Red nucleus cell activity in awake cats during a placing reaction

It is known that the discharges of the posterior red nucleus (RN) cells are modulated during locomation (Orlovsky, 1972). The aims of the present investigation were to look for a possible relationship between the unitary activity of RN cells and movements as well as postural reactions of the forelimbs. In the experimental arrangement used, the cat stood on four transducers which recorded to forces on each limb. The animal was restrained by a hammock supporting a part of the total body weight. A horizontally moving plate touched one forepaw and pushed it backwards until a placing mavement was induced. During the first part of the sequence, the mechanically stimulated limb was actively unloaded just prior to the placing movement, while the contralateral forelimb simultaneously presented a loading postural reaction. The results can be summarized and interpreted in the following way: 1. The RN cells exhibit large variations of discharge frequencies during phasic muscular activities linked to movement and to postural adjustments, but no correlations can be established between tonic cell activity and static postural forces exerted by the limb. 2. During the swing phase, the neuronal activity of some RN cells is maximal during flexion, while for others the maximum is during extension. 3. When there is a modification in frequency at the onset of the placing reaction relative to activity during the unloading reaction, the beginning of this change occurs after the onset of the movement as recorded by a potentiometer placed at the elbow joint. 4. The increases of frequency are higher during reciprocal muscle activation (during the movement) than during coactivation of antagonistic muscles (during the postural loading reaction). 5. During the loading reaction (with mostly extensor activation) the frequency of firing of those neurones discharging during the flexion phase. Thus the data may indicate that the RN is included in a feed-back loop which is linked not to muscle activities but rather to phasic spinal reflexes.