164 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



FIG. 15. Effect of nondepolarizing synaptic blocliing agents 

 on the responses of the eel electroplaque. Direct stimulation of 

 the cell is represented in A to F, neural excitation in A' to C. 

 The initial responses to both stimuli are shown in .4 and A'. At 

 3 min. after substituting a bathing solution containing 5 mg 

 per ml o'-tubocurarinc, the directly elicited spike was unchanged 

 (B), but synaptic excitation was less effective, the spike arising 

 later on the smaller p.s.p. (£')■ A* 5 ""in. (C, C) the directly 

 elicited response was still unaffected, but the p.s.p. had de- 

 creased so much (,C") that it was seen only with repetitive 

 stimulation at 50 per sec, and produced a single small 'spike,' 

 after which it could no longer affect the electrically excitable 

 membrane. The latter, however, remained fully responsise to a 

 direct stimulus 41 min. later (/)), but eventually this responsive- 

 ness decreased (96 min. later, E; and 1 1 o min. after this, F). 

 The resting potential of the cell was unchanged. Calibration 

 100 mv and msec. [From Altamirano et al. (6).] 



the presence of the chemical stunulant, leads to 

 inactivation of the spike-generating membrane as 

 described above. The entire cell may then become 

 inexcitable by direct stimuli (fig. 13). In the case of 

 skeletal muscle fibers, the inactivating depolarization 

 is confined to the regions of the endplates and neuro- 

 muscular transmi.ssion is blocked Ijecause these 

 regions do not generate spikes. Neuromuscular jjlock- 

 ade evoked by 'depolarizing' synap.se activating drugs, 

 and blockade also at neuronal synapses, are usually 

 preceded by a brief period of hyperactivity. The dis- 

 organized contraction of muscles, frequently but 

 incorrectly termed 'fasciculation', is due to the initial 

 excitatory effect of the synaptic depolarization, the 

 individual muscle fibers responding to this stimulus 

 before their spikes are inactivated. Blockade by the 

 truly curarizing drugs, the inacti\ators of synaptic 

 activity, is not preceded l)y the excitatory eflfects. 



Postsynaptic Potential': as \onpropagated 

 ' Standing' Potentials 



The local circuit current of activity, in combination 

 with electrical excitability, makes possible the con- 

 ductile property of electrically excitable, regenera- 

 tively responsive membrane (fig. 8). The all-or-none 

 character of the spike then leads to decreinentless 

 propagation. A consequence of electrical inexcitaljil- 

 itv is that the p.s.p.'s do not set off activity in other 

 portions of synaptic membrane. The electrogenesis is 

 therefore localized and does not propagate except 

 electrotonically as mentioned earlier (fig. 2). This 

 'standing' nature of p.s.p.'s has important physiologi- 

 cal consequences that will be discus.sed later. It also 

 introduces a technical complication in the interpre- 

 tation of potentials recorded from volume conductors. 

 The rules that apply to potentials generated by a 

 travelling impulse (cf. 140, 141) need not hold, 

 particularly since hyperpolarizing as well as depolar- 

 izing p.s.p.'s of the ' standing' variety can be produced 

 at various sites (cf. 161-167).' It is of more than his- 

 torical interest to note that Sherrington and his 

 colleagues sue:gested that the central excitatory state 

 (c.e.s.) "is a specialized manifestation of local exci- 

 tatory state." (44, p. 43). In the present da\- contexts, 

 the central excitatory state may be identified in large 

 measure with occurrence of depolarizing p.s.p.'s, 

 and the central inhibitory state with of hyperpolariz- 

 ing p.s.p.'s. However, phenomena such as desensitiza- 

 tion (p. 157) may obscure or eliminate this parallelism 

 between potentials and excitability. Thus, as appears 

 in figure 11, the depolarized but also desensitized 

 endplate may not respond to a stimulus. Such a condi- 

 tion might lead to blockade of transmission although 

 the background is one of depolarization. Desensitiza- 

 tion of hyperpolarizing synapses has not yet been 

 described, l)ut its occurrence is not unlikely. If it 

 exists, it could provide cases of lifting of inhibitory 

 blockade in the face of a background of hyperpolariza- 

 tion. It will be shown later that the responsiveness of 

 electrically excitable membrane (its local excitatory 

 state) can change without a parallel change of the 

 membrane potential, although the excitability of this 

 meinbrane is also a reflection of the action of tjraded 

 local responses. 



' An extreme example of localized activity which is therefore 

 highly instructive has been reported in the cat cortex (150, fig. 

 19). Within a range of 20 /i in the depth of the cerebral cortex 

 the pattern and degree of electrical acti%ity undergoes great 

 modifications. 



