The Control of Impulse Frequenq< 59 



amplitude of the receptor potential. In so-called * position ' 

 sense organs, on the other hand, the receptor potential is maintained 

 during displacement of the sense organ from some * resting ' 

 position, and impulse frequency appears to be a linear function of 

 receptor potential amplitude. 



No discussion of mechanoreceptor adaptation would be 

 complete without an examination of the classic examples of 

 slowly- and rapidly-adapting neurons associated with the crayfish 

 abdominal stretch-receptors. The relationship between the 

 time-course of the receptor potential and the duration of the 

 applied stimulus has been examined in the slowly-adapting organ 

 by Florey.28 j^ ^^ found that both the onset and the cessation 

 of stimuli were followed by dynamic phases in the time-course of 

 the receptor potential, while the major portion of the response 

 was characteristically steady. The situation in the rapidly- 

 adapting stretch-receptor neuron was found to be different.*^* i®^ 

 The receptor potential in this cell is independent of the duration 

 of the applied stimulus, and it decays with a characteristic time- 

 course after onset of the stretch, possibly due to in-series visco- 

 elastic elements at the insertion of the dendrites into the receptor 

 muscle. Neural adaptation is also pronounced,^* as has been 

 found to occur in the Pacinian corpuscle. Thus, the physiologicall\ 

 characteristics of both the transducer and the spike-generating \ 

 regions of these two sensory cells seem to complement one another/ 

 One other example may be appropriately mentioned at this point 

 to indicate the potential importance of the site for mechano- 

 electric conversion as a potential locus for sensory adaptation. In 

 the dually-innervated tactile hairs found on the crayfish thorax," 

 the rate of sensory adaptation to equivalent deflections of the hair 

 may show great differences between the two neurons. When 

 exposed to a constant source of cathodal current, however, the 

 same two cells adapt at nearly identical rates. 



Adaptation at the non-nervous level is a prominent feature of 

 many primary photore ceptor cells. Although these cells in various 

 arthropod preparations have axons which run to the central^ 

 nervous system, there is some evidence that no impulses are 

 generated or propagated in these structures. Electrical recordings 

 made from the retinula cells of many insects and crustaceans, and 

 from Limulus, disclose only a receptor potential which is evoked 



