The extent of using mobility assistive devices can partly explain fatigue among persons with late effects of polio - a retrospective registry study in Sweden.

BACKGROUND: Fatigue is reported as one of the most disabling symptoms and is common among persons living with late effects of polio. Although fatigue has been studied in the context of people living with late effects of polio, there is a lack of knowledge concerning the association of fatigue and variables of importance for participation in daily life. Therefore, the aim of this study was to explore possible factors associated with fatigue among persons with late effects of polio in Sweden. METHODS: This retrospective registry study consisted of 89 persons with late effects of polio living in Sweden. Fatigue was measured with the Multidimensional Fatigue Inventory (MFI-20) scale, Swedish version. Pearson's correlation coefficient was used to analyse the correlation between the factors and fatigue, and a multiple linear regression was carried out to explore factors for fatigue. RESULTS: Fatigue statistically significantly correlated with age (r = 0.234, p < 0.05) and the use of mobility assistive devices (r = 0.255, p < 0.05). The multiple linear regression model showed that the factors age (ß = 0.304, p < 0.019) and mobility assistive devices (ß = 0.262, p < 0.017) were associated with fatigue among persons living with late effects of polio, and the model partly explained 14% of the variation of fatigue. CONCLUSIONS: Fatigue could partly be explained by the extent of using mobility assistive devices and age. Healthcare professionals should provide and demonstrate the importance of assistive devices to ensure management of fatigue in persons living with late effects of polio.

Evoked thalamic neuronal activity following DRG application of two nucleus pulposus derived cell populations: an experimental study in rats.

PURPOSE: To investigate the effects on evoked thalamic neuronal activity of application of notochordal cells and chondrocyte-like cells derived from nucleus pulposus (NP) onto a dorsal root ganglion (DRG) and to compare these effects with a previously reported increased thalamic activity induced by NP. METHODS: Nucleus pulposus was harvested from tail discs of adult rats and the disc cells were separated into two cell populations, notochordal cells and chondrocyte-like cells. The two cell populations were applied separately, or in combination, to the L4 DRG of anaesthetised female Sprague-Dawley rats during acute electrophysiological experiments. In control experiments, cell suspension medium was applied on the DRG. Recordings from the contralateral thalamus were sampled for 40 min while electrically stimulating the ipsilateral sciatic nerve at above Aδ-fibre thresholds. RESULTS: Application of notochordal cells resulted in a decrease in evoked thalamic activity within 10 min while chondrocyte-like cells did not induce any changes during the 40 min of recording. The difference in evoked thalamic activity 40 min after notochordal and chondrocyte-like cell application, respectively, was statistically significant. Neither an increased concentration of chondrocyte-like cells alone nor a combination of the two cell populations induced any changes in thalamic activity. CONCLUSIONS: Separate exposure of the DRG to the two NP-derived cell populations induced different effects on evoked thalamic activity, but none of the tested cell samples induced an increase in neuronal activity similar to that previously observed with NP. This indicates a high complexity of the interaction between NP and nervous tissue.

Inhibitory inputs to four types of spinocerebellar tract neurons in the cat spinal cord.

Spinocerebellar tract neurons are inhibited by various sources of input via pathways activated by descending tracts as well as peripheral afferents. Inhibition may be used to modulate transmission of excitatory information forwarded to the cerebellum. However it may also provide information on the degree of inhibition of motoneurons and on the operation of inhibitory premotor neurons. Our aim was to extend previous comparisons of morphological substrates of excitation of spinocerebellar neurons to inhibitory input. Contacts formed by inhibitory axon terminals were characterised as either GABAergic, glycinergic or both GABAergic/glycinergic by using antibodies against vesicular GABA transporter, glutamic acid decarboxylase and gephyrin. Quantitative analysis revealed the presence of much higher proportions of inhibitory contacts when compared with excitatory contacts on spinal border (SB) neurons. However similar proportions of inhibitory and excitatory contacts were associated with ventral spinocerebellar tract (VSCT) and dorsal spinocerebellar tract neurons located in Clarke's column (ccDSCT) and the dorsal horn (dhDSCT). In all of the cells, the majority of inhibitory terminals were glycinergic. The density of contacts was higher on somata and proximal versus distal dendrites of SB and VSCT neurons but more evenly distributed in ccDSCT and dhDSCT neurons. Variations in the density and distribution of inhibitory contacts found in this study may reflect differences in information on inhibitory processes forwarded by subtypes of spinocerebellar tract neurons to the cerebellum.

