Initiation of Nerve Impulses in Cochlea and 

 Other MechanO'Receptors' 



HALLOWELL DAVIS 



Central Institute for the Deaf, St. Louis, Missouri 



FROM THE POINT OF VIEW of a iieurophysiologist who is particularly interested 

 in sense organs, the term 'trigger action' might apply to two or three rather 

 different aspects of the activity of the cochlea. It might apply to the release of 

 patterns of activity of the organism as a whole which are 'triggered' by some 

 particular sound. Specilic vigorous reactions to sensory stimulation by sight, 

 sound or smell are certainly commonplace in biology, and certainly the sense 

 organs are the pathway whereby the information that sets off the specific re- 

 actions reaches the central nervous system from the external world. 



At a much simpler level, the level of general physiology, the term 'triggei 

 action' applies very aptly to the stimulation of a nerve impulse by electrical, 

 chemical, mechanical or any other means. Here, too, a response that involves 

 the liberation of energy by the stimulated tissue runs a well-defined, predeter- 

 mined course. It is initiated by a stimulus, or 'trigger.' In this particular in- 

 stance, the reaction is highly stereotyped. Also, many different stimuli can be 

 used, although all of them share the requirement that a certain threshold value 

 of stimulation must be produced if triggering is to occur. 



This type of 'trigger action' occurs in sense organs, and we shall examine one 

 of them in some detail to see how mechanical vibrations can serve to set off 

 nerve impulses. We shall be dealing here with trigger action at the level of the 

 cell rather than at the level of the organism. Both levels of trigger action have in 

 common the release by the organism or cell of more energy than was provided by 

 the stimulus. A stereotyped pattern ensues, and the sequence of events is then 

 determined by conditions other than those which constituted the original 

 trigger. 



The term 'trigger action' certainly implies, although perhaps it does not 

 require, that the amount of energy involved in the response shall be large rela- 

 tive to the amount of energy required to pull the trigger. Such a relation is very 

 clear in the contraction of a muscle when stimulated by either a motor nerve 

 impulse or an electric shock. It is rather less evident in the case of a single 



'Prepared untler Contract N60NR-272 between the Central Institute for the Deaf and 

 the Office of Naval Research. Rejjrodnction in whole or iii part is permitted for an\ |)urpose 

 of the United States Government. 



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