Facilitation from contralateral primary afferents of interneuronal transmission in the Ia inhibitory pathway to motoneurones

Oscillatory Cortical Activity in an Animal Model of Dystonia Caused by Cerebellar Dysfunction

The synchronization of neuronal activity in the sensorimotor cortices is crucial for motor control and learning. This synchrony can be modulated by upstream activity in the cerebello-cortical network. However, many questions remain over the details of how the cerebral cortex and the cerebellum communicate. Therefore, our aim is to study the contribution of the cerebellum to oscillatory brain activity, in particular in the case of dystonia, a severely disabling motor disease associated with altered sensorimotor coupling. We used a kainic-induced dystonia model to evaluate cerebral cortical oscillatory activity and connectivity during dystonic episodes. We performed microinjections of low doses of kainic acid into the cerebellar vermis in mice and examined activities in somatosensory, motor and parietal cortices. We showed that repeated applications of kainic acid into the cerebellar vermis, for five consecutive days, generate reproducible dystonic motor behavior. No epileptiform activity was recorded on electrocorticogram (ECoG) during the dystonic postures or movements. We investigated the ECoG power spectral density and coherence between motor cortex, somatosensory and parietal cortices before and during dystonic attacks. During the baseline condition, we found a phenomenon of permanent adaptation with a change of baseline locomotor activity coupled to an ECoG gamma band increase in all cortices. In addition, after kainate administration, we observed an increase in muscular activity, but less signs of dystonia together with modulations of the ECoG power spectra with an increase in gamma band in motor, parietal and somatosensory cortices. Moreover, we found reduced coherence in all measured frequency bands between the motor cortex and somatosensory or parietal cortices compared to baseline. In conclusion, examination of cortical oscillatory activities in this animal model of chronic dystonia caused by cerebellar dysfunction reveals a disruption in the coordination of neuronal activity across the cortical sensorimotor/parietal network, which may underlie motor skill deficits.

Spinal Control of Locomotion: Individual Neurons, Their Circuits and Functions

Systematic research on the physiological and anatomical characteristics of spinal cord interneurons along with their functional output has evolved for more than one century. Despite significant progress in our understanding of these networks and their role in generating and modulating movement, it has remained a challenge to elucidate the properties of the locomotor rhythm across species. Neurophysiological experimental evidence indicates similarities in the function of interneurons mediating afferent information regarding muscle stretch and loading, being affected by motor axon collaterals and those mediating presynaptic inhibition in animals and humans when their function is assessed at rest. However, significantly different muscle activation profiles are observed during locomotion across species. This difference may potentially be driven by a modified distribution of muscle afferents at multiple segmental levels in humans, resulting in an altered interaction between different classes of spinal interneurons. Further, different classes of spinal interneurons are likely activated or silent to some extent simultaneously in all species. Regardless of these limitations, continuous efforts on the function of spinal interneuronal circuits during mammalian locomotion will assist in delineating the neural mechanisms underlying locomotor control, and help develop novel targeted rehabilitation strategies in cases of impaired bipedal gait in humans. These rehabilitation strategies will include activity-based therapies and targeted neuromodulation of spinal interneuronal circuits via repetitive stimulation delivered to the brain and/or spinal cord.

Pathophysiology of spastic paresis. II: Emergence of muscle overactivity

In the subacute and chronic stages of spastic paresis, stretch-sensitive (spastic) muscle overactivity emerges as a third fundamental mechanism of motor impairment, along with paresis and soft tissue contracture. Part II of this review primarily addresses the pathophysiology of the various forms of spastic overactivity. It is argued that muscle contracture is one of the factors that cause excessive responsiveness to stretch, which in turn aggravates contracture. Excessive responsiveness to stretch also impedes voluntary motor neuron recruitment, a concept termed stretch-sensitive paresis. None of the three mechanisms of impairment (paresis, contracture, and spastic overactivity) is symmetrically distributed between agonists and antagonists, which generates torque imbalance around joints and limb deformities. Thus, each may be best treated focally on an individual muscle-by-muscle basis. Intensive motor training of the less overactive muscles should disrupt the cycle of paresis-disuse-paresis, and concomitant use of aggressive stretch and focal weakening agents in their more overactive and shortened antagonists should break the cycle of overactivity-contracture-overactivity.

