42 The Physiology of Sense Organs 



If an outward current is caused to flow across an area of 

 electrically-excitable membrane (either from an instrumental 

 source or from a region of the cell generating a receptor current), 

 the potential difference across the membrane will at first change 

 with time in an exponential fashion. The final value attained will 

 be dictated by the effective transmembrane resistance and the 

 density of the current flowing across it. The time course of these 

 changes will be determined by the product of membrane resistance 

 and capacitance. However, membrane impedance (RxC) is 

 known to undergo dramatic changes during an action potential: 

 the action currents during the spike first effecting a large depolariz- 

 ing overshoot; this is followed, as has been explained on page 8, \ 

 by a repolarization, which may reach the potassium equilibrium 

 potential, even in the face of a sustained depolarizing influence. 

 r As the membrane impedance returns to its resting values, the 

 \ depolarizing generator currents once more effect an exponentially 

 j rising change in the IR drop across the membrane. In the absence 

 of other factors (such as changes in impulse threshold and develop- 

 I ment of a local membrane response) spike frequency at any level of 

 / depolarization would depend on the membrane time-constant, 

 ^ for it is this factor that governs the rate of change of membrane 

 depolarization following recovery from each action potential. 

 ' Other factors ', however, cannot be ignored; at least two addi- 

 tional properties associated with electrically-excitable membranes 

 complicate the process of frequency control. As Hodgkin^** first 

 discovered in 1938, impulse threshold is attained, not by passive 

 depolarization of the membrane (appearing as an IR drop due to 

 extrinsically generated currents), but only after the development 

 of a local sub-threshold response. This response is initiated 

 by levels of membrane depolarization lower than those required 

 to trigger a full action potential. Two wholly independent 

 properties of the membrane thus jointly determine the rate at which 

 a depolarizing influence arrives at threshold levels for impulse- 

 initiation. The situation is diagramatically illustrated in figure 

 16. In addition to those factors which affect the rate of change in 

 membrane potential of electrically-excitable tissue, the impulse 

 threshold itself undergoes changes in value during and following 

 each spike. Such changes in excitability delineate the relative and 

 absolute refractory periods that occur in impulse-generating 