Do spinocerebellar neurones forward information on spinal actions of neurones in the feline red nucleus?

We recently demonstrated that feline ventral spinocerebellar tract (VSCT) neurones monitor descending commands for voluntary movements initiated by pyramidal tract (PT) neurones as well as locomotor movements relayed by reticulospinal (RS) neurones. The aim of the present study was to examine whether VSCT neurones likewise monitor descending commands from the red nucleus (RN). Extracellular records from the spinal border (SB) subpopulation of VSCT neurons revealed that a third (31%) of SB neurones may be discharged by trains of stimuli applied in the RN. Moreover, when RN stimuli failed to discharge SB neurones they facilitated activation of some of these neurones by RS and/or PT neurones, while activation of other SB neurones was depressed. We propose that the facilitation and depression of actions of RS neurones by RN neurones might serve to reflect a higher or lower excitability of motoneurones and therefore a likely higher or lower efficacy of the RS descending commands, prompting the cerebellum to adjust the activation of reticulospinal neurones. Activation of SB neurones by RN stimuli alone would also allow monitoring and adjusting the RN descending commands. Intracellular records from SB neurones revealed both monosynaptic and disynaptic EPSPs and disynaptic IPSPs evoked by RN stimuli. The disynaptic actions remained following transection of axons of reticulospinal neurones within the medullary longitudinal fascicle (MLF) and were therefore taken to be relayed primarily by spinal neurones, in contrast to EPSPs and IPSPs evoked by PT stimuli found to be relayed by reticulospinal rather than spinal neurones.

Processing information related to centrally initiated locomotor and voluntary movements by feline spinocerebellar neurones.

Feed-back information on centrally initiated movements is processed at both supraspinal and spinal levels and is forwarded by a variety of neurones. The aim of the present study was to examine how descending commands relayed by reticulospinal neurones are monitored by a population of spinocerebellar tract neurones. Our main question was whether a spinal border (SB) subpopulation of ventral spinocerebellar tract (VSCT) neurones monitor actions of reticulospinal neurones with input from the mesencephalic locomotor region (MLR) as well as from pyramidal tract (PT) neurones. In the majority of intracellularly recorded SB neurons, stimuli applied in the MLR and in the medullary pyramids evoked EPSPs in parallel with EPSPs evoked by stimulation of axons of reticulospinal neurones in the medial longitudinal fascicle (MLF). In extracellularly recorded neurones short trains of stimuli applied in the ipsilateral and contralateral pyramids potently facilitated discharges evoked from the MLF, as well as EPSPs recorded intracellularly. In both cases the facilitation involved the disynaptic but not the monosynaptic actions. These results indicate that reticulospinal neurones activating SB neurones (or more generally VSCT neurones) are co-excited by axon-collaterals of other reticulospinal neurones and by fibres stimulated within the MLR and PTs. The study leads to the conclusion that these spinocerebellar neurones monitor descending commands for centrally initiated voluntary as well as locomotor movements relayed by reticulospinal neurones. Thereby they may provide the cerebellum with feed-back information on the likely outcome of these commands and any corrections needed to avoid errors in the issuing movements.

A trans-spinal loop between neurones in the reticular formation and in the cerebellum.

Voluntary limb movements are initiated in the brain but the neurones responsible for activating the muscles (motoneurones and interneurones) are located in the spinal cord. The spinal cord also contains neurones that provide the brain, and especially the cerebellum, with continuous information on effects of the descending commands. We show that one population of such neurones provide the cerebellum with information on how likely the brain's commands (mediated by descending reticulospinal neurones) are to be executed as planned, depending on the degree of inhibition of motoneurones. They may therefore play an important role in preventing errors in activation of motoneurones and thereby help the brain to correct its signals to the spinal cord before such errors have been committed.

Collateral actions of premotor interneurons on ventral spinocerebellar tract neurons in the cat.