Facilitation of transmission via inhibitory pathway 1a to spinal extensor motoneurons in response to stimulation of the forelimb nerves in cats

Activation of forelimb flexor reflex afferent in cats anesthetized with a mixture of Chloralose and Nembutal evoked temporospatial facilitation in the reciprocal inhibitory pathways to spinal extensor motoneurons. The amplitude of disynaptic reciprocal 1a inhibitory postsynaptic potentials (IPSP) evoked in the extensor motoneurons by activation of the most excitable fibers of the nerve supplying the antagonist muscle increased several-fold using conditioning stimulation of the forelimb nerve. Facilitation of 1a IPSP occurred on a background of IPSP evoked by descending inter-limb discharges. Facilitation of 1a IPSP had a latent period of 18-20 msec and could last to 60 msec. The possible role of inhibitory 1a interneurons in interlimb coordination is discussed.

The influence of contralateral primary afferents on Ia inhibitory interneurones in humans

1. Contralateral influences on short latency reciprocal inhibition between wrist extensor and flexor muscles were investigated in twenty-two healthy volunteers. Reciprocal inhibition, probably mediated through the Ia inhibitory interneurone, was measured by conditioning the flexor carpi radialis (FCR) H reflex by weak stimulation of the ipsilateral radial nerve. Maximum reciprocal inhibition occurring at a precise delay between conditioning and conditioned stimulations was taken as the test level of inhibition. 2. Contralateral median or radial nerves were stimulated at short intervals before the onset of reciprocal inhibition. The latter was increased by 8.6% after median nerve stimulation and decreased by 16.5% after radial nerve stimulation. 3. The contribution of sensory fibres in the two nerves to contralateral effects was investigated by stimulating purely sensory branches of the nerves. No clear modification of the contralateral reciprocal inhibition was observed. The effects produced by mixed nerve stimulation are thus likely to have been mediated by Ia fibres. 4. In three hemiplegic patients where reciprocal inhibition was reduced unilaterally, stimulation on the spastic side produced contralateral effects similar to those observed in normal subjects. This result indicates that contralateral effects are not mediated through the Ia inhibitory interneurone ipsilateral to the conditioning stimulus. 5. Since contralateral effects occur after short delays (2 ms, median nerve; 3 ms, radial nerve), we suggest a functional scheme in which the excitability of Ia inhibitory interneurones is modified by contralateral primary afferents via the interneurones activated by group I fibres, probably Ia fibres. The short delays indicate that the interneurone transmitting primary afferent influences to the contralateral side is probably excitatory.

Motor compensatory reactions following a forward fall in subjects with unilateral abolition of the triceps-surae H reflex

The electromyograms of the right and left soleus and tibialis anterior muscles of 6 subjects with unilateral abolition of Achilles tendon reflex due to S1 radiculitis were recorded during a forward fall involving stepping to recover balance. Each subject took part in two series of experiments, one in which the step was performed with the unaffected leg and a second in which the affected leg was used. A unilateral deficiency of peripheral proprioceptive afferents affected ankle muscles EMG activities bilaterally, except for the EMG activity of the soleus of the starting foot. The tibialis anterior of both the affected and the unaffected side, showed either a normal pattern (i.e. a phasic contraction after soleus contraction stopped), or an early contraction. On the stance side, the early contraction was associated with a depressed soleus EMG activity. Some abnormal motor patterns could be due to the ipsilateral deficiency of the Ia inhibitory projection from soleus to tibialis anterior. The presence of abnormal patterns on the unaffected side indicates that the motor activity in one lower limb can be modified by a loss of peripheral afference in the contralateral limb. This suggests that crossed pathways between lower limbs are involved in balance recovery movement.

The measurement of single motor-axon recurrent inhibitory post-synaptic potentials in the cat

