Effects of stimulation of vestibular and neck receptors on Deiters neurons projecting to the lumbosacral cord

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Abstract
1. The activity of lateral vestibular nucleus (LVN) neurons, antidromically identified by stimulation of the spinal cord at T12 and L1, thus projecting to the lumbosacral segments of the spinal cord (lVS neurons), was recorded in precollicular decerebrate cats during rotation about the longitudinal axis either of the whole animal (labyrinth input) or of the body only while the head was kept stationary (neck input). 2. Among the lVS neurons tested for vestibular stimulation, 76 of 129 units (i.e. 58.9%) responded to roll tilt of the animal at the standard parameters of 0.026 Hz, ±10°. The gain and the sensitivity of the first harmonic responses corresponded on the average to 0.47±0.44, SD, impulses·s−1·deg−1 and 3.24±3.15, SD, %/deg, respectively. As to the response patterns, 51 of 76 units (i.e. 67.1%) were excited during side-down and depressed during side-up tilt, whereas 15 (i.e. 19.7%) showed the opposite behavior. In both instances the peak of the responses occurred with an average phase lead of about +21.0±27.2., SD, deg with respect to the extreme side-down or side-up position of the animal. Moreover, the former group of units showed almost a twofold larger gain with respect to the latter group (t-test,p−1·deg−1 and 3.30±3.42, SD, %/deg, respectively, thus being similar to the values obtained for the labyrinth responses. However, 59 of 75 units (i.e. 78.6%) were excited during side-up neck rotation and depressed during side-down neck rotation, while 8 of 75 units (i.e. 10.7%) showed the opposite pattern. In both instances the peak of the responses occurred with an average phase lead of +52.0±18.3, SD, deg for the extreme side-up or side-down neck displacements. Further, the former group of units showed a larger gain than the latter group. 4. Histological controls indicated that 102 of 129 (i.e. 79.0%) lVS neurons tested for labyrinth stimulation and 86 of 109 (i.e. 78.9%) lVS neurons tested for neck stimulation were located in the dorsocaudal part of LVN, the remaining lVS neurons being located in the rostroventral part of LVN. 5. The observation that the predominant response pattern of the lVS neurons to roll tilt was just opposite to that of lVS neurons to neck rotation indicates that the motoneurons innervating ipsilateral hindlimb extensors were excited by an increased discharge of vestibulospinal neurons during side-down tilt but they were disfacilitated by the reduced discharge of vestibulospinal neurons during side-down neck rotation; the opposite would occur during side-up animal tilt or neck rotation. These findings were compared with those of previously recorded LVN neurons, whose descending axons were not identified as projecting to upper or lower segments of the spinal cord. It was then possible to evaluate the role that the LVN exerts not only in the control of the limb but also of the neck extensor musculature.