Katinka Stecina

Modelling spinal circuitry involved in locomotor pattern generation: insights from the effects of afferent stimulation

A computational model of the mammalian spinal cord circuitry incorporating a two‐level central pattern generator (CPG) with separate half‐centre rhythm generator (RG) and pattern formation (PF) networks has been developed from observations obtained during fictive locomotion in decerebrate cats. Sensory afferents have been incorporated in the model to study the effects of afferent stimulation on locomotor phase switching and step cycle period and on the firing patterns of flexor and extensor motoneurones. Here we show that this CPG structure can be integrated with reflex circuits to reproduce the reorganization of group I reflex pathways occurring during locomotion. During the extensor phase of fictive locomotion, activation of extensor muscle group I afferents increases extensor motoneurone activity and prolongs the extensor phase. This extensor phase prolongation may occur with or without a resetting of the locomotor cycle, which (according to the model) depends on the degree to which sensory input affects the RG and PF circuits, respectively. The same stimulation delivered during flexion produces a temporary resetting to extension without changing the timing of following locomotor cycles. The model reproduces this behaviour by suggesting that this sensory input influences the PF network without affecting the RG. The model also suggests that the different effects of flexor muscle nerve afferent stimulation observed experimentally (phase prolongation versus resetting) result from opposing influences of flexor group I and II afferents on the PF and RG circuits controlling the activity of flexor and extensor motoneurones. The results of modelling provide insights into proprioceptive control of locomotion.

Excitatory and inhibitory intermediate zone interneurons in pathways from feline group I and II afferents: differences in axonal projections and input

The aim of the present study was to compare properties of excitatory and inhibitory spinal intermediate zone interneurons in pathways from group I and II muscle afferents in the cat. Interneurons were labelled intracellularly and their transmitter phenotypes were defined by using immunocytochemistry. In total 14 glutamatergic, 22 glycinergic and 2 GABAergic/glycinergic interneurons were retrieved. All interneurons were located in laminae V–VII of the L3–L7 segments. No consistent differences were found in the location, the soma sizes or the extent of the dendritic trees of excitatory and inhibitory interneurons. However, major differences were found in their axonal projections; excitatory interneurons projected either ipsilaterally, bilaterally or contralaterally, while inhibitory interneurons projected exclusively ipsilaterally. Terminal projections of glycinergic and glutamatergic cells were found within motor nuclei as well as other regions of the grey matter which include the intermediate region, laminae VII and VIII. Cells containing GABA/glycine had more restricted projections, principally within the intermediate zone where they formed appositions with glutamatergic axon terminals and unidentified cells and therefore are likely to be involved in presynaptic as well as postsynaptic inhibition. The majority of excitatory and inhibitory interneurons were found to be coexcited by group I and II afferents (monosynaptically) and by reticulospinal neurons (mono‐ or disynaptically) and to integrate information from several muscles. Taken together the morphological and electrophysiological data show that individual excitatory and inhibitory intermediate zone interneurons may operate in a highly differentiated way and thereby contribute to a variety of motor synergies.

Commissural interneurons with input from group I and II muscle afferents in feline lumbar segments: neurotransmitters, projections and target cells

The aim of this study was to analyse neurotransmitter content, projection areas and target cells of commissural interneurons with input from group I and/or II muscle afferents in lumbar segments in the cat. Axonal projections of 15 intracellularly labelled commissural interneurons were reconstructed. Ten interneurons (nine located in laminae VI–VII, one in lamina VIII) were glutamatergic; only one interneuron (located in lamina VIII) was glycinergic. Contralateral terminal projections were found both in motor nuclei and within laminae VI–VIII. In order to identify target cells of commissural interneurons, effects of stimulation of contralateral group I and II muscle afferents were investigated on interneurons within these laminae. Three tests were used: intracellular records from individual interneurons, modulation of probability of activation of extracellularly recorded interneurons and modulation of their actions on motoneurons using disynaptic PSPs evoked in motoneurons as a measure. All these tests revealed much more frequent and/or stronger excitatory actions of contralateral afferents. The results indicate that commissural interneurons with input from contralateral group I and II afferents target premotor interneurons in disynaptic pathways from ipsilateral group I and II afferents and that excitatory disynaptic actions of contralateral afferents on these interneurons are mediated primarily by intermediate zone commissural interneurons. A second group of commissural interneurons activated by reticulospinal neurons, previously described, frequently had similar, but occasionally opposing, actions to the cells described here, thus indicating that these two subpopulations may act on the same premotor interneurons and either mutually enhance or counteract each others actions.

