ECCLES: ELECTRICAL THEORIES OF TRANSMISSION 449 



the currents generated by the pre-synaptic impulses (figure 5) excite 

 the post-synaptic membrane sufficiently to produce the observed synap- 

 tic potentials and initiation of impulses. Such an intensity of action 

 is not normally attained, even by the most efficient ephapse/' *^ but 

 there are possibly three factors increasing the efficiency of the synapse: 



(i) The contact of the pre- and post-synaptic membranes is so inti- 

 mate^° that virtually all the current penetrating the former must pene- 

 trate the latter (in contrast to the estimated value of one third for the 

 ephapse*^) . 



(ii) In the synapse, a large expansion in the area of contact is pro- 

 duced by the swelling and branching of the pre-synaptic terminals; 

 also, with nerve cells, by the multiplicity of closely adjacent synapses.^" 



(iii) If the post-synaptic membrane has special excitatory proper- 

 ties (part 4, C), the efficiency of electrical excitation may be thereby 

 increased. 



By stimulating just beyond the region where a nerve volley is blocked, 

 Hodgkin" (cf. also Lorente de N6'''°) showed that, with critical condi- 

 tions for blockage, the threshold may be lowered to only 10% of nor- 

 mal, i.e., the nerve volley still provides as much as 90% of the threshold 

 electrical stimulus, beyond the blockage. Hence, there is a high prob- 

 ability that, in the absence of block, it provides an electrical stimulus 

 adequate to excite, i.e., that the transmission of nerve impulses is elec- 

 trical. It should be possible to apply a similar test to the curarized 

 end-plate of the isolated nerve-muscle fiber preparation. It has, of 

 course, been shown that there is a lowering of threshold, during the 

 end-plate potential,*^- ^^ as would be expected for a catelectrotonus, 

 however produced. The present test would explore, instead, the brief 

 interval of pre-synaptic current flow, particularly that preceding the 

 origin of the synaptic potential {i.e., AX in figure 7C). Such a test 

 has already given suggestive results with motoneurones,^* but, in order 

 to be convincing, there should be an accurate location of the stimulat- 

 ing electrode on the synaptic region of the post-synaptic membrane, 

 and, at present, this seems possible only with the isolated nerve- 

 muscle fiber (cf. Kufiler^*). An attempt on the whole sartorius was 

 unsuccessful.* 



Crucial testing of the hypothesis will also be provided by further 

 pharmacological experiments on synaptic transmission: particularly 

 the action of anti-cholinesterases, and the effects of various background 

 concentrations of acetylcholine, potassium, and calcium. Predictions 

 leading to tests have already been mentioned in the preceding sections. 



♦ Katz, B.«: K9t;. 



