MECHANORECEPTORS AND BEHAVIOR 



351 



Statistical analyses of lateral-line spontaneous activity have been carried 

 out for Xenopus (Harris and Milne 1966) and for the eel Anguilla (Alnaes 

 1973a), and some data on the dogfish Scyliorhinus are given by Roberts 

 (1972). In the dogfish some units discharge regularly, and nearly all the 

 interspike intervals fall within 40 ms of each other (Figure 9b); others show 

 greater variation, which is more marked after a period of strong mechanical 

 stimulation. Alnaes (1973a) has pointed out that regular firing probably 

 means that there is only a single spike initiation site; in Xenopus, with 

 irregular firing, evidence for multiple initiation sites has recently been pre- 

 sented (Murray and Capranica 1973). 



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20 40 60 80 100 120 140 80 160 240 320 400 480 560 



Intervals (msec) 



Figure 9 Typical record of a spontaneously discharging lateral-line unit from Scylio- 

 rhinus. Histograms of 1000 consecutive impulses from eight different units illustrate the 

 various patterns of spontaneous activity found in the posterior lateral-line organs 

 (Roberts 1972). 



Although it is widely agreed that the stimulus for the hair cell is a shearing 

 displacement of the cupula, how this arises naturally is a question causing 

 much controversy which we shall be trying to answer more fully later. Sand 

 (1937) argued that the consequence of the stimulus, however generated, 

 would be a movement of the fluid in the canals. He compared the sensory 

 hairs to "reeds in a river bed . . . bent while the stream flows"— and so in his 

 established certain important features about lateral-line organization. First, he 

 found, in opposition to previous views, that the sense organ adapted slowly 

 to a sustained stimulus. Second, he showed that some units were excited 

 if the fluid flowed in one direction along the canal but were depressed 

 (he spoke of "inhibition") if the flow went the other way (Figure 10). 



