36 The Physiology of Sense Organs 



inhibiting the discharge which could be recorded from the eye 

 in the dark. Since it has not been possible to obtain intracellular 

 recordings from these interesting types of sensory cells, investi- 

 gations into the nature of the oflF-response are incomplete. It 

 might be mentioned that impulse discharges following periods of 

 synaptic inhibition (the so-called * post-inhibitory rebound ' 

 phenomena) are not uncommon among central neurons. In 

 some instances such discharges may be ascribed directly to conscr- 

 quences of the hyperpolarized state, which produces potassium 

 inactivation in the electrically-excitable membrane and results 

 in a period of depolarization as soon as the anodal generator 

 is removed. The excitatory activity which follows stimulation 

 has also been examined using intracellular techniques in other 

 invertebrate primary photosenspry neurons (fig. 14). The sequence 

 of electrical events which occurs in these cells appears to be 

 similar to the events described above for predominantly mechano- 

 receptor neurons.^* Thus, a depolarizing receptor potential is 

 produced as a consequence of the incidence of electromagnetic 

 energy and is related to stimulus intensity. At a definite threshold 

 amplitude of depolarization, action potentials are generated in 

 electrically excitable regions of the membrane and propagate 

 towards the central nervous system. Where they have been 

 examined, these responses appear to be wavelength-specific. 



Transient periods of hyperpolarization have also been observed 

 in the frog muscle spindle^* after the cessation of a mechanical 

 stimulus. This is shown in figure 15. Katz regarded these 

 periods of hyperpolarization as part of the * dynamic ' component 

 of the receptor potential — analogous to, but different in electrical 

 sign from, the initial high-amplitude phase of the depolarizing 

 response to passive stretch of the spindle. This phenomenon was 

 noticed also in the intracellular records from crayfish stretch 

 receptors, 2' but was not specifically investigated. 



Before examining the control of firing frequency in sensory 

 systems in the next chapter, it may be instructive to consider the 

 terms * receptor potential ' and ' generator potential ' as they are 

 used in the experimental literature, since these have engendered 

 some confusion in the past. The functional arrangement of 

 different sensory systems shows a good deal of variation; two basic 

 plans are adhered to, however: (i) the primary sensory neuron, 