1. Signal averaging was used in forty experiments on low-spinal cats to measure and characterize the oligosynaptic responses of seventy-six motoneurons supplying the medial gastrocnemius muscle to the single impulses of antidromically stimulated single motor axons supplying the same muscle. 2. In thirteen experiments on chloralose-urethane anaesthetized preparations, twelve (43%) of the tested twenty-eight motoneurones exhibited a single-axon recurrent inhibitory post-synaptic potential (recurrent i.p.s.p.), as compared to sixty-four (62%) of the 103 motoneurones tested in twenty-seven animals in the absence of anaesthetic after ischaemic decapitation. 3. Single-axon recurrent i.p.s.p.s most often consisted of a single, long-lasting hyperpolarization. Ten of the recurrent i.p.s.p.s contained a second late peak of hyperpolarization. In another eight of the i.p.s.p.s, a small late depolarization was evident. 4. The distinct profiles of the recurrent i.p.s.p.s were readily distinguished from the relatively flat profiles with low noise levels in the averages of the fifty-five 'no-response' cells. The transmembrane and post-synaptic nature of the i.p.s.p.s was confirmed by extracellular control recordings taken immediately outside seven of the cells with positive responses. In addition, ten cells with positive responses were subjected to current passage during the averaging procedure. In all cases, depolarization increased and hyperpolarization reduced the amplitude of their single-axon recurrent i.p.s.p.s. 5. The mean amplitude of the responses was 12.0 microV in chloralose-urethane preparations as compared to a peak-to-peak noise level less than 6.0 microV in the no-response averages. Corresponding values in ischaemic-decapitate preparations were 46.2 microV and less than 7.5 microV, respectively. 6. Latency, rise-time and half-width (i.e. duration at half-amplitude) values of the i.p.s.p.s were similar for chloralose-urethane and ischaemic-decapitate preparations. The average values in both preparations were 2.5, 5.6 and 19.3 ms, respectively. The latency values indicated both disynaptic and, perhaps, longer components in the recurrent i.p.s.p.s. The rise-time and half-width values were relatively similar to those reported or measured from published records for analogous composite recurrent i.p.s.p.s (i.e. responses to antidromic stimulation of the whole muscle nerve rather than single motor axons). A weak, but significant, correlation between rise-time and half-width was observed for the sixty-six single-axon recurrent i.p.s.p.s with a single negative-going peak.(ABSTRACT TRUNCATED AT 400 WORDS)

Reflex pathways from group II muscle afferents. 2. Functional characteristics of reflex pathways to alpha-motoneurones

The convergence of group II muscle afferents on interneurones in reflex pathways has been elucidated by investigating interaction in transmission to motoneurones. Recording was also made from interneurones activated from group II afferents. Maximal group II EPSPs evoked in motoneurones from different muscles (extensors or flexors and extensors) did not summate linearly but with a deficit of 35-40%. The corresponding deficit in summation with Ia EPSPs was 7%. It is suggested that the difference in deficit is caused largely by occlusion due to shared interneuronal discharge zones and that it gives an approximate minimal measure of the convergence of group II afferents from different muscles on the interneurones. Tests with weak group II volleys from different muscles gave no or little evidence for spatial facilitation in the disynaptic excitatory pathway to flexor motoneurones, and there was no or little temporal facilitation of transmission in this pathway. It is suggested that group II excitation of the interneurones in this pathway depends on few afferents giving large unitary EPSPs. Convergence of cutaneous afferents and joint afferents on the interneurones was evidenced by spatial facilitation from these afferents of group II transmission to motoneurones. Convergence on interneurones in the trisynaptic inhibitory pathway from group II afferents to extensor motoneurones was also investigated with the spatial facilitation technique. There was convergence on common interneurones of group II afferents from different muscles (extensors or flexors and extensors) and from cutaneous afferents as well as joint afferents. Trisynaptic group II IPSPs, including those depending on spatial facilitation from different muscles were resistant to recurrent depression from motor axon collaterals and are therefore not mediated by the reciprocal Ia inhibitory pathway. Interneurones with monosynaptic group II EPSPs were recorded from in the dorsal horn and intermediate region. Graded stimulation revealed large unitary EPSPs from few group II afferents. The EPSP evoked by a single group II afferent may produce firing (extracellular recording). Convergence of monosynaptic group II EPSPs from different muscles was rather limited but could be from flexors and extensors. Extensive multisensory convergence onto some of these interneurones was indicated by di- or polysynaptic EPSPs from group II and III muscle afferents, from joint afferents and from cutaneous afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

Reflex pathways from group II muscle afferents. 3. Secondary spindle afferents and the FRA: a new hypothesis

