82 The Physiology of Sense Organs 



favorable conditions, they can be visualized in living preparations. 

 An electrode can, thus, be inserted into the soma of one of these 

 neurons. Such an intracellular electrode rarely records a pre- 

 potential to an impulse or anjrthing resembling a receptor potential 

 in response to mechanical stimulation of the receptor organ — only 

 propagating impulses are seen. However, the waveform of the 

 latter is strictly dependent upon the direction of approach to the 

 cell body. Antidromically-propagated impulses are invariably 

 inflected on the rising phase of the transient, probably due to a 

 slight delay in conduction at the axon-soma boundary. On the 

 other hand, orthodromic impulses from healthy preparations are 

 rarely inflected, and the rate of potential change on the rising phase 

 of the spike undergoes no obvious variations until it peaks out. 

 These observations lead one to the conclusion that impulses 

 must not originate at an axonal locus (i.e. proximal to the cell body); 

 for, if a receptor potential of sufficient amplitude (2 - 10 mV) to 

 generate impulses had spread to the axon, logically it should have 

 been detected by an electrode in the soma — a region of the cell 

 much closer to the transducer locus, and thus to the source of the 

 receptor currents. Secondly, impulses known to be invading the 

 soma from an axonal (antidromic) direction are always inflected, 

 whereas impulses evoked by natural stimuli are usually uninflected, 

 and must therefore be entering the soma over another path- 

 way. In some instances, cells were injured by the micro- 

 electrode and the character of the response to stimuli deteriorated 

 during the course of an experiment. Initially, this took the form 

 'of a marked inflection appearing on the rising phase of ortho- 

 dromic, as well as antidromic, impulses. The amplitude of the 

 inflected region of the spike was, however, invariably different in 

 the two cases. Characteristically different impulse configurations 

 were thus observable during invasion of the soma by, respectively, 

 orthodromic and antidromic spikes. 



Further support for the concept that the initiation of ortho- 

 dromic impulses occurs at a locus distal to the soma was obtained 

 following electrical stimulation of the distal process and mechanical 

 stimulation of the receptor organ. The results of these experi- 

 ments are shown in figure 36: in it four superimposed traces 

 indicate the variable waveform of impulses recorded from the 

 soma of an injured cell, following mechanical stimulation of the 