Strong evidence that premotor interneurons provide ventral spinocerebellar tract (VSCT) neurons with feedback information on their actions on motoneurons was previously found for Ia inhibitory interneurons and Renshaw cells, while indications for similar actions of other premotor interneurons were weaker and indirect. Therefore the aim of the present study was to reexamine this possibility with respect to interneurons relaying actions of group Ib afferents from tendon organs and group II afferents from muscle spindles. In all, 133 VSCT neurons in the L3-L5 segments (including 41 spinal border neurons) were recorded from intracellularly in deeply anesthetized cats to verify that stimuli applied in motor nuclei evoked monosynaptic inhibitory postsynaptic potentials (IPSPs) attributable to stimulation of axon collaterals of premotor interneurons. IPSPs were found in over two thirds of the investigated neurons. When intraspinal stimuli were preceded by stimuli applied to a muscle nerve at critical intervals, IPSPs evoked from motor nuclei were considerably reduced, indicating a collision of nerve volleys in axons of interneurons activated by group I and group II afferents. In individual VSCT neurons monosynaptic IPSPs were evoked from both biceps-semitendinosus and gastrocnemius-soleus motor nuclei, in parallel with disynaptic IPSPs from group Ib and group II as well as group Ia afferents. These observations indicate that individual VSCT neurons may monitor the degree of inhibition of both flexor and extensor motoneurons by premotor interneurons in inhibitory pathways from group Ib and group II afferents to motoneurons. They may thus be providing the cerebellum with feedback information on actions of these premotor interneurons on motoneurons.

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 other's actions.

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.

Functional differentiation and organization of feline midlumbar commissural interneurones.

Interneurones interconnecting the two sides of the spinal cord (commissural interneurones) are critically important for interlimb coordination, but little is known about their organization. We have examined the inputs to commissural interneurones located in the midlumbar segments with projections to contralateral motor nuclei, aiming to determine whether they form distinct subpopulations. Based on intracellular records from 78 interneurones, two major non-overlapping subpopulations were identified: one monosynaptically excited by group II muscle afferents (n=10), the other monosynaptically excited by reticulospinal neurones (n=52). Monosynaptic input from group I muscle afferents and/or from vestibulospinal tract neurones was found in those with monosynaptic reticulospinal, but not group II input, and in a few other neurones (n=6). Only disynaptic input from these sources was found in the remaining 10 interneurones. Disynaptic excitatory input from ipsilateral and contralateral muscle afferents and from descending tracts was distributed less selectively and might mediate coexcitation of interneurones with monosynaptic afferent or descending input. The dominant disynaptic and polysynaptic input was, however, inhibitory. IPSPs were evoked from the descending tracts in a high proportion of the commissural interneurones that were monosynaptically excited by group II afferents (55%) and from group II afferents in a high proportion of the commissural interneurones that were monosynaptically excited by reticulospinal fibres (78%). This distribution suggests that the two subpopulations are activated differentially, rather than being coactivated, in either centrally initiated movements or reflex adjustments. This would be consistent with the previous demonstration that noradrenaline differentially affects commissural neurones of the two subpopulations.

Ipsilateral actions of feline corticospinal tract neurons on limb motoneurons.

Contralateral pyramidal tract (PT) neurons arising in the primary motor cortex are the major route through which volitional limb movements are controlled. However, the contralateral hemiparesis that follows PT neuron injury on one side may be counteracted by ipsilateral of actions of PT neurons from the undamaged side. To investigate the spinal relays through which PT neurons may influence ipsilateral motoneurons, we analyzed the synaptic actions evoked by stimulation of the ipsilateral pyramid on hindlimb motoneurons after transecting the descending fibers of the contralateral PT at a low thoracic level. The results show that ipsilateral PT neurons can affect limb motoneurons trisynaptically by activating contralaterally descending reticulospinal neurons, which in turn activate spinal commissural interneurons that project back across to motoneurons ipsilateral to the stimulated pyramidal tract. Stimulation of the pyramids alone did not evoke synaptic actions in motoneurons but potently facilitated disynaptic EPSPs and IPSPs evoked by stimulation of reticulospinal tract fibers in the medial longitudinal fascicle. In parallel with this double-crossed pathway, corticospinal neurons could also evoke ipsilateral actions via ipsilateral descending reticulospinal tract fibers, acting through ipsilaterally located spinal interneurons. Because the actions mediated by commissural interneurons were found to be stronger than those of ipsilateral premotor interneurons, the study leads to the conclusion that ipsilateral actions of corticospinal neurons via commissural interneurons may provide a better opportunity for recovery of function in hemiparesis produced by corticospinal tract injury.

