The Depolarizing Nature of the Trigger 23 



An interesting model sense organ was examined in 1946 by 

 Bernhard and Granit,^^ which exhibited electrical characteristics 

 similar to those that had been proposed for primary sensory 

 neurons. The model consisted of a length of cat sciatic nerve 

 which was caused to fire impulses in a repetitive fashion by local 

 cooling of a restricted region near one end. With an electrode 

 pair placed so as to record the steady potential difference between 

 the cooled region of the nerve and another region five centimeters 

 away, it was possible to detect that the former region became 

 electrically negative during the application of the stimulus. This 

 effect was reversible, and it is presumed that the maintained 

 cathodal potential was acting as the generator of the impulse trains^ 

 This finding lent support to the theory that sustained electrical 

 changes in sensory neurons are the natural triggers for impulses 

 arising in these structures. 



It was not until 1950, however, that the first electrical records 

 were obtained which unequivocally demonstrated the presence of a 

 graded and sustained generator potential within a primary sensory 

 neuron. The credit for this discovery belongs to Bernard 

 Katz,^®» ^' who reinvestigated a preparation which had been made 

 famous twenty-five years earlier by Adrian, namely, the frog 

 muscle spindle. Now vertebrate muscle receptor organs are 

 supplied by primary sensory neurons whose axons originate from 

 cells within the central nervous system. Characteristically, the 

 fibers are myelinated except for their terminal portions, which are 

 inserted into the end-organ. In some muscles, which control the 

 movement of the toes in the frog, the number of sensory axons per 

 muscle is small, the individual axons being 30 to 60 microns in 

 diameter. Katz was thus able to account for all the nerve fibers 

 entering the particular muscle he used and, subsequently, to 

 transect all nervous connections, save for a single axon. The inset 

 in figure 7 illustrates the recording arrangement used for the detec- 

 tion of graded electrical activity. If the length of sensory nerve 

 between the spindle end-organ and the active recording lead, £2, 

 was sufficiently short, sustained low-amplitude potential changes, 

 as well as trains of action potentials, were recorded by a direct 

 coupled amplifier as a result of applying stretch to the spindle. 

 Tjrpical records are shown in figure 7. Katz was especially careful 

 in interpreting the records obtained with these procedures. Two 



