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Notes: Abstract 1. The contraction of limb extensor muscles during side-down roll tilt of the animal depends upon an increased discharge of excitatory vestibulospinal (VS) neurons (α-response) and a reduced discharge of inhibitory reticulospinal (RS) neurons of the medulla (β-response), both acting on ipsilateral limb extensor motoneurons. In the decerebrate cat, a modulation of the multiunit EMG activity was clearly present in forelimb extensors, but was extremely weak or absent in hindlimb extensors. 2. Experiments performed in decerebrate cats with the deefferented GS muscle fixed at a constant length have shown that Renshaw (R)-cells, monosynaptically coupled with gastrocnemius-soleus (GS) motoneurons, were either unresponsive or displayed only very weak, small amplitude α-responses to sinusoidal stimulation of labyrinth receptors elicited during slow head rotation after bilateral neck deafferentation. This effect was attributed to excitatory VS volleys acting on GS motoneurons and, through their recurrent collaterals, on the related R-cells. In these instances the recurrent inhibition of the GS motoneurons contributed to the very low gain of the EMG response of the corresponding muscles to labyrinth stimulation. 3. Intravenous injection of an anticholinesterase (eserine sulphate, 0.05–0.1 mg/kg) at a dose that in previous experiments increased the firing rate of medullary RS neurons, while decreasing the decerebrate rigidity, slightly increased the discharge rate of R-cells linked with the GS motoneurons in the animal at rest; these findings suggest that the RS system inhibits the extensor motoneurons by exciting the related R-cells. All the R-cells, which prior to the injection were either unresponsive or showed an α-response to head rotation (at 0.026–0.15 Hz, ±10°), after eserine sulphate showed a β-response for the same parameters of labyrinth stimulation. In particular, a reduced discharge of the R-cells linked with the GS motoneurons occurred during side-down head rotation as shown for the majority of the RS neurons. It appears therefore that the same R-cells, which in the normal decerebrate cat responded to the excitatory VS volleys acting through the GS motoneurons, were now decoupled from their input motoneurons during head rotation, thus behaving as if they underwent the most efficient direct excitatory control of the RS system. The reduced discharge of the R-cells linked with the GS motoneurons during side-down head rotation would lead to disinhibition of these motoneurons, thus enhancing the response gain of the corresponding muscle to labyrinth stimulation. 4. This hypothesis was supported by the results of electromyographic experiments showing that the modulation of the multiunit activity of the GS muscle, which in normal decerebrate cats was negligible or extremely poor in amplitude, greatly increased after injection of the same dose of anticholinesterase. 5. We postulate that the inhibitory RS system, which is under the tonic excitatory control of a cholinergic system, operates through the R-cells as a variable gain regulator, acting at the motoneuronal level during the vestibular reflexes. The results of the present experiments support the general hypothesis on the function recently attributed to the recurrent inhibitory circuit in the spinal cord.