Both dorsal horn and lamina VIII interneurones contribute to crossed reflexes from feline group II muscle afferents.

Previous studies have demonstrated that group II muscle afferents exert powerful actions on contralateral motoneurones and that these actions are mediated primarily via lamina VIII commissural interneurones. We examined whether dorsal horn interneurones also contribute to these actions, as they have been shown to contribute to the actions of group II afferents on ipsilateral motoneurones. We tested the susceptibility of IPSPs and EPSPs evoked from group II afferents in contralateral motoneurones to presynaptic inhibition as an indicator of the relative contribution of dorsal horn interneurones to these PSPs, since the monosynaptic activation of dorsal horn interneurones is more weakly and more briefly depressed by presynaptic inhibition than is the monosynaptic activation of lamina VIII and other intermediate zone and ventral horn interneurones. While the earliest components of IPSPs and EPSPs evoked by group II afferents were abolished by conditioning stimulation of group II afferents, consistent with them being evoked disynaptically by commissural interneurones, trisynaptic components of these PSPs were only partly reduced and are therefore attributed to dorsal horn interneurones. The same conditioning stimuli depressed the disynaptic excitation of lamina VIII commissural interneurones by group II afferents much less effectively than they depressed monosynaptic excitation, indicating that dorsal horn interneurones contribute to this disynaptic excitation. On the basis of these observations we conclude that that dorsal horn interneurones contribute to the late actions of group II muscle afferents on contralateral motoneurones through their disynaptic actions on commissural interneurones.

On organization of a neuronal network in pathways from group II muscle afferents in feline lumbar spinal segments.

The aim of the study was to verify the hypothesis that trisynaptic actions of group II muscle afferents upon motoneurones are, at least in part, mediated by dorsal horn interneurones exciting the same intermediate zone interneurones that are interposed in disynaptic pathways from group II afferents. Population EPSPs (field potentials) and responses of individual interneurones evoked by group II afferents in the dorsal horn and in the intermediate zone were analysed in order to assess the possibility of a causal relationship between them. When direct actions of group II afferents in the intermediate zone were abolished by presynaptic inhibition, distinct later components of field potentials and delayed interneuronal responses were induced at latencies 0.5-1 ms longer than those seen originally. Both the latency and a marked temporal facilitation define these later group II actions as being evoked disynaptically. Under the same conditions, single stimuli activated more than one half of dorsal horn interneurones, and the second and third stimuli activated all of these interneurones. Responses of dorsal horn interneurones preceded disynaptically evoked responses of intermediate zone interneurones. The study indicates that intermediate zone interneurones may be activated by group II afferents both directly and via dorsal horn interneurones and that synaptic actions of group II afferents upon these interneurones, and their subsequent actions upon motoneurones, may be modulated in parallel at the level of intermediate zone and dorsal horn interneurones.

Differential presynaptic inhibition of actions of group II afferents in di- and polysynaptic pathways to feline motoneurones.

The aim of this study was to investigate differences in the effects of presynaptic inhibition of transmission from group II muscle afferents to neurones in the dorsal horn and in the intermediate zone and the consequences of these differences for reflex actions of group II afferents upon alpha-motoneurones. The degree of presynaptic inhibition was estimated from the degree of depression of monosynaptic components of population EPSPs (field potentials) evoked by group II muscle afferents in deeply anaesthetized cats. The decrease in the area of field potentials was considerably larger and longer lasting in the intermediate zone, where they were often obliterated, than in the dorsal horn, where they were reduced to about two-thirds. Presynaptic inhibition of field potentials evoked by other afferents at the same locations was much weaker. Intracellular records from alpha-motoneurones revealed that short latency EPSPs and IPSPs evoked from group II afferents are considerably reduced by conditioning stimuli that effectively depress intermediate zone field potentials of group II origin. The results of this study lead to the conclusion that strong presynaptic inhibition of transmission to intermediate zone interneurones allows a selective depression of disynaptic actions of group II muscle afferents on alpha- and gamma-motoneurones, mediated by these interneurones, and favours polysynaptic actions of these afferents.

