VISUAL INPUT AFFECTS THE RESPONSE TO ROLL IN RETICULOSPINAL NEURONS OF THE LAMPREY

A body orientation with the dorsal side up is usually maintained by la mpreys during locomotion. Of crucial importance for this is the vestib ular-driven control system. A visual input can affect the body orienta tion: illumination of one eye during swimming evokes roll tilt towards the source of light. The aim of the present study was to investigate the interaction of visual and vestibular inputs in reticulospinal (RS) neurons of the brainstem. The RS system is the main descending system transmitting information from the brainstem to the spinal cord. The r esponse of neurons in the middle rhombencephalic reticular nucleus to a unilateral non-patterned optic input was investigated, as well as th e influence of this input on the response of RS neurons to vestibular stimulation (roll tilt). Experiments were carried out on a brainstem p reparation with intact labyrinths and, in some cases, intact eyes. Ill umination of one eye or electrical stimulation of the optic nerve (10 Hz) resulted in an activation of RS neurons preferentially on the ipsi lateral side of the brainstem. The same result was obtained after abla tion of the optic tectum, demonstrating that there are asymmetrical vi sual projections to the lower brainstem which do not involve the tectu m. Stimulation of the optic nerve strongly affected the vestibular res ponse in RS neurons. As a rule RS neurons are silent at the normal (do rsal-side-up) orientation of the brainstem and become active with cont ralateral roll tilt. During continuous optic nerve stimulation, howeve r, the RS neurons on the side of stimulation fire during normal orient ation of the brainstem, and the response to contralateral roll tilt in creases considerably in many neurons. The effects of the optic input i n contralateral-RS neurons were less consistent. Any asymmetry in the signals transmitted to the spinal cord by the two (left and right) sub -populations of RS neurons can be expected to evoke a correcting motor response aimed at turning the body around its longitudinal axis to a position at which the symmetry between left and right RS neurons is re stored. Normally, the symmetry will occur when the dorsal side is upwa rds, but with a unilateral visual input it will occur instead at some degree of ipsilateral roll.