Convergence of neck and macular vestibular inputs on vestibulospinal neurons projecting to the lumbosacral segments of the spinal cord.

Abstract

The activity of LVN neurons was recorded in decerebrate cats and analyzed during separate stimulation of macular vestibular and neck receptors elicited by sinusoidal rotation about the longitudinal axis at 0.026 Hz, 10 degrees peak amplitude. Of 119 LVN units examined, the great majority, i.e. 106, were vestibulospinal neurons antidromically identified following stimulation of the spinal cord at T12-L1, thus projecting to the lumbosacral segments of the spinal cord (IVS neurons); the remaining 13 units were nonantidromically activated. Among the 119 LVN neurons, 77 (64.7%) responded with a periodic modulation of their firing rate to roll tilt of the animal and 81 (68.1%) responded to neck rotation. Convergence of macular and neck inputs was found in 58/119 (48.7%) lateral vestibular neurons; in these units, the gain as well as the sensitivity of the first harmonic of responses corresponded on the average to 0.58 ± 0.45, S.D. imp./sec/deg and 4.39 ± 3.58, S.D.%/deg for the neck responses and 0.52 ± 0.49, S.D. imp./sec/deg and 3.85 ± 3.35, S.D.%/deg for the macular responses, respectively. In addition to these convergent units, 19/119 (16.0%) and 23/119 (19.3%) lateral vestibular units responded to selective stimulation either of macular receptors or of neck receptors only. These units, which showed on the average an higher firing rate and a lower conduction velocity of the corresponding vestibulospinal axons than the convergent units, displayed a significantly lower response gain and sensitivity to animal tilt and neck rotation with respect to those obtained from convergent units. Most of the convergent lateral vestibular units were maximally excited by the direction of stimulus orientation, the first harmonic of responses showing an average phase lead of +51.4 degrees with respect to neck position and +21.9 degrees with respect to animal position. Two populations of convergent neurons were observed. The first group of units (53/58, i.e. 91.4%) showed reciprocal (out-of-phase) responses to the two inputs in that they were mainly excited during side-down animal tilt and side-up neck rotation. The remaining group of units (5/58, i.e. 8.6%) showed parallel (in phase) responses to the two inputs and they were mainly excited by side-up neck rotation and animal tilt. Interestingly, the former group of units displayed an average gain and sensitivity to the labyrinth and neck inputs which were more than twice higher than the values obtained from the latter group of units.(ABSTRACT TRUNCATED AT 400 WORDS)