Responses of lateral reticular neurons to convergent neck and macular vestibular inputs.

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Abstract
Convergence of neck and macular vestibular inputs within the lateral reticular nucleus (NRL) and the lateral aspect of the main reticular formation (RF) of the medulla was investigated in decerebrate cats following sinusoidal rotation around the longitudinal axis either of the body only, while maintaining the head stationary (neck input), or of the whole animal (macular input). Neck and macular inputs were elicited by rotating the head and the body remained fixed in the horizontal position. Among the 102 lateral reticular units tested at the frequency of rotation of 0.026 Hz (5-10.degree. peak amplitude), 51 units (i.e., 50%) responded to neck and macular inputs, 10 units (9.8%) responded to the neck input only, 7 units (6.9%) to the macular input only and the remaining 34 units (33.3%) were unresponsive to either type of stimulation. The 1st harmonic of responses to the neck input and the macular input showed an average phase lead of about +38.degree. with respect to neck position and +14.degree. with respect to animal position. The sensitivity of the units, expressed in percentage changes of the mean firing rate/degree of displacement, corresponded on the average to 5.5 .+-. 5.1 (SD) for the neck resonses and 3.2 .+-. 2.8 (SD) for the macular responses. Two distinct populations of neurons were observed. The 1st group of units (36/48, i.e., 75%) received reciprocal influences from neck and macular receptors. These units were mainly excited during side-down neck rotation, but were inhibited during side-down tilt of the whole animal. The sensitivity of the neck responses was only slightly higher than that of the macular responses. The 2nd group of units (12/48, i.e., 25%) received parallel influences from the 2 types of receptors. These units were mainly excited during both side-up neck rotation and side-up tilt of the whole animal. The sensitivity of the neck responses within this group was much higher than that of the macular responses. The 1st population of units received a somatosensory input from the hindlimbs, which was not the case for the other one. The 1st group of units was distributed within the whole extent of the NRL; the 2nd group was concentrated in the dorsomedial, magnocellular part of this nucleus. The response characteristics of lateral reticular neurons to the combined neck and macular inputs, elicited during head rotation, closely corresponded to those obtained under the assumption of a vectorial summation of the individual neck and macular responses. The units that showed a reciprocal pattern of convergence displayed small amplitudes and large phase leads of the responses with respect to head position, when both types of receptors were simultaneously stimulated. The units that showed a parallel pattern of convergence displayed large amplitude and small phase leads during head rotation. The functional significance of these 2 populations of neurons is discussed relative to the known tonic effects of neck and macular vestibular inputs on the limb, neck and extrinsic eye musculature.