MECHANORECEPTORS AND BEHAVIOR 365 



The time of efferent nerve activity— The observation that the efferent 

 neurons reside in the lateral-line lobes might imply that the efferent system 

 forms a feedback loop so that strong stimulation of the hair cells would 

 achieve a modifying inhibition. However, at present there is no unambiguous 

 evidence for this type of circuit. Certainly, electrical stimulation of the 

 afferent fibres will provoke an efferent discharge (Roberts and Russell 1972; 

 Paul and Roberts 19776), but natural lateral-line stimulation does not evoke 

 efferent nerve activity (Figure 22B). Indeed experiments designed to ex- 

 amine the relationship between natural lateral-line stimulation and efferent 

 activity showed the ineffectiveness of lateral-line stimuli (Roberts and 

 Russell 1972) and gave the clear impression that the type of stimulus that 

 was successful in evoking efferent action (mostly vestibular and touch) was 

 normally followed by some movement of the fish (Figure 22C). In the shark- 

 like elasmobranchs these movements depend on two muscle systems that are 

 brought into action at different times— steady, rhythmic movements involv- 

 ing only the peripheral red muscle system and briefer, larger unsustained 

 movements, such as "escape" movements, produced by the extensive white 

 musculature. 



Vigorous brief movements of the fish are a frequent response to strong 

 tactile stimulation and are immediately preceded by and accompanied by 

 activity of the efferent neurons (Figure 23A, 23B).At these times there is a 

 general correlation between the frequency of the efferent nerve activity and 

 the amplitude of movement. The movement of the body alone, though, is 

 insufficient to stimulate the efferent system, because passively induced body 

 movements are not accompanied by efferent activity (Figure 24C). The ef- 

 ferent neurons are spontaneously active, discharging a few impulses at low 

 frequency (5-10 imp/s), during rhythmical swimming movements that in- 

 volve red muscle fibres (Figure 23C, D). 



The consequence of efferent nerve activity— -The demonstration that 

 electrical stimulation of the efferent fibres at frequencies above 40 Hz causes 

 a pronounced inhibition of lateral-line activity, as well as the finding that 

 efferent discharge frequencies of this order are obtained naturally in actively 

 swimming fish, implies that a swimming dogfish should show reduced sensi- 

 tivity to lateral-line stimulation. This has been directly tested by Russell and 

 Roberts (1974), who recorded total nerve activity from the intact buccal 

 nerve, set up in response to a vibrating probe placed close to the head canals 

 in stationary and in swimming fish. Figure 24 shows that the amplitude of 

 the response in swimming fish is attenuated by as much as 50% and that this 

 reduction is absent when the lateral-line nerve is transected centrally, to 

 eliminate the efferent downflow. 



A theory of efferent nerve action— -These experiments on the timing 

 of the efferent activity have clearly established two important points relevant 

 to efferent function: (1) a true feedback system cannot exist, and (2) ef- 

 ferent activity is closely associated with body movement. 



During steady movement it is now clear that the efferent system cannot 

 function to counteract the rhythmical afferent activity generated during 



