Relative contribution of Ia inhibitory interneurones to inhibition of feline contralateral motoneurones evoked via commissural interneurones

Abstract

The aim of the study was to examine to what extent the crossed inhibition of hindlimb lumbar alpha-motoneurones is evoked via interneurones that mediate reciprocal inhibition between flexors and extensors (Ia inhibitory interneurones), and to what extent via other spinal interneurones. The crossed inhibition was evoked by reticulospinal and vestibulospinal tract fibres, stimulated in the contralateral medullary longitudinal fascicle and the lateral vestibular nucleus, respectively, or by group II muscle afferents in the contralateral quadriceps nerve. The components of the IPSPs recorded in motoneurones that were mediated by Ia inhibitory interneurones were identified by their depression following activation of Renshaw cells. Trisynaptic components of IPSPs of reticulospinal and vestibulospinal origin, and polysynaptic (but not trisynaptic) components of IPSPs from group II afferents were found to be depressed in the majority of the motoneurones, while disynaptic components, those due to direct actions of inhibitory commissural interneurones, were not depressed. These results indicate that the coordination of left and right hindlimb movements based on crossed inhibition from reticulospinal and vestibulospinal neurones, depends on the degree of activation of Ia inhibitory interneurones by muscle spindle afferents and on their inhibition by Renshaw cells. Our results also indicate that Ia inhibitory interneurones do not operate as last-order inhibitory interneurones in crossed trisynaptic pathways from group II afferents, even though they mediate inhibition evoked by interneurones in shared polysynaptic pathways of crossed flexor and extensor reflexes coactivated by group II and other high-threshold muscle, skin and joint afferents.

Figure 1. Diagram of the hypothetical network of neurones mediating di- and trisynaptic IPSPs evoked from the reticular formation, the vestibular nuclei and group II afferents in contralateral motoneurones and the location of the stimulation sites

A, the neuronal network of inhibition evoked via either inhibitory commissural interneurones (C shaded) or excitatory commissural interneurones (C white) and Ia inhibitory interneurones (In) which are excited by group Ia afferents and inhibited by Renshaw cells (R). The arrowhead indicates the site of conditioning stimulation of Q motoneurones axons after transection of the L5 and L6 dorsal roots. Open circles represent excitatory interneurones. Filled circles represent inhibitory interneurones. B–D, stimulation sites, marked by the electrolytic lesion made after one of the experiments with the locations of marking lesions in the remaining experiments superimposed on the same scanned sections (posterior 9–10 for MLF, 7–8.5 for LVN and 3–4 at the border of the brachium conjunctivum, BC; at one additional location) according to the Horsley–Clarke coordinates (Berman, 1968). IO, inferior olive; Pyr, pyramid; IN, nucleus interpositus; FN, nucleus fastigius; SO, superior olive; co Ex, contralateral extensors; co Fl, contralateral flexors.

Figure 2. Selective depression of trisynaptic IPSPs evoked from the MLF in contralateral motoneurones by Renshaw cells

A, IPSPs evoked in a Sart motoneurone by a train of MLF stimuli which were sufficiently strong to evoke not only disynaptic but also trisynaptic IPSPs. B, IPSPs evoked by the same stimuli preceded by conditioning stimulation of the ipsilateral Q nerve (i Q) to activate Renshaw cells (after the dorsal roots were transected). Lower traces in A and B are records of afferent volleys from the surface of the spinal cord. All are averages of 10 successive records. C, superimposed expanded records of the IPSPs in A and B and the difference between them. Broken vertical lines indicate the onset of the disynaptically and trisynaptically evoked IPSPs. D–F, similar records from another Sart motoneurone in which only disynaptic IPSPs were evoked by weaker stimuli. Note in F that activation of Renshaw cells had no effect on the IPSP evoked by the thirdd stimuli and, if anything, increased IPSPs evoked by the second stimuli. Double arrows indicate the time windows within which the areas of the early parts of the conditioned IPSPs and of the differences between the test and conditioned IPSPs were measured (for the comparisons summarized in Table 1, column 9).

