The Depolarizing Nature of the Trigger 19 



the only logical explanation for observations that localized regions 

 of axons in the vicinity of the stimulating cathode remained 

 hyperexcitable for varying periods following a sub-threshold 

 conditioning shock. Since the period of raised sensitivity was too 

 long to be explained by the residual polarization from the con- 

 ditioning shock — a phenomenon resulting from the finite time 

 taken to recharge the nerve membrane capacitance — it was 

 concluded that local sub-threshold activity of the fiber was 

 responsible for the period of heightened excitabiUty. Within the 

 same year, Hodgkin^" was able, by electrical records from single 

 crustacean axons, to provide direct proof of the existence of this 

 type of activity. Many crustacean peripheral nerves are only 

 loosely invested with connective tissue, and, as a result, individual 

 nerve fibers can be rather easily separated from the main bundle. 

 Preparations of this kind are particularly well suited for extra- 

 cellular recordings of low-amplitude transmembrane events; the 

 axons are unusually large, being 30-50 microns in diameter, and 

 the proportion of active to inactive tissue is consequently greater 

 than that found in multifibered preparations. Now, since inactive 

 tissue not only adds nothing to the electrical potentials generated 

 during membrane activity, but provides for a lower extracellular 

 resistance in the environment of the active tissue, focal activity 

 along a nerve with a good deal of extraneous tissue is subjected 

 to an amount of short-circuiting by non-active conductor; in 

 single-fiber preparations short-circuiting of this sort is con- 

 siderably reduced, and the signal-to-noise ratio is correspondingly 

 increased. 



Figure 6 illustrates some of Hodgkin's original records. With 

 small shocks, both anodal and cathodal pulses produced identically- 

 shaped potential changes at the neighboring recording electrode. 

 In both cases, the smooth exponential decay of potential due to 

 small current pulses is diagnostic of a recharging membrane capa- 

 citance, following the transient alteration of the resting potential 

 by the impressed stimulus. Larger cathodal shocks (which were, 

 however, still sub-threshold for impulse initiation) produced in 

 the membrane close to the point of stimulus application an 

 electrical response which had a time-course undeniably longer 

 than that occasioned by the passive decay of charge. This effect 

 could not be evoked when stimulus polarity was reversed, i.e. 



