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Notes: Abstract 1. Contraction of ipsilateral limb extensors during side-down roll tilt of the head, leading to selective stimulation of labyrinth receptors, is attributed to an increased discharge of excitatory vestibulospinal (VS) neurons (α-responses) and a decreased discharge of medullary inhibitory reticulospinal (RS) neurons (β-responses), both of which act on ipsilateral extensor motoneurons. 2. Experiments were performed in decerebrate cats, with the de-efferented gastrocnemius-soleus (GS) muscle fixed at a constant length, to find out whether Renshaw (R) cells linked with GS motoneurons responded to labyrinth stimulation elicited by head rotation, while the neck had been bilaterally deafferented. We hoped in this way to clarify the role and the mechanism by which these inhibitory interneurons act on limb extensor motoneurons during the vestibular reflexes. 3. 72.7% of the R-cells, disynaptically excited by group I volleys elicited by single shock stimulation of the GS nerve, weakly responded to head rotation at frequencies of 0.026–0.15 Hz and at a peak amplitude of 10°. 4. For the frequency of head rotation of 0.026 Hz, ±10°C, most of the GS R-cells increased their firing rate during side-down head displacement (α-responses); some responses were related to head position, but others showed some phase lead or lag with respect to head position. The gain of the first harmonic of these unit responses was very low and corresponded on the average to 0.084±0.062, S.D. imp./s/deg, while the sensitivity corresponded to 2.14±2.35, S.D.%/deg (base frequency, 6.85±5.97, S.D.imp./s). These responses were attributed to the activity of VS neurons, the increased discharge of which during side-down head rotation exerts a weak excitatory influence on a limited number of GS motoneurons and, through their recurrent collaterals, on the related R-cells. 5. The modulation of the firing rate of R-cells coupled with the GS motoneurons increased linearly by increasing the peak amplitude of displacement from 5° to 20° at the frequency of 0.026 Hz, so that the response gain remained almost unchanged. 6. An increase in frequency of head rotation from 0.026 to 0.32 Hz at a fixed amplitude of 10°, thus changing the maximal angular acceleration from 0.26°/s2 to 41.7°/s2, reversed the response pattern of R-cells reported above. The resulting β-responses, which also showed some phase lead or lag with respect to head position, were attributed to vestibular activation of RS neurons. These neurons may directly excite the R-cells linked with the GS motoneurons, so that their reduced discharge during side-down head rotation would lead to a decrease in firing rate of the corresponding inhibitory interneurons. Interestingly, the response gain and sensitivity of these R-cells first decreased but then increased after reversal of the response pattern, by increasing frequency of head rotation. The critical frequency at which a cancellation of the response occurred before the response pattern was reversed, ranged between 0.051 and 0.15 Hz. 7. In conclusion, R-cells linked with the GS motoneurons displayed an α-pattern of response during low frequency head rotation, suggesting that they were largely activated by the recurrent collaterals of the corresponding motoneurons driven by the VS volleys. The same R-cells, however, exhibited the opposite β-pattern of response at higher stimulus frequencies, thus behaving as if they were decoupled from their input motoneurons and controlled more directly by the medullary RS neurons. In these instances, activation of GS motoneurons during side-down head rotation could be attributed not only to an increased discharge of the excitatory VS neurons, but also to a reduced discharge of the R-cells driven by the medullary RS neurons. This system may thus contribute to the gain regulation of the EMG responses of hindlimb extensors to labyrinth stimulation.

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.