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MECHANICAL AND ACOUSTICAL SENSES 



no impact on movement. Also, of course, the spinal dogfish, in which the 

 brain had been destroyed and therefore the lateral-line centres obliterated, 

 swam steadily. Nevertheless, all neurophysiologists who have worked with 

 the isolated lateral-line organ have been impressed by its extreme sensitivity 

 and by the obvious fact that body movements must have an excitatory 

 impact. This problem was examined in the dogfish by Roberts (1972), who 

 showed that, as expected, the lateral line was indeed stimulated by loco- 

 motory movements, for whereas in a stationary fish the lateral-line organ 

 would be discharging steadily, in the swimming fish (the continuous condi- 

 tion in many open-ocean sharks) the swimming stimulus came to dominate 

 totally lateral-line activity, which consisted of bursts of activity separated by 

 periods of silence (Figure 27). At the time of this study it was not known 

 how much the efferent activity would modify this locomotory discharge, but 

 we now know this impact to be insignificant. Stimulus detection in these 

 fishes therefore must be carried out against a continuously modulating back- 

 ground. 



Recordings taken from the swimming dogfish show that certain features 

 of the rhythmical discharge are correlated with body movements, so that the 

 lateral line is capable of providing proprioceptive information. It does not 

 follow, of course, that this information is used by the fish, for it is clear 

 from what is known about the properties of the lateral-line lobe that much 

 of this pattern would be lost even at the first synapse, because of the wide- 

 spread smoothing and because of the low-pass properties of the centre. It is 

 most likely, therefore, that the impact of the lateral line is that of a sus- 

 tained tonic input. 



The Role of Proprioceptors in Coordination of Movement 



The importance of mechanoreceptors for the coordination of body move- 

 ment in animals is a perennial topic for debate among comparative physiol- 

 ogists and has at times featured work done on the elasmobranch fishes. The 



Figure 27 Continuous record of activity of single lateral-line unit in a dogfish that 

 started to swim (at arrow); bottom trace is from the movement transducer (Roberts 1972). 



