

 action potential taken together are not inconsistent with the above 

 hypotheses, and that the action potential is prima-facie evidence of the 

 transmission of nerve impulses. The simplicity of the hypotheses and 

 the possibility of correlating future experimental data with the theoreti- 

 cal analysis will probably lead to an understanding of the actual mechan- 

 ism of nerve conduction in biophysical terms. 



Action Potentials in the Optic Nerve 



If two non-polarizable electrodes are placed on an excised eye, one 

 electrode upon the cut face of the optic nerve and the other on the 

 cornea, and then connected in series with an electrometer, the usual 

 demarcation current is observed. If light is allowed to penetrate to the 

 retina, the electrometer will indicate a change in the magnitude of the 

 current, whose direction of flow is from the fundus to the cornea and 

 therefore in the same direction as the demarcation current. These 

 electric currents are a valuable index of the activity of the retina, but 

 they do not indicate what retinal activity is propagated to the brain. 



The effect of allowing visual radiant energy to illuminate the retina 

 is to set up a series of nerve impulses in the retina which will be detected 

 as action potentials in the optic nerve. Hartline [1938] has shown that 

 the pattern of the discharge of impulses in a single optic-nerve fiber in the 

 vertebrate eye, as a result of subjecting the retina to illumination, is 

 similar to that for the whole optic nerve as originally obtained by Adrian 

 and Matthews [1927] from the eye of the eel and later by Granit [1933] 

 from the optic nerve of a mammal. 



The optic nerve in a 3-lb Conger vulgaris specimen is about 15 mm long 

 and thinner than sciatic nerve of the frog. The retina has the normal 

 structure of the teleost eye, containing both rods and cones, and differs 

 little from the mammalian retina except in the presence of the campanula 

 and the absence of anything corresponding with the fovea. The num- 

 ber of fibers in this optic nerve is about 10,000. 



The isolated eye and its optic nerve were mounted in a light-tight box 

 at 16° C, and the nerve was supported by two electrodes. A source of 

 illumination outside the box could be focused on the retina. 



