HARRY GRUNDFEST 



B- rs. C 



127 



Fig. 2. Responses of eel electroplaque to paired stimuli. Above: A direct stimulus was 

 followed by a maximal stimulus to a nerve trunk supplying the cell with a nerve. Belou:- Both 

 stimuli were applied to the same nerve, the first strong, the second somewhat above threshold 

 for a spike. Recording conditions are shown in the diagram. While the cell is still refractory to 

 a second direct stimulus following the conditioning activity, it can develop a p.s.p., seen on 

 the falling phase of the conditioning spikes (A). At successively longer intervals between the 

 stimuli (B-D) the p.s.p. increases and evokes graded responses which are largest at site 2. 

 Still in the relatively refractory period, the neural stimulus then evokes small, delayed spikes 

 (F-G). The testing neurally evoked responses are seen in isolation (H). Below: With the con- 

 ditioning stimulus also a neural volley (A'), the second stimulus does not evoke a p.s.p., 

 because the nerve is in absolute refractoriness. During its recovery (B'-D') a p.s.p. develops 

 and grows, then causes spikes (E'-F'). The electroplaque is still relatively refractory, as may 

 be seen from the small spikes in E', but homosynaptic facilitation causes their earlier, almost 

 synchronized development on the p.s.p. in comparison with the effect of the weak testing 

 volley in isolation (C). 100 mv and msec, calibration on each of the recording traces (from 

 ref. 4). 



an explosive mechanism is imposed (7, 98). Graded membrane responsiveness 

 to electrical stimuli might result if its transducer elements differed widely in 

 their electrical threshold. A small imposed membrane depolarization then would 

 initiate transducer action only in the most excitable elements, but the resulting 



nomenon is the change in electrical excitability of uterine muscle during physiological or in- 

 duced hormonal changes (17, 55). The anestrous muscle responds to electrical stimuli (17) 

 or to neural (166) only with local activity. In the estrous phase, or after treatment with 

 estrone, the response becomes propagated. Uterine muscle can be converted from propagating 

 responsiveness (estrogen-dominated, ref. 55) to the non-propagating (progesterone-domi- 

 nated). The relation between the different types of electrical responsiveness and coupling of 

 these to the mechanical response of muscle were considered briefly by .A.ltamirano el al (4), 

 but cannot be discussed here. 



