450 ANNALS NEW YORK ACADKMY OF SCIENCES 



111 addition, iiiHtlifiiiatical treatment of the flow of penetrating current 

 at a schematized synapse should be possible, and would give more pre- 

 cise predictions for experimental testing. Since the hypothesis is 

 based on the investigations on nerve and muscle fibers outlined in 

 PART 3 (particularly the ephaptic experimentsj , further developments 

 of this work are of immediate relevance as tests of the hypothesis, 

 providing data on which will depend its development, or modification, 

 or rejection in whole or in part. 



Finally, it may be stated that a recommendation for the hypothesis 

 is its systematization of synapses and neuro-muscular junctions in a 

 series (neuro-muscular junctions, ganglionic synapses, and central 

 synapses) , exhibiting a progressive replacement of acetylcholine trans- 

 mission by electrical transmission. To the beginning of such a series 

 could be added those special modifications of ganglionic and neuro- 

 muscular synapses, seen, respectively, with the chromaffin organs (su- 

 prarenal medulla) and electric organs, where synaptic transmission 

 seems to be wholly due to acetylcholine. 



9. SUMMARY 



• Transmission of impulses across synapses of the spinal cord, sympa- 

 thetic ganglia, and skeletal muscle, involves a dual problem: (a) the 

 setting up of synaptic, catelectrotonic potentials in the post-synaptic 

 cell; and (b) the initiation of impulses in the post-synaptic cell by such 

 potentials. Evidence is given that, in their present form, both the 

 chemical (acetylcholine) and electrical hypotheses relating to Prob- 

 lem (a) are unsatisfactory. Furthermore, recent experiments are 

 cited which indicate that acetylcholine plays a negligible part as a 

 synaptic transmitter with motoneurones ; a subsidiary role with sym- 

 pathetic ganglia, and possibly also with skeletal muscle. Hence, it is 

 desirable to attempt a more precise formulation of the electrical hy- 

 pothesis. 



. The present attempt is based, mainly, on four lines of recent inves- 

 tigation : 



(1) The electrical properties of surface membranes: resistance, elec- 

 tromotive force, capacity, and rectification. Inductance is neglected, 

 because it is doubtful if the high values of cephalopod axons obtain for 

 vertebrate nerve and muscle. 



(2) The existence of active load responses. These may be very 

 large in refractory, or anesthetized, or deteriorated nerve. 



(3) The electrical actions occnrring (uto.s.s artificial synapses [ephap- 

 ses) . There are shown to be three main types of ejih apses, the synapse 