A comparison of postactivation depression of synaptic actions evoked by different afferents and at different locations in the feline spinal cord.

Postactivation depression of synaptic actions of group I and II muscle afferents and low threshold cutaneous afferents was compared with depression of actions of group Ia afferents on alpha-motoneurones in cats deeply anaesthetised with pentobarbital and alpha-chloralose. The depression was analysed on field potentials (population EPSPs). The degree of depression was evaluated by analysing changes in the monosynaptic components of the field potentials, in areas within 0.4- to 0.6-ms-long time windows from their onset. When intervals between successive stimuli used to evoke field potentials were reduced from 10 s to 0.4 s, the potentials evoked by Ia afferents in motor nuclei were depressed as described previously. Field potentials evoked by group II afferents and cutaneous afferents in the dorsal horn were similarly depressed. In contrast, monosynaptic components of field potentials evoked in the intermediate zone, by group I or II afferents, were only marginally affected. Postactivation depression of synaptic actions of group I afferents in the intermediate zone was not enhanced when test stimuli were applied 30-40 ms after a train of four conditioning stimuli. These observations indicate that the degree of postactivation depression may differ depending on the type of afferent. In addition, if postactivation depression depends on intrinsic properties of afferent terminals, differences in the degree of depression of postsynaptic potentials evoked by the same group of afferents at different locations may indicate that properties of terminals contacting different neurones may differ.

Spinal interneurones; how can studies in animals contribute to the understanding of spinal interneuronal systems in man?

The first part of this review deals with arguments that the essential properties and organization of spinal interneuronal systems in the cat and in man are similar. The second part is concerned with the possibility that some interneuronal systems may be responsible for motor disturbances caused by spinal cord injuries and that these interneurones may be defined. This possibility is discussed with respect to the hyperexcitability of alpha-motoneurones and the exaggeration of stretch reflexes in spastic patients. To this end, what is known about cat spinal interneurones and about the neuronal basis and pharmacological treatment of spasticity, is put together. Interneurones in di- and trisynaptic reflex pathways from group II muscle afferents are singled out, since they are depressed by the alpha(2) noradrenaline receptor agonists clonidine and tizanidine, which is a critical feature of interneurones expected to contribute to exaggerated stretch reflexes which are reduced by alpha(2) noradrenaline receptor agonists. Recent observations that reflex excitation of extensor motoneurones from group II afferents is enhanced in spastic patients and that the pathologically strong reflex actions of group II afferents are reduced by clonidine and tizanidine support this proposal. On the other hand, a lack of effect of clonidine and tizanidine upon other types of excitatory or inhibitory interneurones argues against any major contribution of such interneurones to the abnormally strong responses of alpha-motoneurones to muscle stretch.

Areas of operation of interneurons mediating presynaptic inhibition in sacral spinal segments.

Sources of primary afferent depolarization (PAD) of skin afferents in the sural (Sur) nerve and of group-II muscle afferents in the posterior biceps and semitendinosus (PBST) nerve were compared at several sites, about 2 mm apart, within the L7-S2 segments in order to define areas of projection of sacral interneurons mediating PAD of these afferents. Just rostral to the pudendal nucleus, strong PAD of Sur afferents was evoked by stimulation of skin nerves, while stimulation of muscle nerves had only marginal effects. This indicates that sacral PAD interneurons co-excited by skin and muscle afferents operate primarily within the regions overlying the pudendal nucleus. Furthermore, PAD evoked by muscle afferents was weaker over the rostral part of the pudendal nucleus than over the caudal part of this nucleus, where hamstring afferents became its main source, both in Sur and in PBST group-II afferents. By correlating the relative strength of PAD at the levels of the rostral and caudal parts of the pudendal nucleus with the previously established input from muscle and cutaneous afferents to interneurons at these levels, it is therefore proposed that sacral PAD interneurons operate over shorter distances than indicated by previous experiments: over either rostral or caudal parts of the pudendal nucleus, i.e., about 2 mm, rather than over the whole length of this nucleus, i.e., 4-5 mm. Sacral PAD interneurons may, thus, modulate synaptic transmission to even more spatially restricted neuronal populations than previously proposed.