A hypothesis is forwarded regarding the role of secondary spindle afferents and the FRA (flexor reflex afferents) in motor control. The hypothesis is based on evidence (cf. Lundberg et al. 1987a, b) summarized in 9 introductory paragraphs. Group II excitation. It is postulated that subsets of excitatory group II interneurones (transmitting disynaptic group II excitation to motoneurones) may be used by the brain to mediate motor commands. It is assumed that the brain selects subsets of interneurones with convergence of secondary afferents from muscles whose activity is required for the movement. During movements depending on coactivation of static gamma-motoneurones impulses in secondary afferents may servo-control transmission to alpha-motoneurones at an interneuronal level. The large group II unitary EPSPs in interneurones are taken to indicate that, given an adequate interneuronal excitability, impulses in single secondary afferents may fire the interneurone and produce EPSPs in motoneurones; interneuronal transmission would then be equivalent to that in a monosynaptic pathway but with impulses from different muscles combining into one line. It is postulated that impulses in the FRA are evoked by the active movements and that the role of the multisensory convergence from the FRA onto the group II interneurones is to provide the high background excitability which allows the secondary spindle afferents to operate as outlined above. The working hypothesis is put forward that a movement governed by the excitatory group II interneurones is initiated by descending activation of these interneurones, but is maintained in a later phase by the combined effect of FRA activity evoked by the movement and by spindle secondaries activated by descending activation of static gamma-motoneurones. As in the original "follow up length servo" hypothesis (Rossi 1927; Merton 1953), we assume that a movement at least in a certain phase can be governed from the brain solely or mainly via static gamma-motoneurones. However, our hypothesis implies that the excitatory group II reflex connexions have a strength which does not allow transmission to motoneurones at rest and that the increase in the gain of transmission during an active movement is supplied by the movement itself. Group II inhibition. It is suggested that the inhibitory reflex pathways like the excitatory ones have subsets of interneurones with limited group II convergence. When higher centres utilize a subset of excitatory group II interneurones to evoke a given movement, there may mobilize inhibitory subsets to inhibit muscles not required in the movement.(ABSTRACT TRUNCATED AT 400 WORDS)

Reciprocal inhibition during the tonic stretch reflex in the decerebrate cat

1. The aim of this study was to investigate post-synaptic reciprocal Ia inhibition during the stretch reflex; particularly the extent to which an increased Ia excitation of the Ia inhibitory interneurones will be counteracted by recurrent inhibition from motor axon collaterals. For this purpose we investigated depression of monosynaptic test reflexes antagonist flexors (reciprocal inhibition) during static stretch of quadriceps or triceps surae in unanaesthetized decerebrate cats. 3. With increasing stretch of the extensor muscle there was first a linear augmentation of reciprocal inhibition, but along with the stretch reflex in the extensor a plateau appeared in the inhibition of the flexors, although the extensor stretch reflex (judged by the e.m.g.) increased with further stretching. Within the range of stretching of triceps surae which gave increased stretch reflexes the plateau in the reciprocal inhibition was usually maintained, while during stretching of quadriceps a second phase of augmenting reciprocal inhibition often appeared. Stretch beyond the level which increased the stretch reflex activity gave augmenting reciprocal inhibition both in case of quadriceps and triceps surae. 3. Excitability measurements from central terminals of Ia afferents revealed that the increasing reciprocal inhibition during increasing stretch reflex activity in quadriceps was associated with a primary afferent depolarization in knee flexor Ia afferents; there was no corresponding effect in ankle flexor Ia afferents during stretch reflexes in triceps surae. 4. The primary afferent depolarization evoked in knee flexor Ia afferents by electrical nerve stimulation was then compared with the presynaptic inhibition of knee flexor monosynaptic test reflexes produced by the same stimuli. The results suggest that the second phase of increasing reciprocal inhibition in knee flexors is due to presynaptic inhibition and accordingly that the depth of post-synaptic reciprocal inhibition remains constant at different degrees of stretch reflex activity in both knee and ankle extensors. 5. It is postulated that during increasing stretch reflex activity the increment in Ia excitation and recurrent inhibitio; on to the Ia inhibitory interneurones almost exactly balance each other. It is suggested that recurrent inhibition of Ia inhibitory interneurones may serve as a segmental autoregulatory mechanism to keep 'alpha-gamma-linked reciprocal inhibition' at a constant depth during different levels of agonist activity.

Convergence on interneurones mediating the reciprocal Ia inhibition of motoneurones. II. Effects from segmental flexor reflex pathways

Interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as "Ia inhibitory interneurones") were recorded in the lumbar spinal cord of the cat. Volleys in ipsilateral and contralateral high threshold muscle afferents, cutaneous and high threshold joint afferents evoked a mixture of polysynaptic excitation and inhibition. These effects were ascribed to pathways activated by flexor reflex afferents (FRA) and in addition a specific ipsilateral low threshold cutaneous pathway. Ia inhibitory interneurones excited monosynaptically from flexor nerves received stronger net excitation by volleys in ipsilateral FRA than did extensor coupled interneurones, while the opposite pattern was seen from the contralateral FRA. These patterns are similar to those found in flexor and extensor motoneurones respectivey. The FRA inhibition in Ia inhibitory interneurones was partly mediated by "opposite" Ia inhibitory interneurones, i.e. those which are mediating the Ia inhibition of Ia inhibitory interneurones. The extent to which the FRA inhibition is transmitted by Ia inhibitory interneurones was roughly estimated by its susceptibility to recurrent depression by antidromic ventral root stimulation. The main conclusion is that most segmental pathways seem to evoke their effects in parallel to motoneurones and Ia inhibitory interneurones which are monosynaptically linked to the same muscle. The functional importance of this conclusion is discussed in a following report.

