330 



HARRY GRUNDFEST 



Stimuli has been provided recently (Fuortes, 1959; Rushton, 1959). Under 

 the combined effects of polarizing currents and illumination the observed 

 changes in the frequency of the impulses of the Limulus final path neuron 

 can be described quantitatively (Fig. 3). The system has an equivalent circuit 

 of two parallel membrane components. The resistance of only one decreases 

 and only with light, not by depolarization of the membrane, i.e. it is elec- 

 trically inexcitable. 



Thus, the functional organization of a primary receptor cell, like the cray- 

 fish stretch receptor, or of a final common path sensory neuron, like the 

 Limulus eccentric cell, is the same as that of a generalized neuron (Fig. 4). 



specific 



y 



Excitants 



INPUT ; CONDUCTILE 



I ( Electrically 

 ^^ ' Excitable) 



Graded 



Electrogenesis ^ ■: 



(Con be / „ , ., „ 

 sustoin-/ Pulsatile Responses 



edl/' [All or none) 



OUTPUT 



Secretory Secretory 

 Activity 



Graded Products 

 Sustained 



AExcitatorA 

 \ Inhibitory / 



GENERATOR 

 ACTIVITY 



""S^ 



CONDUCTILE 

 ACTIVITY 



Secretory 

 Activity 



TERMINAL 

 ELECTROGENESIS 7 



Fig. 4. Diagrammatic representation of functional components and electrical 

 responses of a receptor cell or neuron. The electrically inexcitable input produces 

 electrogenesis graded in proportion to its specific stimulus and sustained as long 

 as the latter is applied. The possibility of hyperpolarizing electrogenesis is shown 

 but is not further considered. The depolarization at the input, operating upon 

 the conductile electrically excitable component, can evoke spikes in the latter 

 coded in number and frequency in proportion to the depolarization. These 

 signals, propagated to the output, there command secretory activity, roughly 

 proportional to the information encoded in their message and sustained as long 

 as the message demands. The transmitter released at the output can initiate a 

 synaptic transfer by operating upon the depolarizing input of another cell. The 

 possibility of a special output electrogenesis is indicated but is not further con- 

 sidered. (From Grundfest, 1957b.) 



A specifically sensitive membrane at the input responds with a depolarizing 

 electrogenesis which is graded and produces coded messages of spikes. The 

 input membrane in most receptors is probably electrically inexcitable (Grund- 

 fest, 1956, 1957b, c,d). Thus, the conclusion that the olfactory epithelium 

 in vertebrates is electrically inexcitable (Grundfest, 1957b) has recently been 

 confirmed (Ottoson, 1959). Parenthetically, it may be noted that this is the 

 only primary receptor type among the cranial special senses of the verte- 

 brates. However, at least one type of receptor ought to respond to electrical 



