Asymmetry in the pitch control system of the lamprey caused by a unilateral labyrinthectomy.

A postural control system in the lamprey is driven by vestibular input and maintains a definite orientation of the animal during swimming. After a unilateral labyrinthectomy (UL), the lamprey continuously rolls toward the damaged side. Important elements of the postural network are the reticulospinal (RS) neurons that are driven by vestibular input and transmit commands for postural corrections to the spinal cord. We characterized the effect of UL on vestibular responses in RS neurons elicited by rotation of the animal in the pitch plane. The activity of RS neurons was recorded from their axons in the spinal cord before and after UL. The neurons can be classified into the Up and Down groups activated preferentially with nose-up or nose-down rotation, respectively. After UL, vestibular responses in the group Up changed only slightly on the damaged side and disappeared almost completely on the opposite side. In the group Down, responses on both sides persisted after UL. These results indicate that the left and right subgroups of the group Up neurons receive excitatory input mainly from the contralateral labyrinth. In contrast, the group Down neurons receive excitatory input from both labyrinths. We conclude that the UL-induced changes in vestibular responses to pitch tilt will disturb the normal activity of the postural control system. The UL-induced asymmetry in the bilateral activity of the group Up neurons seems to be an important factor contributing to the loss of equilibrium in UL animals and to their rotation during swimming.