2 The Physiology of Sense Organs 



nerve cells. Information about a stimulus is thus pulse-coded, 

 and the correct response which an animal makes to an environ- 

 mental change depends upon the ability of the central nervous 

 system to interpret correctly the changes in interval between, 

 and/or the number of, impulses which arrive through the indivi- 

 dual axonal pathways from each receptor system. These impulses, 

 or action potentials, are self-propagating potential changes of 

 fixed amplitude. Among animal groups, they have been evolved 

 and maintained as an apparently universal mechanism for rapidly 

 and precisely conveying information over substantial inter- 

 organic distances (fig. i). 



Fig. I. An action potential evoked in a crayfish primary 

 sensory neuron by stimulating its axon with a brief electrical 

 shock. The resting potential of the cell, indicated by the 

 distance between the horizontal straight line and the 

 electrical trace, was about 80 millivolts in magnitude. The 

 action potential overshoots the zero potential level by about 

 20 millivolts at its peak amplitude and then decays, as is 

 typical with many crustacean neurons, without any notice- 

 able undershoot of the resting potential level. The duration 

 of the wave-form at the half-amplitude level is o*8 milli- 

 seconds. 



Like many other cell types, neurons and sensory cells maintain 

 a potential difference across their plasma membrane. Nerve 

 axons are roughly circular in cross-section, and they are hundreds 

 or thousands of times longer than they are wide. Changes in 

 potential at one site along a nerve are accompanied by current 

 flow across the membrane at that point. If the ratio of the elec- 

 trical resistance of the membrane at all points on the axon to that 

 of the internal cjrtoplasm (axoplasm) were infinitely large, the 

 currents originating at the source would spread for large distances 

 within the axis cylinder, and the potential changes engendered by 

 their movements across the electrical resistance of distant mem- 



