442 ANNALS NEW YORK ACADEMY OF SCIENCES 



The further problem of synaptic transmission concerns the initia- 

 tion of propagated impulses by the catelectrotonic synaptic potential 

 (see PART 1, Problem (b)). The observations of Kuffler'" on the iso- 

 lated neuro-muscular junction indicate that the initiation of the im- 

 pulse occurs in the muscle membrane adjacent to the end-plate region 

 (cf. FIGURE 6d). No such intimate observations have been made for 

 synapses in ganglia and the spinal cord, but it may be assumed that 

 the catelectrotonic potentials from diverse synaptic regions sum 

 by electrotonic spread. So far, only the over-all, summed potential 

 has been observed after electrotonic spread along the axon (the so- 

 called synaptic potential.^' ''*' ^2. 23, 25) j^ setting up the discharge of 

 impulses, this synaptic potential appears to act just as a catelectro- 

 tonus, the discharge occurring at a critical degree of depolarization. 

 The synaptic potential provides a satisfactory explanation of all the 

 phenomena hitherto attributed to the central excitatory state.- 22,25 

 The "detonator response"^"' ^^ need no longer be considered as a sepa- 

 rate entity, for that hypothesis was based on experiments now explic- 

 able, in part, by the flow of penetrating current, as in figure 5, and, in 

 part, by the postulated local response of the post-synaptic mem- 

 brane.^^' ^^' ^^ 



Thus, the sequence of events in synaptic transmission is envisaged 

 as: 



(1) Impulse in pre-synaptic nerve fiber generates a current which 

 gives a diphasic effect at the synaptic region of the post-synaptic cell, 

 with a total duration of probably not more than 1 msec, in mammalian 

 muscle and the spinal cord; initial anodal focus, with cathodal sur- 

 round; more intense cathodal focus, with anodal surround. 



(2) This cathodal focus sets up a brief and intense local response at 

 the synaptic region. 



(3) From this local response, a catelectrotonus spreads decrementally 

 over the post-synaptic cell membrane. 



(4) A propagated impulse is set up in the post-synaptic cell, if this 

 catelectrotonus is above a critical value. If it is below, then, as the 

 local response subsides, the catelectrotonic surround decays passively. 



6. APPLICATIONS OF THE HYPOTHESIS 



The hypothesis offers an explanation of the following observations 

 on synaptic transmission: 



A. Irreversibility of Synaptic Transmission 



This may be explained in the following three ways: (i) An impulse, 

 artificially set up in the post-synaptic cell and fired antidromically at 



