Principles of Stimulus Coding 5 



the membrane reaction. Whatever the causes, it is clear that 

 distortion will occur only during extreme loading of the response 

 mechanisms, and over most of the effective range of frequencies 

 all impulses will resemble one another with regard to spike height 

 and waveform. This situation would certainly not be true of 

 the type of analog-responding system dependent upon a voltage 

 booster principle mentioned above. In an analog system, any 

 errors of amplification along the path taken by a signal would 

 occur as a similar percentage in all signals, not just large ones, and 

 distortion would therefore affect the detection of even the smallest 

 stimulus, and would be to some extent inherently unpredictable 

 to compensatory central nervous mechanisms. This last point is 

 probably important. It is perfectly reasonable to assume that the 

 central nervous system might weight high-frequency impulse 

 trains differently from those showing more moderate rates of 

 impulse recurrence, but it is difficult to envisage any way that the 

 higher nervous centers would know how to detect error-laden 

 analog signals from those supplying a true representation of the 

 conditions of stimulation. 



<] Fig. 2. (A) The hypothetical recording and stimulating situ- 

 ations used to obtain the relationships graphically illustrated in 

 (B) and (C). / indicates a current-passing electrode inserted 

 into a nerve axon, and e indicates an electrode to record the trans- 

 membrane potential at any point. (B) The time-course and final 

 value of potential change recorded at the three loci illustrated in (A) 

 due to a rectangular pulse of (maintained) current passed through 

 the stimulating electrode. (C) A graph illustrating the decline in 

 recorded potential along an axon with increasing distance from the 

 source of injected current. The space constant, A, is assigned 

 to that distance within which the recorded potential declines to 

 37 % of its value at the point of current injection. (D) An electrical 

 model of the axon membrane. External longitudinal resistance is 

 assumed to be negligible. 



ri, internal longitudinal resistance; rm, transmembrane re- 

 sistance; Cm, transmembrane capacitance; Sg, resting e.m.f. 



A current which is injected across the membrane, as in (A), 

 flows through the circuit composed of in-parallel resistive and 

 capacitative membrane elements. Depending on its direction of 

 flow, the current will generate a potential difference across the 

 transmembrane resistance, which either sums with or subtracts 

 from the resting potential. (From Woodbury and Patton,^"* 

 Fig. 36, c, d, and Fig. 186.) 



