86 The Physiology of Sense Organs 



clearly illustrated in figure 38. As mentioned above, the axon, 

 soma, and some of the distal process of the bipolar thoracic 

 sensory neurons in the crayfish are electrically excitable and 



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50 mV 



50 msec. 



JUL JLli LI 



Fig. 38. Results of an experiment illustrating the differential responsive- 

 ness of neighboring regions on a sensory neuron. An intracellular 

 recording electrode was within the expanded region of the axon, well 

 central to the location of the soma. A pair of extracellular stimulating 

 electrodes was moved along the neuron, and stimuli were given at 

 several locations, three of which are shown in the diagram. When the 

 electrode pair was in the region of the soma-dendrite boundary, a 3 5 -msec, 

 current pulse elicited a train of seven impulses. At two regions on the 

 axon central to the soma, identical stimuli evoked, respectively, two and 

 one impulses each, at the onset and cessation of the current pulse. (From 

 Mellon and Kennedy,^' Fig. 10.) 



support impulse propagation. The response of this electrically- 

 excitable membrane to prolonged electric currents can be quanti- 

 tatively different from one region of the cell to the next, however. 

 Thus, while many impulses may be generated when the current 

 is applied to the distal process, only one arises when the stimulating 

 current is confined to the axon region. On the other hand, axons 

 of central neurons in arthropods have been shown to receive 

 synaptic contacts from other nerve cells, so that both electrically- 

 excitable and inexcitable regions must exist in close proximity 

 and may even form a functional mosaic. Electrophysiological 

 techniques with much greater resolution than now exists will be 

 needed to provide accurate functional maps of neural membranes. 

 The development of such refined techniques may make it possible 

 to recognize correlations between various physiological properties 

 of the membrane and its fine structure. 



