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HANDBOOK OF PHVSIOLOGV 



NEUROPHYSIOLOGY I 



tentials is related to the more or less steady resting 

 potential as shown in figure 8. The majority of units 

 penetrated show spikes larger than the corresponding 

 resting potentials. Occasionally, small or large spikes 

 are recorded with negligible resting potential, and 

 many of these have a diphasic positive-negative 

 shape as shown in 9^. Spikes accompanying large 

 steady resting potentials may be either brief like 

 those recorded from dorsal or ventral root fibers, as in 

 figure gfi, or of longer duration, as in figure gC. 



50 



100 mV 



FIG. 8. Plot of spike amplitude against steady voltage as 

 measured from presumed e.xtracellular potential level. One 

 hundred and sixty-seven units from penetrations of cats' spinal 

 cords. Open circles from units identified as primary afferent 

 fibers; tilled circles from other units. [From Frank & Fuortes 

 (26).] 



Damage to Penetrated Units 



A unit may be considered to have been seriously 

 damaged when the potentials recorded from it de- 

 crease rapidly and are small and drawn out; and 

 when the pattern of activity recorded differs from that 

 obtained before penetration. Minor damage cannot 

 be recognized, and the degree of abnormality due to 

 insertion of the microelectrode can only be postulated 

 in a number of cases. 



The assumption that small spikes are recorded from 

 damaged structures seems to be contradicted by the 

 observation that elements producing small spikes 

 may respond with normal patterns to orthodromic 

 stimulation. However, this is probably due to the 

 fact that damage can occur at the place of recording 

 (axon) without involving the structures responsible 

 for the generation of the response (.soma and den- 

 drites). 



Primary Sensory Fibers 



When the microelectrode penetrates a primary 

 afferent fiber after it has entered the central nervous 

 system, the unit can still be provisionally identified as 

 sensory by several features, a) The action potential 

 should have about the same shape, size and duration 

 as those recorded with microelectrodes in peripheral 

 nerves or dorsal roots. Of course, fine afferent branches 

 may not meet this requirement and other axons may 

 not be excluded. h~) If conduction latency is less than 

 about 0.5 insec, it may i)e presumed that no synapse 

 is tra\'ersed. This criterion cannot be used if long con- 

 duction paths are involved. Also this presumption ex- 

 cltides the possibility of very fast synapses, such as 



FIG. 9..-!. Diphasic potential recorded from a cat's motoneuron just prior to penetration as indicated 

 by subsequent sudden development of negative resting potential. Artifact indicates shock to dorsal 

 root. Calibration : 20 mv. Time : i msec. B. Brief spike presumably from inside an axon in the cat's 

 spinal cord. Calibration: 20 rav. Time i msec. C. .Action potential from a cat's motoneuron following 

 first dorsal and then ventral root stimuli. Calibration: ^o mv. Time: i msec. 



