INITIATION OF LOCOMOTION BY LATERAL-LINE PHOTORECEPTORS IN LAMPREY - BEHAVIORAL AND NEUROPHYSIOLOGICAL STUDIES

The lateral line system of Iampreys includes photoreceptors distribute d in the skin of the tail region, These are innervated by the trunk la teral line nerves, and the afferents terminate bilaterally in the medi al octavolateral nucleus, crossing the midline through the cerebellar commissure, Stimulation of the dermal photoreceptors by tail illuminat ion initiates locomotion, The present study was performed to character ize the response to illumination in larval and adult lampreys in detai l and to elucidate the neuronal pathways responsible for the activatio n of locomotion, In both larval and adult quiescent lampreys, the resp onse to unilateral illumination of the tail was found to consist of an initial turn followed by rectilinear swimming, The sign and magnitude of the turning angle were not correlated with the laterality of the o ptic stimulus, In mechanically restrained lampreys, spinalized at the level of segments 15-20, tail illumination evoked a complex motor resp onse in the rostral part of the body, with switches between different patterns of coordination (turns in different directions, locomotion, a nd turns combined with locomotion), Thus, the response to tail illumin ation is not a simple reflex, but includes a behavioural choice. Retic ulospinal neurones play a crucial role in the initiation of locomotion in lampreys. The response to unilateral tail illumination in rhombenc ephalic reticular cells was studied with extracellular single-unit rec ordings, It was found that neurones in the middle and posterior rhombe ncephalic reticular nuclei were activated bilaterally, Tonic activity or slow bursts (<0.5Hz) were evoked, in some cases lasting up to 60s a fter the stimulation, The response remained bilateral after transectio n of one lateral line nerve and the cerebellar commissure, Afferents f rom one side can thus activate reticulospinal cells on both sides thro ugh a pathway outside the cerebellar commissure, This bilateral activa tion of reticulospinal neurones is presumably responsible for the acti vation of spinal locomotor networks, without any directional bias to t he left or the right side, and for the rectilinear swimming observed i n behavioural experiments. In the caudal part of the termination area of the lateral line nerve afferents, neurones with contralateral proje ctions were retrogradely stained with horseradish peroxidase. These ne urones appear to be likely candidates for mediating the contralateral effects of the lateral line fibres.