272 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



B 



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FIG. 1 1. Responses obtained in cat's spinal cord and identiKed 

 as from interneurons by criteria discussed in text. A. Spike 

 duration about i msec, after prespilce potential, presumably a 

 soma response. B. Spike duration about 0.3 msec, with no 

 prespike potential, presumably from an axon. Brief postspike 

 hyperpolarization is a frequent but not a constant finding and 

 may indicate damage done to a fiber by the microelectrode. 

 Calibration: 50 mv. Time: 1 msec. [From Frank & Fuortes 

 (26).] 



tials which he interprets as indicating antidromic 

 conduction along the dendrites for a distance in excess 

 of I mm. But the diphasic responses of figure 9^4 can 

 be interpreted as due to the sum of conductive and 

 reactive currents outside .some inactive membrane, 

 according to Freygang (28). If this interpretation is 

 correct, then at least some of the dendrites of moto- 

 neurons probably do not participate in the actively 

 conducted action potential. 



Interneurons 



It is convenient, for the gross identifications possible 

 in the spinal cord with these techniques, to define an 

 interneuron as a postsynaptic unit which does not 

 .send its axon to ventral roots. The criteria for deciding 

 if a unit is postsynaptic are the latency of its response 

 and whether it responds with more than one impulse 

 to a single afferent volley. There is no doubt that if we 

 accept spontaneously firing units which cannot be 

 driven by the electrical stimuli available we may in- 

 clude some primary afferents from distant receptors 

 as interneurons, but most of these can be eliminated 

 bv the regularity of their firing.' Figure 1 1 shows two 



' For a discussion of possible confusion between interneurons 

 and primary afferents conducting dorsal root reHexes see Frank 

 & Fuortes C26"). 



typical units satisfying the above criteria. Many work- 

 ers have studied patterns of activity of single cells 

 with extracellular electrodes and some have reported 

 patterns of intracellular potentials. For references to 

 these, see especially Chapters II and I\' of this 

 volume and their bibliographies. 



One class of interneurons deserves mention in a dis- 

 cussion of neuron identification. Occasionally units 

 are penetrated which respond to a single ventral root 

 volley with a very high frequency train of spikes in- 

 stead of the single action potential shown by moto- 

 neurons. Eccles et at. (19) have named these Renshaw 

 cells in honor of Birdsey Renshaw (49) who predicted 

 the existance of such cells making synapses with the 

 axon collaterals of motoneurons. The function of these 

 cells has not been established, but Eccles et al. (19) be- 



20 



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1-2 

 msec 



1-5 



1-8 



2-1 



FIG. 12. Charts illustrating distribution of duration of spikes 

 recorded from different structures in the cat's spinal cord. 

 Abscissa: spike duration in msec; ordinates: number of spikes 

 within 0.3 msec, groups. A. Spikes recorded from dorsal and 

 ventral root fibers. B. Spikes recorded from motoneuron somata 

 or dendrites identified by criteria given in text. C. Spikes re- 

 corded from postsynaptic elements of the cord other than 

 those of .1 and B. [From Frank & Fuortes (26).] 