Parallel reflex pathways from flexor muscle afferents evoking resetting and flexion enhancement during fictive locomotion and scratch in the cat

Reflex actions of muscle afferents in hindlimb flexor nerves were examined on ipsilateral motoneurone activity recorded in peripheral nerves during midbrain stimulation‐evoked fictive locomotion and during fictive scratch in decerebrate cats. Trains of stimuli (15–30 shocks at 200 Hz) were delivered during the flexion phase at intensities sufficient to activate both group I and II afferents (5 times threshold, T). In many preparations tibialis anterior (TA) nerve stimulation terminated ongoing flexion and reset the locomotor cycle to extension (19/31 experiments) while extensor digitorum longus (EDL) stimulation increased and prolonged the ongoing flexor phase activity (20/33 preparations). The effects of sartorius, iliopsoas and peroneus longus muscle afferent stimulation were qualitatively similar to those of EDL nerve. Resetting to extension was seen only with higher intensity stimulation (5T) while ongoing flexor activity was often enhanced at group I intensity (2T) stimulation. The effects of flexor nerve stimulation were qualitatively similar during fictive scratch. Reflex reversals were consistently observed in some fictive locomotor preparations. In those cases, EDL stimulation produced a resetting to extension and TA stimulation prolonged the ongoing flexion phase. Occasionally reflex reversals occurred spontaneously during only one of several stimulus presentations. The variable and opposite actions of flexor afferents on the locomotor step cycle indicate the existence of parallel spinal reflex pathways. A hypothetical organization of reflex pathways from flexor muscle afferents to the spinal pattern generator networks with competing actions of group I and group II afferents on the flexor and extensor portions of this central circuitry is proposed.

Neuronal relays in double crossed pathways between feline motor cortex and ipsilateral hindlimb motoneurones

Coupling between pyramidal tract (PT) neurones and ipsilateral hindlimb motoneurones was investigated by recording from commissural interneurones interposed between them. Near maximal stimulation of either the left or right PT induced short latency EPSPs in more than 80% of 20 commissural interneurones that were monosynaptically excited by reticulospinal tract fibres in the medial longitudinal fascicle (MLF). The EPSPs were evoked at latencies that were only 1–2 ms longer than those of EPSPs evoked from the MLF, compatible with a disynaptic coupling between PT fibres and these commissural interneurones. EPSPs evoked by PT stimulation were frequently associated with IPSPs which either followed or preceded the EPSPs. The latencies of the IPSPs (on average about 1 ms longer than latencies of the earliest EPSPs) indicated that they were mediated via single additional inhibitory interneurones. Records from a sample of nine commissural interneurones from a different population (with monosynaptic input from group I and/or II muscle afferents, and disynaptically excited from the MLF) suggest that actions of PT fibres on such interneurones are weaker because only four of them were excited by PT stimuli and at longer latencies. By demonstrating disynaptic coupling between PT neurones and commissural interneurones via reticulospinal fibres, the results provide a direct demonstration of trisynaptic coupling in the most direct pathways between PT neurones and ipsilateral motoneurones, and thereby strengthen the proposal that the double crossed pathways between PT neurones and ipsilateral motoneurones might be used to replace crossed actions of damaged PT neurones.

Modulation of spontaneous locomotor and respiratory drives to hindlimb motoneurons temporally related to sympathetic drives as revealed by Mayer waves.

