Excitation Triggers in Post-Jimctional Cells 



HARRY GRUNDFEST 



Department of Neurology, College of Physicians and Surgeons, 

 Columbia Universitx, New York Cilv 



PHYSICAL METHODS often |)ermit the elect rophysiologist to determine with 

 high precision the onset, magnitude and time course of bioelectric activity 

 of cells. These methods can also provide considerable information regarding 

 some of the phenomena underlying the responses. Thus, although the membrane 

 molecular changes involved in electrogenesis are not yet known, electro- 

 physiological data to some extent provide specifications as to the nature of their 

 trigger mechanisms. Electrogenic responses all have one feature in common. 

 They are all produced by changes in membrane permeability and these change 

 the electrical state of the polarized cell membrane. Specific types of membrane 

 molecular structure, the electrochemical conditions, and probably other factors, 

 determine the nature of the various types of electrical activity of electrogenic 

 cells. 



EXCITABLE MEMBRANE AS TRANSDUCER 



The bioelectric response is essentially the utilization of potential energy of 

 the cell, and this process may be termed transducer action. The overt initial 

 change is an alteration of the ionic permittivity of the membrane in response 

 to a specific stimulus. Emphasis must be placed on the ionic aspects because 

 ion movements are probably the only means for carrying a current across the 

 non-metallic barrier of the membrane.^ However, as with other types of trans- 

 ducers (e.g. the piezo-electric crystal) the effectiveness of biological transducer 

 action is enhanced by amplification. In the cell the two processes may be 

 closely linked by an interlocking mechanism (129) and are usually considered 

 together (27, 57, 183). 



a) Transducer Action. The change in permittivity which results during the 

 response temporarily disturbs the steady state inequalities which exist in the 



' The possibility of electron transfers playing a role in bioelectrogenesis is an intriguing 

 subject (of. 47). However, while such systems may be theoretically possible when operating 

 in combination with ionic movements, they must be examined rigorously when considered 

 in the absence of the latter. For example, a scheme in which it is claimed (54, p. loi) that 

 oxidation-reduction potentials "can be measured with non-metallic, i.e., calomel, electrodes" 

 consists of an ordinary redox cell, but with the metal forming the membrane between the 

 two solutions. This transposition does not alter the principle (cf. 108) of action of the cell, 

 although giving the appearance of electron transfer without metals. 



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