Effects of galvanic vestibular stimulation on postural limb reflexes

25 26 Quadrupeds maintain the dorsal side up body orientation due to the activity of the 27 postural control system driven by limb mechanoreceptors. Binaural galvanic vestibular 28 stimulation (GVS) causes a lateral body sway towards the anode. Previously we have 29 shown that this new position is actively stabilized, suggesting that GVS changes a set30 point in the reflex mechanisms controlling body posture. The aim of the present study 31 was to reveal the underlying neuronal mechanisms. Experiments were performed on 32 decerebrate rabbits. The vertebral column was rigidly fixed, whereas hindlimbs were 33 positioned on a platform. Periodic lateral tilts of the platform caused postural limb 34 reflexes (PLRs): activation of extensors in the loaded and flexing limb, and decrease in 35 extensor activity in the opposite (unloaded and extending) limb. Putative spinal 36 interneurons were recorded in segments L4-L5 during PLRs with and without GVS. We 37 have found that GVS enhanced PLRs on the cathode side and reduced them on the anode 38 side. This asymmetry in PLRs can account for changes in the stabilized body orientation 39 observed in normal rabbits subjected to continuous GVS. Responses to platform tilts 40 (frequency modulation) were observed in 106 spinal neurons, suggesting that they can 41 contribute to PLRs generation. Two neuron groups were active in opposite phases of the 42 tilt cycle of ipsi-limb: F-neurons in flexion phase, and E-neurons in extension phase. 43 Neurons were driven mainly by afferent input from ipsi-limb. If one supposes that F and 44 E-neurons contribute, respectively, to excitation and inhibition of extensor motoneurons, 45 one can expect that the pattern of response to GVS in F-neurons will be similar to that in 46 extensor muscles, while E-neurons will have opposite pattern. We have found that about 47 40% of all modulated neurons meet this condition, suggesting that they contribute to the 48 generation of PLRs and to the GVS-caused changes in PLRs. 49