EXCITATORY AND INHIBITORY PROCESSES 293 



The mechanisms underlying the production of the generator potential are, 

 to a large extent, unknown. It is possible that conditions present in other 

 receptor organs may also occur in crayfish and lobster stretch receptors. 

 Thus, an inward flux of Na+ ions may occur during stretch and this ionic 

 movement might be responsible for setting up the generator potential. Gray 

 and Sato (1953) have been able to obtain changes in the magnitude of the 

 generator potential in Pacinian corpuscles by altering the sodium content of 

 the bathing solution. Similar studies in the crustacean preparation are lacking. 



The problem of how rhythmic activity is initiated in these receptors was 

 thoroughly studied by Eyzaguirre and Kufiler (1955a). This aspect of the 

 problem has been extensively reviewed by a number of authors (Eccles, 

 1957; Kufiler, 1958, 1960; Edwards, 1960; Grundfest, 1956, 1957, 1959). 

 Consequently, it is felt that further writing on this subject is, at present, 

 unnecessary. 



The Origin of the Conducted Nerve Impulse 



Eyzaguirre and KuflHer (1955a, b), recording intracellularly from the cell 

 soma, showed that the slowly adapting neuron discharged impulses if its 

 resting potential was reduced by about 8 to 12 mV. Rapidly adapting cells, 

 on the other hand, needed a reduction in resting potential of about 18 to 

 22 mV before firing occurred. With this evidence it was concluded that the 

 low firing level presented by the slowly adapting cells indicated that impulses 

 arose at some distance from the recording lead, presumably somewhere along 

 the dendrites or near the dendrite-soma junction. Similarly the high firing 

 level presented by rapidly adapting cells seemed to indicate that the nerve 

 impulses arose near the recording lead, that is at the soma. This view seemed 

 to be consistent with the finding that antidromic impulses blocked at or near 

 the axon hillock were capable of eliciting a local response in the cell which, 

 if it attained a magnitude of about 20 mV, was capable of eliciting propagated 

 action potentials in the soma. 



A diff"erent view has been proposed by Edwards and Ottoson (1958) working 

 with the slowly adapting neuron of the lobster. These authors recorded the 

 sensory discharges by means of extracellular microelectrodes, the preparation 

 being immersed in a large volume of fluid. With this system they were able to 

 locahze a region in the axon where the nerve impulses apparently originated 

 several hundred microns from the center of the neuron soma (Fig. 6). 



The method employed by Edwards and Ottoson yields more accurate 

 information, when trying to localize the origin of the nerve impulse, than 

 that employed by Eyzaguirre and Kuffler (1955a, b). However, it is felt that 

 more work is needed in order to locate accurately the site of initiation 

 of the sensory impulses for the foUowing reasons: (1) it is not known whether 

 the site of origin is constant throughout a wide range of stretch or whether 



