The activity of lateral-line efferent neurones in stationary and swimming dogfish.

The hair cells of the acoustico-lateralis sensory system receive both afferent and efferent innervation. This dual innervation was first demonstrated in the case of the organ of Corti, where the efferent nerve fibres constitute the olivo-cochlear bundle (Rasmussen, 1942). An efferent pathway has also been traced along cranial nerve VIII to the vestibular system (Rasmussen & Gacek, 1958; Gacek, i960; Rossi & Cortesina, 1963) and in the frog it has been shown that these efferent fibres originate from Purkinje cells in the auricular lobe of the cerebellum (Hillman, 1969; Llinas & Precht, 1969). The function of the efferent system has not been established, although it is generally agreed that efferent fibres can exert an inhibitory effect on the sense organs which they innervate (Galambos, 1956; Fex, 1959; Sala, 1965; Llinas & Precht, 1969). The idea of an inhibitory function was developed from experiments in which the efferent fibres were stimulated synchronously with trains of pulses, but it is difficult to judge the significance of these results because of the lack of information about the activity patterns of efferent neurones under natural conditions. In this paper the natural pattern of efferent activity is described in the case of the lateral-line sensory system of dogfish. A preliminary report of this work has already been published (Roberts & Russell, 1970). The lateral-line organ of fishes and amphibia is the simplest example of an acoustico-lateralis receptor; it lies on the body surface or just beneath it in a fluidfilled canal and is sensitive to 'near-field' water displacements (Harris & van Bergeijk, 1962). Electron-microscopical studies have shown that efferent synapses similar to those found at the base of hair cells in the ear (Engstrom & Wersall, 1958) are also present at the base of hair cells in lateral-line organs (Hama, 1965; Flock, 1965, 1967) including those of sharks and dogfish (Hama, 1970; Roberts & Ryan, 1971). As in other acoustico-lateralis receptors, electrical stimulation of lateral-line efferent nerve fibres suppresses impulses in the afferent pathway (Katsuki, Hashimoto & Yanagisawa, 1968; Russell, 1968; Russell & Roberts, 1972). Dogfish were chosen for this study because of the relative ease with which lateralline activity can be monitored from a swimming preparation (Roberts, 1972).