Convergence on interneurones mediating the reciprocal Ia inhibition of motoneurones. III. Effects from supraspinal pathways

Supraspinal effects were investigated in interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as Ia inhibitory interneurones). It was revealed that volleys in the vestibulospinal tract may evoke mono- and disynaptic EPSPs in interneurones monosynaptically excited from extensor muscles, i.e. extensor coupled Ia inhibitory interneurones. Flexor coupled interneurones instead received disynaptic inhibition. Volleys in the rubrospinal tract evoked a dominating polysynaptic excitation, usually mixed with inhibition, in flexor as well as extensor coupled interneurones. Disynaptic rubrospinal EPSPs and IPSPs were also revealed. The pyramidal tract also gives rise to a dominating polysynaptic excitation, usually mixed with inhibition, in flexor as well as extensor coupled Ia inhibitory interneurones. Rubrospinal and pyramidal volleys were shown to facilitate transmission in various segmental reflex pathways to the Ia inhibitory interneurones. A detailed comparison reveals a striking parallelism of segmental and supraspinal effects on alpha-motoneurones and Ia inhibitory interneurones connected to the same muscles. This considerably strengthens the hypothesis of an "alpha-gamma-linkage in the reciprocal inhibition".

Convergence on interneurones mediating the reciprocal Ia inhibition of motoneurones. I. Disynaptic Ia inhibition of Ia inhibitory interneurones

Interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as "Ia inhibitory interneurones") were recorded in the lumbar spinal cord of the cat. It was revealed that the Ia inhibitory interneurones themselves receive disynaptic Ia inhibition. The muscles from which this inhibition is evoked are strictly antagonistic to those supplying their Ia excitation. Similar to the Ia inhibition in motoneurones the Ia inhibition in the Ia inhibitory interneurones is decreased when preceded by an antidromic stimulation of ventral roots. Furthermore, transmission of Ia inhibition to the Ia inhibitory interneurones is facilitated from ipsilateral and contralateral primary afferents as well as several supraspinal pathways analogous to earlier findings for the Ia inhibition of motoneurones. The pattern and control of the Ia inhibition of motoneurones and of Ia inhibitory interneurones display so striking similarities that it is suggested that identical interneurones are responsible. The conclusion thus emerges that "opposite" Ia inhibitory interneurones (i.e. interneurones monosynaptically connected to antagonistic muscles) are mutually inhibiting each other. The functional significance of this organization is discussed.

Effects from the vestibulospinal tract on transmission from primary afferents to ventral spino-cerebellar tract neurones

Convergence of vestibulospinal and segmental effects onto spinal interneurones which project to the ventral spino-cerebellar tract (VSCT) neurones has been studied by intracellular recording in VSCT cells. The disynaptic Ia IPSPs evoked in a group of VSCT neurones from the quadriceps nerve are monosynaptically facilitated by the vestibulospinal tract while there was no facilitation of Ia IPSP evoked from a flexor nerve. These results support the view that Ia inhibition to VSCT cells and motoneurones is mediated by common interneurones. The disynaptic inhibition evoked in other VSCT cells from the vestibulospinal tract is facilitated by volleys in the contralateral flexor reflex afferents (FRA) or bilaterally from the FRA. It is postulated that these actions are mediated by collaterals of the interneurones responsible for the analogous effects in motoneurones. Findings are reported suggesting that the monosynaptic vestibulospinal EPSP in VSCT cells in most cases is collateral to the excitatory input to the last order interneurones of reflex pathways from the FRA to motoneurones and only exceptionally to the corresponding input to Ia inhibitory interneurones. In many VSCT cells the vestibulospinal tract evoked disynaptic EPSPs which are facilitated from the FRA; the functional significance of this action is uncertain. The results are consistent with the hypothesis that VSCT neurones signal information on interneuronal transmission to motoneurones.