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HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



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TABLE I. Characteristics and Properties of Differently 

 Excitable Electrogenic Membrane 



FIG. 4. Differences between electrically inexcitable and ex- 

 citable membrane. A : The slow muscle fiber of the frog is not 

 electrically excitable and produces no spikes, even when the 

 membrane is strongly depolarized at beginning of (a). It 

 develops p.s.p. on stimulation of the nerve during the applica- 

 tion of the electrical pulse. The response at the resting potential 

 («), a depolarizing p.s.p., is increased when the membrane is 

 hyperpolarized by the applied pulse (/, g). The p.s.p. is de- 

 creased by depolarizing the membrane (</) and is reversed in 

 direction by strongly depolarizing the membrane (a to f)- The 

 magnitude of the reversed depolarizing p.s.p. increases as the 

 interior of the membrane is driven beyond an equilibrium po- 

 tential given approximately by the pulse in c. [From Burke & 

 Ginsborg (35).] B: Responses of a cat motoneuron to ortho- 

 dromic stimuli show essentially the same behavior of the p.s.p.'s, 

 but are complicated by the appearance of a spike and the inac- 

 tivation of electrically excitable membrane. The response at the 

 resting potential ( — 66 mv) is a depolarizing p.s.p. which does 

 not elicit a spike. Hyperpolarization of the membrane caused 

 little change in the p.s.p. Depolarizations to —60 mv and —42 

 mv summed with the excitatory effect of the p.s.p., evoking 

 spikes. These are no longer produced by the p.s.p.'s at the rest- 

 ing potential — 32 mv, etc. These depolarizations, after evoking 

 spikes by the electrical stimuli, then inactivated the spike- 

 generating membrane. The p.s.p.'s decreased and at a mem- 

 brane potential of +3 mv disappeared but reappeared in 

 reversed sign as the internal face of the membrane was made 

 more positive. [From Eccles (60).] 



cally excitable. Other properties of p.s.p 's that dis- 

 tinguish them from spikes are also referable to this 



Spike 

 (Electrical^ Excitable) 



P.s.p.'s 



(Electrically Inexcitable) 



A. Characterisii 

 Transducer action; 



(i) Sequential increase of 

 Na^ and K+ conduct- 

 ances and Na* inactiva- 

 tion 



(ii) Rates determined by 

 membrane potential 

 Electrical response: 

 (i) Begins with graded de- 

 polarization, develops 

 overshoot 

 (ii) All-or-none response 



Two types: 



a) increased conductances 



for all ions 

 A) specific increase in K+ 

 and/or Cl~ conduct- 

 ances 

 Rates not determined by 

 membrane potential 



Two types: 



a) depolarizing 



6) hyperpolarizing 

 Graded response 



B. Direct Evidence Jor Characteristic Differences 



Developed only by neural or 



(i) Spike absent | 

 (ii) Spike presentj 



chemical stimuli 



C. Consequences of Characteristic Differences 



(i) Always in depolarizing 

 direction 



(ii) Hindered or blocked by 

 hyperpolarization 



(iii) Excited, then blocked 

 by depolarization 



(iv) Pulsatile, relatively 



fixed duration inde- 

 pendent of stimulus 

 (v) "Vanishingly brief la- 

 tency 



(vi) Relatively inert to 

 chemicals 



(vii) Decrementless propaga- 

 tion 



Of either sign, electrochemi- 



cally reversible 

 Electrochemical gradation 



Electrochemical gradation 



May be prolonged, sustained 

 while stimulus lasts 



Appreciable, irreducible la- 

 tency 



Sensitive in two ways: re- 

 sponse may be 

 a) evoked by synapse acti- 

 vators; 

 i) depressed or blocked 

 by inactivators 



Nonpropagated, 'standing' 

 potential 



single, fundamental difference in their modes of 

 excitation. These correlations are summarized in table 

 I, and form the content of this section (cf. also 97). 



Mechanisms of Bioelectrogenesis 



The means by which a cell can generate electrical 

 activity are restricted in variety by the nature of the 

 physiological and electrochemical systems of living 

 tissues (91, 112, 113). Conductile and transmissional 



