LAUSR

We recently demonstrated in decerebrate and conscious cat preparations that hindlimb somatosensory inputs converge with vestibular afferent input onto neurons in multiple CNS locations that participate in balance control. While it is known that head position and limb state modulate postural reflexes, presumably through vestibulospinal and reticulospinal pathways, the combined influence of the two inputs on the activity of neurons in these brainstem regions is unknown. In the present study, we evaluated the responses of vestibular nucleus (VN) neurons to vestibular and hindlimb stimuli delivered separately and together in conscious cats. We hypothesized that VN neuronal firing during activation of vestibular and limb proprioceptive inputs would be well-fit by an additive model. Extracellular single-unit recordings were obtained from VN neurons. Sinusoidal whole-body rotation in the roll plane was used as the search stimulus. Units responding to the search stimulus were tested for their responses to 10° ramp-and-hold roll body rotation, 60° extension hindlimb movement, and both movements delivered simultaneously. Composite response histograms were fit by a model of low and high pass filtered limb and body position signals using least squares nonlinear regression. We found that VN neuronal activity during combined vestibular and hindlimb proprioceptive stimulation in the conscious cat is well-fit by a simple additive model for signals with similar temporal dynamics. The mean R2value for goodness of fit across all units was 0.74 ±0.17. It is likely that VN neurons that exhibit these integrative properties participate in adjusting vestibulospinal outflow in response to limb state.