ECCLES: ELECTRICAL THEORIES OF TRANSMISSION 445 



local response provides the active region for setting up a spreading 

 catelectrotonus — the synaptic potential. Thus, the synaptic delay (in 

 FIGURE 7, c and d), is represented by the interval AX. The value of 

 0.6 msec, which has been observed for this synaptic delay at mamma- 

 lian neuro-muscular junctions'*^' ^* and motoneurone synapses"^ (frog, 

 1.3 msec.) accords well with tlie duration of the pre-synaptic action 

 potential (figure 7a), when it is remembered that there is probably 

 some slowing of time-course, as the impulse propagates into the fine 

 pre-synaptic terminals.'*® As shown in figure 7d, a further delay, XZ, 

 usually 0.2 to 0.3 msec, is involved in the building up of the synaptic 

 potential to the threshold, for initiating an impulse at Z."- ^^' ^^ 



On the basis of figure 7, the hypothesis offers a satisfactory explana- 

 tion of all the experimental findings on synaptic delay. For example: 

 (i) By facilitation, synaptic delay cannot be shortened below a limit- 

 ing value of about 0.5 msec, for central synapses.^'-* In figure 7b and 

 7c, AY would be the minimal interval at which excitation could occur, 

 under optimal conditions of facilitation, (ii) Synaptic delay can, 

 however, be further shortened by the direct excitatory action of a pre- 

 ceding subliminal induction shock. '"^ By its depolarizing action, the 

 shock would diminish the time lag between reversal of current and re- 

 versal of potential, and so shorten synaptic delay to less than AY 

 (figure 7b). (iii) The longer synaptic delay with sympathetic gang- 

 lia (about five times longer) correlates with the longer duration of the 

 pre-synaptic spike.^" which sets the time scale throughout figure 7. 

 (iv) The upper limiting value of synaptic delay, for example, about 

 1.0 to 1.5 msec, for mammalian central synapses'^' "• ^^' ®°' " and neuro- 

 muscular junctions,-*'' ^^ has been correlated with the time of the rising 

 phase of the synaptic potential,^'*' ^'^ and thus, according to figure 7d, 

 to the duration of the local response of the post-synaptic membrane 

 (cf. C, below), (v) Synaptic delay (neuro-muscular in frog)"^ has, 

 as would be expected from figure 7, approximately the same tempera- 

 ture coefficient (2.1) as has the duration of the spike potential. 



C. Time-Course of the Active Phase of the Synaptic Potential 



The time-course of a local response is but little slower than the spike 

 potential.'*^' ^-' °^' ^°' ''^ According to the hypothesis, therefore, the 

 brief phase of active polarization (determined by analysis of the synap- 

 tic potential) should have a time-course somewhat slower than the 

 spike of the post-synaptic cell. This accords well with the findings on 

 ganglion cells,^^ motoneurones,^^ and muscles.^® Furthermore, the tem- 

 perature coefficient of this "active phase" is approximately the same 

 as for a spike.^^ 



