INTRODUCTION 3 



Receptors change reversibly when energy is apphed to them (i.e., 

 they are sensitive, they detect), they do work (i.e., they are responsive)] 

 they generate a message to be transmitted beyond their boundaries. 

 Thus, there are three levels at which the physiology of receptors may 

 be considered. 



Considered first as detectors, receptors would not make much 

 information available to an organism if they were so imperfect as to be 

 insensitive to any but the greatest energy changes or so perfect as to 

 be sensitive to every single elementary particle. Furthermore, their 

 usefulness would be limited if they were so indiscriminate as to detect 

 equally all kinds of particles and so simple as to detect only the pres- 

 ence or absence of a stimulus. From the point of view of an animal's 

 survival, there must be an optimum sensitivity, a capacity for discrim- 

 inating different kinds of stimuli, and a capacity for measuring not 

 only on-off, but rate of change, magnitude of change, absolute change, 

 and direction of change. 



It is characteristic of protoplasm that it has moderate sensitivity to 

 many forms of stimuli and may be sensitive to more than one para- 

 meter of the stimulus. But receptors have become specialized in that 

 they possess enhanced sensitivity to some particular form of energy 

 and to some particular parameter. First, there is development of 

 special characteristics at the molecular level. The presence of a pig- 

 ment to absorb radiant energy in some particular wavelength 

 (rhodopsin in the rods and cones of the vertebrate eye) or the presence 

 of molecules to uncouple the potential energy in sugar molecules 

 (sweet taste receptors) are more well-known examples. 



In addition to specialized sensitivity per se, receptors have also 

 become surrounded with accessory structures whose presence 

 modifies in various fashions the incident energy. A striking example is 

 the complicated accessory structures of the auditory labyrinth of the 

 vertebrate ear. These accessory structures may not only affect the 

 sensitivity of the combined system but may also determine which 

 parameter (magnitude, rate of change, etc.) of a stimulus may be used. 

 Thus, refinement of the receptor from a generalized non-discriminat- 

 ing sensing element with many imperfections has come about through 

 evolutionary specialization of the sensing part of the receptor itself 

 and the structures with which it has become associated. 



In addition to looking at the sensitivity of a receptor, one must also 

 look at its responsiveness. In transforming the energy that comes to 

 it to do some form of work, the receptor does not maintain a one-to- 

 one ratio between input and output. A great deal of integration 



