MECHANORECEPTORS AND BEHAVIOR 363 



facilitate and be passed by the secondary neurons. A steady input, even if not 

 patterned, will nevertheless have an overall impact on the excitability of the 

 secondary neurons, as was shown by Alnaes (19736), who studied statis- 

 tically the spontaneous discharge of the secondary cells in Anguilla and 

 showed that this ceased immediately when the posterior nerve was cut. 



Sensory Centres of the Vestibular System— The labyrinth receives its 

 innervation from nerve VIII, which in elasmobranchs, as in all vertebrates, 

 has two roots. The anterior root supplies the sense organs of the anterior 

 vertical canal, the horizontal canal, and the utriculus. The posterior branch 

 supplies all the other sense organs. 



In the region of the entry point of the nerve, three vestibular nuclei can be 

 recognised: the superior vestibular nucleus and the magnocellularis nucleus 

 (Smeets and Nieuwenhuys 1976) and a ventral vestibular nucleus 

 (Montgomery 1977). In a combined anatomical and electrophysiological 

 study (Montgomery 1977), it was found that vestibular afferent fibres mono- 

 synaptically excite neurons in these nuclei. 



Efferent Innervation of Acoustico-Lateralis Receptors— An important 

 aspect of the innervation of many acoustico-lateralis receptors is the efferent 

 supply. Efferent nerve fibres are absent from the labyrinth of Myxine 

 (Lowenstein and Thornhill 1970), and from the lateral line of the lamprey 

 (Yamada 1973), although not from the labyrinth (Lowenstein et al. 1968), 

 and are rare in the eel (Yamada and Hama 1972). They have been reported 

 so far for nearly all elasmobranch organs— the labyrinth (Lowenstein et al. 

 1964), the lateral-line (Roberts and Ryan 1971), and Savi's vesicles (Nickel 

 and Fuchs 1974)— but have not been sighted in the pit organ or in the 

 ampullae of Lorenzini. 



Although efferent fibres have been detected at the periphery, both elec- 

 trophysiologically and with the electron microscope, there is considerable 

 uncertainty about the central location of their cell bodies, even in the well- 

 studied case of the mammalian cochlea (see Klinke and Galley 1974). The 

 possibilities for the elasmobranch lateral line have recently been examined 

 by Paul and Roberts (19776), who, using electrophysiological techniques, 

 showed that the efferent neurons of the anterior lateral-line nerves are in the 

 rostral region of the anterior lateral-line lobe. By backfilling the axons of 

 these cells with cobalt salts they were able to identify them as multipolar 

 neurons, with axons that entered the lateral-line nerve bundle. 



The impact of efferent nerve activity— Wherever it has been studied it 

 has been shown that stimulation of the efferent fibres leads to an inhibition 

 of impulse activity in the primary afferent fibres. For example, in Scylio- 

 rhinus, electrical stimulation of the efferent fibres has a clear effect on the 

 afferent discharge, the outcome depending on whether the unit is sponta- 

 neously active (Russell and Roberts 1972). If, as is generally the case, the 

 afferent unit is discharging spontaneously, efferent stimulation reduces or 

 inhibits this discharge during the stimulating burst and for up to 200 ms 

 afterwards (Figure 20). The inhibition is usually evident only if the stimulating 