Figure 3. Depression of trisynaptic components of IPSPs evoked from the LVN in contralateral motoneurones by Renshaw cells

A, an electrode track (indicated by the broken line) at Horsley–Clarke plane P7.0. The arrow indicates the site selected for testing effects of LVN stimuli. The scale is in millimetres, according to Horsley–Clarke's coordinates. FN, fastigial nucleus; H–C, Horsley–Clarke; P, posterior. B, IPSPs recorded in a Sart motoneurone from the indicated depths along the electrode track shown in A. The lowermost trace is the cord dorsum potential evoked by stimuli applied at depth −2. C–F, intracellular records from two Sart motoneurones (with cord dorsum records in bottom traces in D and F); averages of 40 and 20 records, respectively. In C and E IPSPs were evoked by trains of test stimuli applied in the LVN and by conditioning stimuli alone (black) and IPSPs evoked when LVN stimuli were preceded by stimulation of motor axons in the Q nerve (grey). In D and F the right parts of records from A and B (twice expanded) are superimposed and aligned with differences between IPSPs evoked by the test stimuli alone and by joint actions of the test and conditioned stimuli from which responses to conditioned stimuli alone were subtracted. The first broken lines indicate the first components of the descending volleys from the LVN in the cord dorsum records; the second and third broken lines indicate the onsets of the test IPSPs and of the deviations between the test and conditioned IPSPs. Other indications are as in Fig. 2. Note that only the later components of IPSPs in C and D were depressed.

Figure 4. Spatial facilitation between synaptic actions of MLF fibres and of group Ia afferents

The upper records in each pair are from a Sart (flexor) motoneurone, as shown by antidromic activation in E, lower records being from the cord dorsum. A–C, effects of separate and joint stimulation of the MLF and group Ia afferents in the Q (extensor) nerve. The grey trace in C is a sum of the upper traces in A and B, showing that the facilitation of the IPSP at a premotoneuronal level was considerably larger. D, IPSP evoked by near-maximal stimulation of group Ia afferents.

Figure 5. Effects of MLF stimulation on a Ia inhibitory interneurone

A,B and D upper traces, single sweep extracellular records from the interneurone; lower traces are from the cord dorsum. C and E, the two upper traces are peristimulus time histograms (PSTHs) of responses of the interneurone and the bottom traces are cumulative sums of these responses. A and B, identification of the interneurone as the Ia inhibitory interneurone, showing responses to a high frequency of stimuli, responses to weak stimulation of the quadriceps (Q) nerve and inhibition of these responses following stimulation of a ventral root (VR) (grey trace in B). C, PSTHs and cumulative sums of responses to 20 consecutive stimuli applied alone, as in B (black) and when they were preceded by VR stimuli (grey). D, sample record of responses to near-threshold Q stimuli preceded by MLF stimuli. E, PSTHs and cumulative sums of responses to 20 test Q 1.1 T stimuli alone, and to test stimuli preceded by conditioning stimulation of the MLF, aligned with records in D. The number of spikes was counted within 2 ms windows indicated by broken lines.

Figure 6. Effects of activation of Renshaw cells on inhibition evoked by contralateral group II muscle afferents

Left and right panels show records from two Sart motoneurones (upper traces, averages of 10 consecutive records) and from the surface of the spinal cord (lower traces). A and D, test IPSPs evoked from contralateral group II afferents. B and E, IPSPs evoked by the same test stimuli following conditioning stimulation of the ipsilateral deafferented Q nerve (grey traces) superimposed on recurrent IPSPs (black traces) evoked by conditioning stimuli. C and F, superimposed records of the test (black) and conditioned (grey) IPSPs, at higher resolution, after subtraction of the recurrent IPSPs from the conditioned responses, together with the differences between these records. Broken lines in C and F indicate onset of the IPSPs and of the components depressed by Renshaw cells. Other indications as in Fig. 2.

Figure 7. Diagram of lines of interneurones mediating inhibition of contralateral motoneurones from the reticular formation, lateral vestibular nucleus and group II afferents

Excitatory and inhibitory neurones are represented by open and shaded circles, respectively. In, Ia inhibitory interneurones; R, Renshaw cells; C, commissural interneurones. For other indications see Fig. 1 and the text.