In this study we investigated how the networks mediating respiratory and locomotor drives to lumbar motoneurons interact and how this interaction is modulated in relation to periodic variations in blood pressure (Mayer waves). Seven decerebrate cats, under neuromuscular blockade, were used to study central respiratory drive potentials (CRDPs, usually enhanced by added CO2) and spontaneously occurring locomotor drive potentials (LDPs) in hindlimb motoneurons, together with hindlimb and phrenic nerve discharges. In four of the cats both drives and their voltage-dependent amplification were absent or modest, but in the other three, one or other of these drives was common and the voltage-dependent amplification was frequently strong. Moreover, in these three cats the blood pressure showed marked periodic variation (Mayer waves), with a slow rate (periods 9–104 s, mean 39 ± 17 SD). Profound modulation, synchronized with the Mayer waves was seen in the occurrence and/or in the amplification of the CRDPs or LDPs. In one animal, where CRDPs were present in most cells and the amplification was strong, the CRDP consistently triggered sustained plateaux at one phase of the Mayer wave cycle. In the other two animals, LDPs were common, and the occurrence of the locomotor drive was gated by the Mayer wave cycle, sometimes in alternation with the respiratory drive. Other interactions between the two drives involved respiration providing leading events, including co-activation of flexors and extensors during post-inspiration or a locomotor drive gated or sometimes entrained by respiration. We conclude that the respiratory drive in hindlimb motoneurons is transmitted via elements of the locomotor central pattern generator. The rapid modulation related to Mayer waves suggests the existence of a more direct and specific descending modulatory control than has previously been demonstrated.

Uncrossed actions of feline corticospinal tract neurones on lumbar interneurones evoked via ipsilaterally descending pathways

Effects of stimulation of ipsilateral pyramidal tract (PT) fibres were analysed in interneurones in midlumbar segments of the cat spinal cord in search of interneurones mediating disynaptic actions of uncrossed PT fibres on hindlimb motoneurones. The sample included 44 intermediate zone and ventral horn interneurones, most with monosynaptic input from group I and/or group II muscle afferents and likely to be premotor interneurones. Monosynaptic EPSPs evoked by stimulation of the ipsilateral PT were found in 12 of the 44 (27%) interneurones, while disynaptic or trisynaptic EPSPs were evoked in more than 75%. Both appeared at latencies that were either longer or within the same range as those of disynaptic EPSPs and IPSPs evoked by PT stimuli in motoneurones, making it unlikely that premotor interneurones in pathways from group I and/or II afferents relay the earliest actions of uncrossed PT fibres on motoneurones. These interneurones might nevertheless contribute to PT actions at longer latencies. Uncrossed PT actions on interneurones were to a great extent relayed via reticulospinal neurones with axons in the ipsilateral medial longitudinal fascicle (MLF), as indicated by occlusion and mutual facilitation of actions evoked by PT and MLF stimulation. However, PT actions were also relayed by other supraspinal or spinal neurones, as some remained after MLF lesions. Mutual facilitation and occlusion of actions evoked from the ipsilateral and contralateral PTs lead to the conclusion that the same midlumbar interneurones in pathways from group I or II muscle afferents may relay uncrossed and crossed PT actions.

Differential modulation by monoamine membrane receptor agonists of reticulospinal input to lamina VIII feline spinal commissural interneurons

