EXCITATORY AND INHIBITORY PROCESSES 



295 



reached (Fig. 7). This possibihty has been questioned in the case of mam- 

 mahan motoneurons. 



Another factor that must be taken into consideration when analyzing the 

 site of origin of the nerve impulses in these cells is that the configuration of 

 the neurons varies greatly not only from one species to another, but within 

 one species in different animals (see Alexandrowicz, 1951, 1952; Florey and 

 Florey, 1955). It is conceivable, therefore, that the site of origin of nerve 

 impulses might not be constant and that it would be closely dependent on cell 

 geometry, diameter and length of the dendrites, and on the morphology of 

 that portion of the axon closer to the cell soma. 



Fig. 7. Orthodromic and antidromic impulses recorded at various points on the 



cell body. Time intervals 0- 1 msec. Scale 0-5 mV. (From Edwards and Ottoson, 



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



Some Characteristics of Antidroniically Evoked Potentials 



Further information on the behavior of the crayfish stretch receptor 

 neuron has been obtained by stimulating the axon and recording potential 

 changes intracellularly from the cell body. These potentials have been 

 analyzed and compared with propagated activity produced by stretch- 

 deformation of the dendrites. 



When a receptor muscle is not stretched, the sensory neurons do not 

 discharge, and they generally show membrane potentials between 70 and 

 80 mV. Antidromic impulses set up centrally in the axon cause in the cell 

 soma an action potential which rises to its peak in about 0-6 msec. This 

 potential overshoots the zero baseline level by about 20 mV. The recovery 

 process consists of a rapid falling phase followed by a slower, gradually 

 decaying component which has a total duration of 10-20 msec. This slow 

 repolarization has been called after-negativity since it is not known to what 

 extent it can be related to other well known after-potentials. The situation is 

 different if a cell is stretched and its membrane potential is reduced to a new 

 level. An antidromic soma-dendrite impulse then acquires a "positive" 



