I20 PHYSIOLOGICAL TRIGGERS 



ionic distributions between the cell interior and its exterior (109), and which 

 are probably brought about by several types of secretory activity or 'pumping' 

 (23, 114, 171, 181). A flow of ions results and this changes the membrane po- 

 tential of the cell from its resting value of about 50 to 100 millivolts, inside 

 negative. The electrogenic response may decrease (or even reverse) the resting 

 potential, or increase it, thus producing depolarization or hyperpolarization of 

 the membrane. 



Specific characteristics of different electrogenic responses are probably 

 determined by specificities in the type of transducer action with respect to one 

 or another ion, and to the kinetics of these actions. While the response develops 

 on the basis of potential energy stored during the resting state, the electrogenic 

 transducer action probably involves molecular reorientations within the 

 membrane, the nature of which is at present unknown. These changes may be 

 considered chemical reactions. However, in phenomena which occur at surface 

 films, the distinction between chemical and physical processes becomes vague. 



Thus, according to the remarkably comprehensive theory of Hodgkin and 

 Hu.xley (iii), electrical excitation of an axon leads to a small depolarization 

 and outward movement of K+. This causes about a thousand-fold increase of 

 sodium permittivity (or conductance). If this effect is considered as caused by 

 mobilization of 'carrier' molecules, the process appears to be chemical. It may 

 however, be due to a 'valving' caused by the reorientation of molecular pores 

 in the membrane (149). This could be considered essentially a physical process. 



b) Amplification. The amount of energy which may be required to initiate 

 the transducer action can be small in comparison with the subsequent release 

 of potential energy during the response. The squid giant axon is excited by an 

 electrical stimulus of only io~'^ to io~^ coulombs/cm- (96, 112), while the ionic 

 transports which occur during the spike involve about io~^ coulombs/cm'-. 

 The amplification between input (stimulus) and output (response) is, therefore, 

 100- to 1000-fold. Similarly, the end-plate potential (e.p.p.) is associated with 

 a change in charge of about io~^ coulombs (38, 71) which is sufficient to trigger 

 the spike of the muscle fiber. The latter in turn triggers the much larger re- 

 lease of energy of the mechanical response. 



c) Excitation in Conduction and Transmission. A nerve or muscle can re- 

 spond to artificially imposed stimuli, of which the electrical are most convenient 

 to produce and to apply in measurable quantity. In their normal functioning 

 in the organism, however, the excitation of electrogenic cells takes a different 

 form. Sensory nerves are excited by various more or less specific stimuli, and 

 conduct messages bearing this information to the central nervous system. 

 Motor and autonomic nerves, on the other hand, carry centrifugally messages 

 which arise at their central terminations. A transfer is then made to the effectors 

 such as muscle or gland cells. In addition, within tlie nervous system itself, 