Noradrenaline and serotonin have previously been demonstrated to facilitate the transmission between descending reticulospinal tracts fibres and commissural interneurons coordinating left–right hindlimb muscle activity. The aim of the present study was to investigate the contribution of subclasses of monoaminergic membrane receptors to this facilitation. The neurons were located in Rexed lamina VIII in midlumbar segments and identified by their projections to the contralateral gastrocnemius–soleus motor nuclei and by lack of projections rostral to the lumbosacral enlargement. The effects of ionophoretically applied membrane receptor agonists [phenylephrine (noradrenergic α1), clonidine (noradrenergic α2), 8‐OH‐DPAT (5‐HT1A, 5‐HT7), 2‐me‐5‐HT (5‐HT3), 5‐me‐5‐HT (5‐HT2) and α‐me‐5‐HT (5‐HT2)] were examined on extracellularly recorded spikes evoked monosynaptically by electric stimulation of descending reticulospinal fibres in the medial longitudinal fascicle. Application of α1 and 5‐HT2 agonists resulted in a facilitation of responses in all investigated neurons while application of α2, 5‐HT1A/7 and 5‐HT3 agonists resulted in a depression. These opposite modulatory effects of different agonists suggest that the facilitatory actions of noradrenaline and serotonin on responses of commissural interneurons reported previously following ionophoretic application are the net outcome of the activation of different subclasses of monoaminergic membrane receptors. As these receptors may be distributed predominantly, or even selectively, at either pre‐ or postsynaptic sites their differential modulatory actions could be compatible with a presynaptically induced depression and a postsynaptically evoked enhancement of synaptic transmission between reticulospinal neurons and commissural interneurons.

Premotor interneurones contributing to actions of feline pyramidal tract neurones on ipsilateral hindlimb motoneurones

The aim of the study was to analyse the potential contribution of excitatory and inhibitory premotor interneurones in reflex pathways from muscle afferents to actions of pyramidal tract (PT) neurones on ipsilateral hindlimb motoneurones. Disynaptic EPSPs and IPSPs evoked in motoneurones in deeply anaesthetized cats by group Ia, Ib and II muscle afferents were found to be facilitated by stimulation of the ipsilateral, as well as of contralateral, PT. The ipsilateral actions were evoked by either uncrossed or double‐crossed pathways. The results show that interneurones mediating reflex actions of muscle afferents may be activated strongly enough by PT stimulation to contribute to movements initiated by ipsilateral PT neurones and that PT actions relayed by them might be enhanced by muscle stretches and/or contractions. However, in some motoneurones disynaptic IPSPs and EPSPs evoked from group Ib or II afferents were depressed by PT stimulation. In order to analyse the basis of this depression, the transmitter content in terminals of 11 intracellularly labelled interneurones excited by PT stimulation was defined immunohistochemically and their axonal projections were reconstructed. The interneurones included 9 glycinergic and 2 glutamatergic neurones. All but one of these neurones were mono‐ or disynaptically excited by group I and/or II afferents. Several projected to motor nuclei and formed contacts with motoneurones. However, all had terminal projections to areas outside the motor nuclei. Therefore both inhibitory and excitatory interneurones could modulate responses of other premotor interneurones in parallel with direct actions on motoneurones.

Ipsilateral actions from the feline red nucleus on hindlimb motoneurones

The main aim of the study was to investigate whether neurones in the ipsilateral red nucleus (NR) affect hindlimb motoneurones. Intracellular records from motoneurones revealed that both EPSPs and IPSPs were evoked in them via ipsilaterally located premotor interneurones by stimulation of the ipsilateral NR in deeply anaesthetized cats in which only ipsilaterally descending tract fibres were left intact. When only contralaterally descending tract fibres were left intact, EPSPs mediated by excitatory commissural interneurones were evoked by NR stimuli alone while IPSPs mediated by inhibitory commissural interneurones required joint stimulation of the ipsilateral NR and of the medial longitudinal fascicle (MLF, i.e. reticulospinal tract fibres). Control experiments led to the conclusion that if any inadvertently coactivated axons of neurones from the contralateral NR contributed to these PSPs, their effect was minor. Another aim of the study was to investigate whether ipsilateral actions of NR neurones, pyramidal tract (PT) neurones and reticulospinal tract neurones descending in the MLF on hindlimb motoneurones are evoked via common spinal relay neurones. Mutual facilitation of these synaptic actions as well as of synaptic actions from the contralateral NR and contralateral PT neurones showed that they are to a great extent mediated via the same spinal neurones. A more effective activation of these neurones by not only ipsilateral corticospinal and reticulospinal but also rubrospinal tract neurones may thus contribute to the recovery of motor functions after injuries of the contralateral corticospinal tract neurones. No evidence was found for mediation of early PT actions via NR neurones.