294 



C. EYZAGUIRRE 



Fig. 6. Tracings of orthodromic impulses set up by stretch in the lobster stretch 

 receptor recorded with external leads at the various indicated points on the 

 nerve cell. One electrode was kept fixed on the cell at £" while the other was pulled 

 to the other positions. Point A was about 1 -3 mm from the cell body axon boun- 

 dary. B was about 500 /< distant. Time intervals are 0- 1 msec. Scale = 0-5 mV. 

 Negativity is recorded upwards. (From Edwards and Ottoson, J. Phvsiol. 

 {London) 143 : 138-148, 1958.) 



this point shifts depending on the intensity of the generator potential. It is 

 probable, however, that during moderate stretch the point of origin does not 

 shift appreciably judged by the relative constancy of the firing level. Never- 

 theless, when strong stretch is apphed the firing level changes (Fig. 3); this 

 might indicate that the point of origin might be shifted appreciably. (2) It is 

 not known whether or not the site of origin of the nerve impulses in rapidly 

 adapting receptors is similar to that of slowly adapting organs. The available 

 evidence seems to indicate that in rapidly adapting cells the site of origin is 

 closer to the soma than is the case with the slowly adapting receptor. (3) No 

 studies of this kind have been performed in crayfish since the only evidence 

 available refers to the slowly adapting receptor of lobster. 



The fact that nerve impulses can start in the axon at some distance from 

 the soma means that the cell body has a higher threshold, probably due to a 

 larger membrane resistance. Differences in the intrinsic properties of the 

 soma membrane and of the fiber have been postulated by Edwards and 

 Ottoson (1958). There is no doubt, however, that the cell soma of the crusta- 

 cean stretch receptor produces propagated spikes once its threshold is 



