Responses of Renshaw cells coupled with hindlimb extensor motoneurons to sinusoidal stimulation of labyrinth receptors in the decerebrate cat.

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 (alpha-responses) and a decreased discharge of medullary inhibitory reticulospinal (RS) neurons (beta-responses), both of which act on ipsilateral extensor motoneurons. 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. 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 degrees. For the frequency of head rotation of 0.026 Hz, +/- 10 degrees C, most of the GS R-cells increased their firing rate during side-down head displacement (alpha-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. 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 degrees to 20 degrees at the frequency of 0.026 Hz, so that the response gain remained almost unchanged. An increase in frequency of head rotation from 0.026 to 0.32 Hz at a fixed amplitude of 10 degrees, thus changing the maximal angular acceleration from 0.26 degrees/s2 to 41.7 degrees/s2, reversed the response pattern of R-cells reported above. The resulting beta-responses, which also showed some phase lead or lag with respect to head position, were attributed to vestibular activation of RS neurons.(ABSTRACT TRUNCATED AT 400 WORDS)