Chapter 12 Convergence of macular vestibular and neck inputs on vestibulospinal and reticulospinal neurons projecting to the lumbosacral cord

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The activity of excitatory LVN neurons as well as of reticular neurons located in the inhibitory area of the medullary RF, the majority of which were antidromically identified as projecting to the lumbosacral segments of the spinal cord, was recorded in decerebrate cats. These neurons were tested during sinusoidal stimulation of macular vestibular, neck or combined receptors by using sinusoidal rotations about the longitudinal axis at the standard parameters of 0.026 Hz, ± 10°. A large proportion of LVN neurons as well as of medullary RF neurons responded with a periodic modulation of their firing rate to individual stimulation of vestibular and neck receptors. For both populations of convergent neurons the gain of the neck responses was higher on the average than that of the macular responses; moreover, the neck responses showed a larger phase lead with respect to position than the responses of the same neurons to the macular input at the standard parameters of stimulation. As to the response pattern, the majority of the convergent vestibular (53/58, i.e. 91.4%) and reticular neurons (58/71, i.e. 81.7%) showed reciprocal ('out of phase') responses to the two inputs. However, while the VS neurons were mainly excited during side-down animal tilt and side-up neck rotation, the RS neurons showed the opposite response patterns. The response characteristics of vestibular and medullary reticular neurons to the combined macular and neck inputs, elicited during head rotation, closely corresponded to those predicted by a vectorial summation of the individual macular and neck responses. The findings are discussed in relation to the possibility that both excitatory VS neurons as well as presumably inhibitory RS neurons contribute to the postural adjustments of the limb musculature during the vestibular and neck reflexes.

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