I: Principles of Stimulus Coding 



The detection of environmental stimuli by sensory systems may 

 be conveniently regarded as three sequentially arranged processes; 

 (i) the absorption of stimulus energy, (2) the utilization of absorbed 

 energy to effect microstructural changes in specific regions of the 

 membrane of the sensory cell, and (3) the initiation of nerve 

 impulses. Physiological studies of various sensory receptors have 

 now made it clear that these events may occur either in single cells 

 (primary sensory neurons) or in special two-cell liaisons, as is the 

 case with some visual systems in invertebrates and arthropods and 

 in cells of the vertebrate acoustico-lateralis system. In the latter 

 instances, cells modified for the absorption of external stimulus 

 energy may be incapable of supporting propagated action potentials 

 and respond electrically only by relatively slow potential changes 

 which can be continuously graded in amplitude. These types of 

 cells (hereafter referred to as sensory cells) make functional 

 contact at the periphery with second-order neurons, and con- 

 ducted impulses which are generated in the latter pass on to the 

 central nervous system. Both types of system have been inten- 

 sively investigated during recent years, and these studies have 

 served to emphasize many of the common features of receptor 

 mechanisms. Thus, functional distinctions based solely on the 

 point of origin of conducted impulses may not be especially 

 instructive. 



Whether a sensory system is modified to detect external 

 energy sources, or changes in the internal environment, the 

 magnitude and rate of change of a stimulus is communicated to 

 the central nervous system by the frequency and number of 

 impulses elaborated at the periphery in primary or second-order 



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