NERVOUS SYSTEM AND BEHAVIOUR 



419 



nervous impulse, therefore, is derived from the movement of ions down the 

 concentration gradients. The axon possesses some active secretory mechan- 

 ism, which restores and maintains the ionic gradient across the membrane 

 by pumping out sodium. The secretory mechanism for discharging sodium 

 appears to be coupled with one that controls potassium influx. The organic 

 anions within the fibre are non-diffusible, and the chloride distributes 

 itself according to the Donnan equilibrium. 



The changes in Na + and K+ per impulse are minute relative to the total 

 concentrations of ions within the fibre. Even in the absence of some 



Fig. 10.3. Action Potential Recorded between the Inside and 

 the Outside of the Giant Axon of the Squid Loligo 

 Time marker, 500 c/s. The vertical scale indicates the potential of the internal elec- 

 trode (in volts). (From Hodgkin and Huxley (47).) 



restoring mechanism the axon is able to transmit many impulses before it 

 is fatigued. This effect depends on the size of the axon: a large fibre has a 

 greater potassium store, and hence fatigues less readily than one of small 

 diameter. 



Transmission of an impulse is explained on the basis of the local-circuit 

 theory. On applying a stimulus to a nerve there is a local build-up at the 

 cathode of electrotonic potential, which is the local response to the 

 stimulus. If the latter is above threshold the local potential gives rise to a 

 propagated response, the action potential. In this process the action poten- 

 tial continually stimulates adjoining regions of the fibre by current flow 

 away from the active point, and a self-propagating disturbance arises, 

 which proceeds along the fibre to its terminus (30, 31, 43, 46, 46a, 47, 48, 

 49, 50, 51, 60, 63, 109). 



