MECHANORECEPTORS AND BEHAVIOR 



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Figure 23 Efferent nerve activity accompanying body movement. In all records the bot- 

 tom trace is the electromyogram recorded from muscle fibres of the body; The top trace 

 is the record of an efferent fibre. (A) Responses to tactile stimulation to the body (marked 

 by horizontal bars). (B) Vigorous tactile stimulation is accompanied by large movements 

 and high-frequency efferent discharges. (C, D) Rhythmic activity of the efferent system 

 accompanying steady swimming movements recorded from two fish (Roberts and Russell 

 1972). 



swimming (see Roberts 1972 and later), for, not only do the efferent fibres 

 discharge at such a slow rate, but also, very careful patterning of the dis- 

 charge would be needed to ensure that the efferent impact coincided with 

 the stimulus evoked by movement for all frequencies of locomotion. In fact, 

 the evidence suggests that all the efferent axons discharge simultaneously. 



During violent movements, in either escape or attack it is clear that the 

 efferent discharges do achieve rates sufficient to reduce sense organ sensi- 

 tivity. As it is also evident that the lateral line is strongly stimulated (Roberts 

 1972) during movement, it has been suggested (Russel 1971a; Roberts and 

 Russell 1972) that the function of the efferent system might be to prevent 

 short-term fatigue occurring in the receptors, so that the lateral-line system 

 would be immediately responsive the moment any violent movement ceased 

 (Figure 25). In the case of Scyliorhinus, an escape movement provoked by 

 (say) a pinch on the tail consists of a few brisk movements followed by a 

 glide, and we would expect the lateral-line sensitivity to be reduced just 

 before and during the tail beat but to have returned to normal for the glide. 



