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Figure 20 Impact of efferent nerve action on spontaneous afferent activity (Scylio- 

 rhinus). (A) Total suppression of spontaneous afferent impulse activity brought about by 

 electrical stimulation of efferent nerve fibres; the upper trace shows afferent unit activity, 

 and the lower trace shows the stimulus given to the efferent fibres (in this case a train of 

 pulses at 42 s" 1 ). (B) Samples of afferent unit activity, showing the impact of efferent 

 nerve stimulation (marked on upper trace) at 100 s -1 (Russell and Roberts 1972). 



bursts contain high-frequency shocks (>40 s" 1 ), but effects with lower 

 frequencies have been seen. Successive stimulations have a declining impact 

 (Figure 21), perhaps because of "fatigue" at the efferent terminals. If the units 

 show no resting activity a stimulating train to the efferent nerve is followed 

 by a brief afferent discharge (three or four impulses) about 500 ms after the 

 stimulus train has ceased; this discharge is presumably a postinhibitory 

 rebound. 



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Figure 21 Variability and decline of the inhibitory effect of electrical stimula- 

 tion of efferent fibres in Scyliorhinus. Complete inhibition (100%) indicates that 

 the unit was totally inactive during the stimulation period; at 0% the afferent 

 fibre was discharging at its resting frequency (15 s -1 ) (Russell and Roberts 1972). 



