Convergence and interaction of neck and macular vestibular inputs on reticulospinal neurons

Extracellular recordings were obtained in decerebrate cats from neurons located in the inhibitory area of the medullary reticular formation, namely in the medial aspects of the nucleus reticularis gigantocellularis, magnocellularis and ventralis. Of 127 medullary reticular units examined, 77 were reticulospinal neurons antidromically identified following stimulation of the spinal cord at T12-L1; the remaining 50 neurons were not activated antidromically. Unit firing rate was analyzed under separate stimulation of macular vestibular, neck, or combined receptors by using sinusoidal rotations about the longitudinal axis at 0.026 Hz, 10 peak amplitude. Among the 127 reticular units, 84 (66.1%) responded with a periodic modulation of their firing rate to roll tilt of the animal and 93 (73.2%) responded to neck rotation. Convergence of macular and neck inputs was found in 71/127 (55.9%) reticular neurons; in these units, the gain as well as the sensitivity of the first harmonic of response corresponded on the average to 0.49 +/- 0.41, SD imp/s/deg and 5.10 +/- 4.27, SD %/deg for the neck responses and to 0.40 +/- 0.39, SD imp/s/deg and 3.90 +/- 3.80, SD %/deg for the macular responses, respectively. Most of the convergent reticular units were maximally excited by the direction of stimulus orientation, the first hormonic or responses showing an average phase lead of about +42.7 with respect to neck position and +24.9 with respect to animal position. Two populations of convergent neurons were observed. The first group of units (58/71, i.e. 81.7%) showed reciprocal ("out of phase") responses to the two inputs in that they were mainly excited during side-down neck rotation, but inhibited during side-down animal tilt. The remaining group of units (13/71, i.e. 18.3%) showed parallel ("in phase") responses to the two inputs and they were mainly excited by side-down neck rotation and animal tilt. The response characteristics of medullary reticular neurons to the combined neck and macular inputs, elicited during head rotation, closely corresponded to those predicted by a vectorial summation of the individual neck and macular responses. In particular, "out of phase" units displayed small amplitudes and large phase leads of the responses with respect to head position, when both types of receptors were costimulated. In contrast, "in phase" units displayed large amplitude and small phase leads during head rotation.(ABSTRACT TRUNCATED AT 400 WORDS)