Visual input affects the response to roll in reticulospinal neurons of the lamprey

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Abstract
A body orientation with the dorsal side up is usually maintained by lampreys during locomotion. Of crucial importance for this is the vestibular-driven control system. A visual input can affect the body orientation: 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 response of neurons in the middle rhombencephalic reticular nucleus to a unilateral nonpatterned optic input was investigated, as well as the influence of this input on the response of RS neurons to vestibular stimulation (roll tilt). Experiments were carried out on a brainstem preparation with intact labyrinths and, in some cases, intact eyes. Illumination of one eye or electrical stimulation of the optic nerve (10 Hz) resulted in an activation of RS neurons preferentially on the ipsilateral side of the brainstem. The same result was obtained after ablation of the optic tectum, demonstrating that there are asymmetrical visual projections to the lower brainstem which do not involve the tectum. Stimulation of the optic nerve strongly affected the vestibular response in RS neurons. As a rule RS neurons are silent at the normal (dorsal-side-up) orientation of the brainstem and become active with contralateral roll tilt. During continuous optic nerve stimulation, however, the RS neurons on the side of stimulation fire during normal orientation of the brainstem, and the response to contralateral roll tilt increases considerably in many neurons. The effects of the optic input in 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 restored. Normally, the symmetry will occur when the dorsal side is upwards, but with a unilateral visual input it will occur instead at some degree of ipsilateral roll.

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