54 The Physiology of Sense Organs 



amplitude, and not in the simple linear fashion which is actually J> 

 observed. This apparent jcontradictioajnight be explained by 

 /Increases in the length of the refractory peripd,lvhich indeed have 

 Tjegn shown to occur at higher impulse frequencies. Such an effect 

 might counteract the tendency of a neuron to reach the firing 

 threshold at larger depolarizations. At the present time, however, 

 any attempt to explain the linear relationship must be purely 

 speculative, for there is no coherent experimental evidence which 

 bears on this question. 



An additional influence on impulse frequency control in 

 primary sensory neurons may occur because of variations in 

 stimulus-evoked depolarization, for it is unlikely that a naturally- 

 induced receptor potential would have a rectangular waveform.-/"^ 

 Adaptation, therefore, may be as much the result of a decay in 

 amplitude of the receptor potential as it is of processes occurring 

 in the electrically-excitable membrane regions. In particular, 

 many primary mechanoreceptor neurons are maximally sensitive 

 to movement rather than static displacement, and the receptor 

 potential set up by transient excursions of the end organ rapidly 

 decay when the velocity of the excursions declines. Thus, Katz 

 observed a dynamic phase of the receptor potential in the frog 

 muscle spindle, in response to sudden stretching of the organ^ 

 More characteristically phasic responses may be obtained from the 

 Pacinian corpuscle'^ and some insect movement receptors.^"' 

 There is no reason to suppose that the factors responsible for 

 adaptation of the receptor potential in primary sensory cells are 

 any less complicated and diverse than those involved in the 

 gradual reduction of activity occurring in electrically-excitable 

 membranes. The loss of effective energy absorption, and/or its 

 transfer to electrogenic mechanisms within the cell, might operate 

 to diminish the magnitude of the slow electrical response by a 

 stimulus of unvarying strength. This problem has now been 

 directly investigated in the Pacinian corpuscle by Mendelson and 

 LOEWENSTEIN.^2' '2 In this mechanoreceptor, a single sensory 

 nerve-ending functions as the receptive element. The endings 

 may be found and removed most easily from mammalian visceral 

 mesentery, but they commonly exist in other parts of the body, 

 including the joints and skin. A diagram of the sensory structure 

 may be seen in figure 23. Most noticeable is the heavy capsule 



