276 



HANDBOOK OF PHVSIOLOOV 



NEUROPHYSIOLOGY I 



two main techniques which have been used. Coombs 

 et al. (14) used a double-barrelled micropipette (fis;. 

 17) passing stimulating or polarizing currents through 

 one barrel while recording the intracellular potential 

 with the other (barrel. Araki & Otani (8) (fig. 18) and 

 Frank & Fuortes (27) (fig. 19) used a bridge circuit 

 to penuit simultaneous stimulation and recording 

 through the same microelectrode tip. The.se articles 

 should be consulted for details and limitations of the 

 two techniques. 



By means of these techniques it has been possible to 

 study the excitabilities of penetrated units. Differen- 

 tiation of axons from .somata is often possible on the 

 basis of their excitabilities. For example large axons in 

 the cat's spinal cord have a rheobasic current of 

 about 1.7 X I o~' ainp., while units identified as moto- 

 neuron somata or dendrites require an average of 



about 7 X io~' amp. through the micropipette to 

 reach threshold. 



Cells firing with regular trains of impulses appear 

 to generate their own impulses at some particular site 

 of origin. When depolarizing currents are applied 

 through the micropipette placed near such a locus the 

 rate of firing is increased in proportion to the applied 

 current. Since the applied current decrements rapidly 

 along an axon and presumably also along a dendrite, 

 current through the pipette will not affect the firing 

 rate when the micropipette is at a distance from this 

 site of origin. It is therefore possible to tell whether 

 the locus of recurrent firing in a cell is near to or far 

 from the tip of the micropipette. If it is accepted that 

 such a locus is normally situated near the axon hil- 

 lock, then the response of a repetitively firing unit to 

 applied polarizing currents can be used to infer 

 whether it is an axon or a soma. 



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