156 HANDBOOK OF PHYSIOLOGY -^ NEUROPHYSIOLOGY I 



FIG. 7. Absence of refractoriness in postsyn- 

 aptic responses in the giant neuron of Aplysia. 

 A : A single shock to the presynaptic nerve first 

 evokes a long-lasting p.s.p. out of which rises 

 the spike of the giant neuron. B: A second stim- 

 ulus, exciting the cell during its refractory pe- 

 riod, adds a potential (solid line beginning at 

 arrow) to the initial response (broken line). 

 The difference (dotted line, below) is due to 

 the second p.s.p. C: The second stimulus was 

 delivered somewhat later. The added potential 

 also shows a local response (prep.") which was 

 initiated by the p.s.p. in the electrically ex- 

 citable membrane during the relatively refrac- 

 tory period. D: At a longer interval, a second 

 stimulus evokes the full response as in A. [Froin 

 Arvanitaki & Chalazonites (11).] 



B 



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looms 



ducer action of the membrane can then act as a further 

 stimulus to the latter. The positive feedback of the 

 effect leads to a regenerative sequence and to the 

 explosive, all-or-none spike. Since the transducer 

 actions of electrically inexcitable membrane are not 

 affected by the electrogenesis of the p.s.p.'s, feedback 

 either positive (in the case of the depolarizing), or 

 negative (for the hyperpolarizing p.s.p.'s) is lacking. 

 Because of the absence of electrical feedback p.s.p.'s 

 of either sign are thus produced that are graded in 

 proportion to the availability of the specific excitants 

 of the respective transducer actions. 



Ot/wr Consequences of Electrical Incxcitability 



a) susT.oiiNED ELECTROGENESIS. Thc transducer actions 

 of the spike generator are a sequence of potential- 

 determined events, the first (.sodium conductance) 

 tending to cause the depolarizing electrogenesis, 

 others (potassium conductance, sodium inactivation) 

 tending to terminate it and to restore the resting 

 potential. The sensitivity of these processes to the 

 changes in membrane potential produced by the 

 electrogenesis itself thus leads to a self-limiting event, 

 the spike, of rather constant duration with which is 

 also associated refractoriness (113). Not being elec- 

 tricallv excitable, the transducer actions of the syn- 



aptic membrane are relatively in,sensitive to the 

 changes of membrane potential. Hence, p.s.p.'s 

 may be sustained as long as the excitant of the trans- 

 ducer action is available (fig. g) since they are not 

 subject to refractoriness (figs. 6, 7) nor inactivation. 

 The transducers of most types of sensory membrane 

 are probably also electrically inexcitable (94, 95, 97). 

 The sustained graded electrogenesis which can de- 

 velop to a sustained stimulus is the means for trans- 

 mitting information by a train of pulsatile spikes, 

 coded as to frequency and number in some relation 

 to the intensity and duration of the stimulus (97, 103; 

 fig. 10; cf. fig. 13). The transducers of some mechano- 

 sensitive organs, at least, also have chemical sensitivity 

 (94, 96, 97), indicating further their relations with 

 chemically sensitive synaptic membrane. 



Although the postsynaptic membrane, in contrast 

 to the electrically excitable, is capable of sustained 

 electrogenesis, its responsiveness to a steady stimulus 

 mav be affected in various ways. These reflect the 

 labilitN of thc membrane in the face of the very chemi- 

 cal agents by which it is excited (95, 96). An example 

 is the gradual diminution or even disappearance of 

 synaptic electrogenesis when a muscle or autonomic 

 ganglion is continuously acted upon h\ acet\ Icholine 

 or other agents (123, 127, 129, 187). 



