MECHANORECEPTORS AND BEHAVIOR 369 



5s 



Figure 26 Single unit of posterior lateral-line nerve of Scyliorhinus exposed to strong 

 mechanical stimulation for duration indicated by horizontal bar. When the stimulus ceased, 

 spontaneous activity was absent for 2.5 s. 



cessation of spontaneous activity and then by a low discharge rate, with 

 great variability. 



Efferent activity in the vestibular system— At present no data are 

 available from the elasmobranch labyrinth on the role of efferent activity, 

 although from the limited information now available on the vestibular ef- 

 ferent fibres of the frog and goldfish, discussed by Klinke and Galley (1974), 

 there also appears to be a relationship between the efferent system and body 

 movement. 



A recent claim has been made in the case of the labyrinth of the frog that 

 "receptor-to-receptor" fibres are present and promote sensory interaction 

 between adjacent receptors (Gribenski and Caston 1974). Interconnecting 

 fibres, initially on the basis of light microscopy (Peters 1971) but now on 

 the basis of electrophysiology and the electron microscope (Spath and 

 Lehmann 1975), have also been claimed for the lateral-line system. If these 

 claims are substantiated the problem of afferent coding in these systems 

 becomes much more complicated than at present thought; further examina- 

 tion of these difficult problems is urgently needed. 



The Significance of Mechanoreceptors in the Behaviour 

 of Elasmobranchs 



It is very difficult to assess correctly the biological function of a particular 

 sense organ. For olfaction, vision, hearing, and touch we have our own 

 subjective experiences, which can at times be applied to comparative animal 

 studies but provide no clues about sense organs we lack. In such circum- 

 stances, as with the lateral-line system, do we invoke an additional sensation, 

 a sixth sense, as was suggested by Leydig? 



The basic experimental approach to the problem of assigning function to a 

 sense organ system is to observe the impact on behaviour of (1) removal of 

 the sense organ or (2) its denervation ("de-afferentation"), usually ac- 

 companied by (3) controlled natural stimulation or (4) electrical stimulation 

 of sensory nerves. No single method is individually conclusive, but taken 

 together with the results from electrophysiological studies, which delineate 

 the likely modality of the system, these methods give an insight into an 

 animal's sensory capabilities. 



The first approach, the ablation or denervation of a sense organ or its 

 brain centre, has been much favoured, but of all the methods it is perhaps 



