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2: The Depolarizing Nature 

 of the Trigger 



Changes in electrical potential which are associated with excitation 

 in receptor structures have been known for over a hundred years. 

 Holmgren^* is usually credited with the discovery in 1865 of the 

 electroretinogram — ^the slow graded change in potential difference 

 across the retinal layer of the eye which results from photic 

 stimulation. Since Holmgren's time, amplitude-graded potential 

 changes have been recorded from other sensory systems having a 

 large population of receptor cells, notably the vertebrate ear and 

 organs of the lateral line in fish.^^ In all these cases, where 

 numerous cells contribute to the recorded signal, the synchronous 

 activation by an appropriate stimulus produces sizeable potential 

 gradients across the sensory structures, and they can be detected 

 with relatively unsophisticated techniques. 



The recognition that nerve cells, as distinct from non-neural 

 sensory cells, are also able to support graded electrical activity has 

 come about only within the last thirty years. In the early 1930s 

 it seems to have been appreciated by physiologists that some sort 

 of graded sustained electrical change within a neuron might 

 account for the control of impulse frequency and number. The 

 first experimental evidence, however, directly implicating graded 

 neuronal activity was not obtained until 1937. In that year and 

 the next, the results of research efforts in two separate laboratories 

 provided evidence to support the contention that nerve axons are 

 intrinsically capable of generating localized electrically-evoked 

 activity which can be graded in amplitude by varying the intensity 

 of the stimulus. Katz^' deduced the presence of such activity as 



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