16 The Physiology of Sense Organs 



strength, but also of the period since the application of the 

 stimulus. Sensory neurons whose output frequency declines 

 most rapidly with time, or which respond only during actual 

 changes in stimulus strength, are arbitrarily classified as phasic. 

 Tonic sensory cells, on the other hand, are those which reproduce 

 the time-course of stimulus application more faithfully (even if, in 

 some cases, this involves a steady-frequency output for hours). 

 As a general rule, however, all neurons fire with higher impulse 

 frequencies immediately following stimulus onset. The subse- 

 quent decline in frequency is usually taken as the neural correlate 

 of sensory adaptation (a phenomenon which has been recognized 

 for a hundred years) and almost certainly results from at least two 

 independent processes (Chapter 3). Some sense organs (usually 

 classed as tonic) have, in fact, two distinct phases of activity — an 

 initial burst of impulses, followed by a phase which declines at a 

 much slower rate (fig. 5). However, the two phases may not show 



280 r- 



240 - 



200 - 



160 



120 - 



100 200 300 400 



TIME(msecO AFTER APPLICATION OF STIMULUS 



500 



Fig. 5. Impulse frequency as a function of time in a primary water- 

 sensitive neuron of the blowfly, Phormia. Both the initial high-frequency 

 phase and the steady phase of the response are reduced by the addition 

 of small amounts of sodium chloride (0-05 -0-3 m), but to different 

 extents. (From Evans and Mellon, ^^ Fig, 6.) 



