Modifications of vestibular responses of individual reticulospinal neurons in lamprey caused by unilateral labyrinthectomy.

A postural control system in the lamprey is driven by vestibular input and maintains the dorsal-side-up orientation of the animal during swimming. After a unilateral labyrinthectomy (UL), the lamprey continuously rolls toward the damaged side. Normally, a recovery of postural equilibrium ("vestibular compensation") takes about 1 mo. However, illumination of the eye contralateral to UL results in an immediate and reversible restoration of equilibrium. Here we used eye illumination as a tool to examine a functional recovery of the postural network. Important elements of this network are the reticulospinal (RS) neurons, which are driven by vestibular input and transmit commands for postural corrections to the spinal cord. In this study, we characterized modifications of the vestibular responses in individual RS neurons caused by UL and the effect exerted on these responses by eye illumination. The activity of RS neurons was recorded from their axons in the spinal cord by chronically implanted electrodes, and spikes in individual axons were extracted from the population activity signals. The same neurons were recorded both before and after UL. Vestibular stimulation (rotation in the roll plane through 360 degrees ) and eye illumination were performed in quiescent animals. It was found that the vestibular responses on the UL-side changed only slightly, whereas the responses on the opposite side disappeared almost completely. This asymmetry in the bilateral activity of RS neurons is the most likely cause for the loss of equilibrium in UL animals. Illumination of the eye contralateral to UL resulted, first, in a restoration of vestibular responses in the neurons inactivated by UL and in an appearance of vestibular responses in some other neurons that did not respond to vestibular input before UL. These responses had directional sensitivity and zones of spatial sensitivity similar to those observed before UL. However, their magnitude was smaller than before UL. Second, the eye illumination caused a reduction of the magnitude of vestibular responses on the UL side. These two factors tend to restore symmetry in bilateral activity of RS neurons, which is the most likely cause for the recovery of equilibrium in the swimming UL lamprey. Results of this study are discussed in relation to the model of the roll control system proposed in our previous studies as well as in relation to the vestibular compensation.