HARRY (iRUNDFEST I43 



quantity is involved. In other cases, either under nonnal conditions when the 

 transmitter is stable and present in relatively large quantities or when a rapidly 

 inactivated transmitter is protected against destruction (e.g. as is acetylcholine 

 by anticholinesterases), the transducer response may persist and the p.s.p. 

 duration may then be long (38, 72). In most known cases, the p.s.p. lasts many 

 limes longer than the spike of the same cell, but in two known cases their dura- 

 tions are approximately equal (squid, ref. 29; eel electroplaque. ref. 4). However, 

 the significance of this is not known.^ 



j) Role of P.S.P. in Transmission. If depolarizing, the p.s.p. acts as an 

 excitant to adjacent electrically excitable membrane (providing the cell has this) 

 just as an external electrical stimulus or the electro tonic potential of the local 

 circuit does. Small excitatory p.s.p. would tend to elicit the local, graded re- 

 sponse and this w'ould sum with the p.s.p. over an area of the membrane. As 

 the p.s.p. is increased, so is the local response until their sum causes sufficient 

 depolarization to trigger the spike (fig. 9). 



The inhibitory p.s.p. would of itself cause only conductance changes charac- 

 teristic of electrotonically anodally polarized membrane (e.g., 62), but these 

 would be opposed by the increased conductance of the activated postsynaptic 

 membrane. In the presence of membrane depolarization it would depress the 

 latter, both by the algebraic summation of opposite membrane charge and also 

 by tending to poise the membrane at its own equilibrium potential, thus pre- 

 venting development of the depolarizing electrogenic response. In the crayfish 

 stretch receptor, the latter action produced near the sites of the mechano- 

 transducer potential generator decreases the electrotonic spread of the generator 

 e.m.f. to the electrically excitable portions of the cell (134). 



k) Synaptic Delay. The response of muscle or nerve to electrical stimulation 

 occurs with vanishingly small latency. Junctional excitation, however, is always 

 associated with a considerable minimum latency, the synaptic delay. This 

 interval, observed in 1882 by Bernstein (100), ranges from about 0.5 milli- 

 seconds in cat motoneurons to values of ten or more times longer, as in sympa- 

 thetic ganglia (66). Slowed conduction in the terminals of the prejunctional 

 nerve fiber may account for some of the synaptic delay (140). However, in the 

 eel electroplaque the stimulus can be applied to the nerve terminals themselves, 

 yet the responses evoked in this way always have a latency of i or more milli- 

 seconds (4) and must be interpreted otherwise. It appears likely (100, loi) that 



'The time constant of the eel electroplaque membrane is 0.1-0.2 msec, or less (2, 130), 

 but that of the squid axon membrane is about i msec. The p.s.p. of the latter is reported to 

 have a hyperpolarizing phase (29) as does the spike. However, the graded response also has 

 this phase (11, 125) and the apparent hyper-polarization of the p.s.p. may have been caused 

 by addition of local (i.e., graded, electrically excitable) response to the synaptic. The responses 

 of electric organs of Torpedo and Malaptenirus, which are elicited only by neural stimuli are 

 therefore p.s.p.'s. The durations of their discharges are only 2-3 msec, as in Electrophorus 

 (cf. 100). 