Effects of monoamines on interneurons in four spinal reflex pathways from group I and/or group II muscle afferents.

Effects of locally applied serotonin (5-HT) and noradrenaline (NA) were tested on extracellularly recorded responses of single spinal interneurons in deeply anaesthetized cats. These effects were tested on: (i) interneurons mediating reciprocal inhibition from group Ia afferents; (ii) interneurons mediating non-reciprocal inhibition from group Ia and Ib afferents; (iii) intermediate zone interneurons co-excited by group I and II afferents; and (iv) dorsal horn interneurons excited by group II afferents. Effects of monoamines were tested on responses evoked at latencies compatible with monosynaptic coupling. Responses evoked by group Ia and/or Ib muscle afferents were facilitated in all of the tested interneurons both by NA and 5-HT. Responses evoked by group II muscle afferents were depressed in the majority of the interneurons but were facilitated in some of them. 5-HT depressed these responses in all dorsal horn interneurons and in one subpopulation of intermediate zone interneurons, while it facilitated them in another subpopulation of intermediate zone interneurons. NA depressed them in all intermediate zone interneurons and in one subpopulation of dorsal horn interneurons, while it facilitated them in another subpopulation of dorsal horn interneurons. The results of this study lead to the conclusions that: (i) modulation of synaptic actions of muscle spindle and tendon organ afferents on spinal interneurons by 5-HT and NA is related to both the type of the afferent and the functional type of the interneuron; and that (ii) 5-HT and NA counteract each others' actions on some interneuronal types but mutually enhance them on the others.

Modulation of responses of four types of feline ascending tract neurons by serotonin and noradrenaline.

Modulation of responses of four types of ascending tract cells by noradrenaline and serotonin was compared in order to investigate how information forwarded by these cells may be gated by monoaminergic tract neurons. Spinocervical tract, postsynaptic dorsal column and dorsal spinocerebellar tract neurons located in Clarke's column and in the dorsal horn were identified by their axonal projections. Noradrenaline and serotonin were applied ionophoretically close to a selected neuron, and their effects were tested on extracellularly recorded responses of this neuron to electrical stimulation of low-threshold skin afferents and group II muscle spindle afferents. The modulatory actions of noradrenaline and serotonin were estimated from changes in the number of responses evoked by 30 successive stimuli, the minimal latencies of these responses, and their firing frequency. All four populations of ascending tract neurons investigated were modulated by serotonin and noradrenaline, but not in the same way. The responses were most often depressed by noradrenaline and facilitated by serotonin, but in some types of neuron they were affected in the same direction. Transmission from low-threshold skin and group II muscle afferents changed in the same direction in some types of neuron but in the opposite direction in other types. The results indicate that transfer of information from skin and group II muscle afferents to supraspinal centres may be gated by descending monoaminergic pathways in a highly differentiated manner, and is adjusted to the requirements of various behavioural situations.

Effects of serotonin on dorsal horn dorsal spinocerebellar tract neurons.

Effects of ionophoretic application of serotonin and of one of its agonists were tested on responses of dorsal horn dorsal spinocerebellar tract neurons evoked by electrical stimulation of peripheral nerves. Both drugs depressed monosynaptically evoked actions of group II muscle afferents; they decreased the number and/or increased the latency of spike potentials evoked by these afferents. In contrast, synaptic actions of low-threshold cutaneous afferents (mono- or oligosynaptic) were facilitated in the majority of the neurons, as judged by decrease in the latency of spike potentials evoked by stimulation of a cutaneous nerve and/or an increase in the number of these potentials. It is proposed that facilitatory actions assist in maintaining tonic discharges of dorsal spinocerebellar tract neurons in some movements and that the selective control of group II input is used to correlate activity of spinal and supraspinal neurons. Both actions may be subserved by tight contacts between serotoninergic nerve fibres and dorsal spinocerebellar tract neurons, which have been revealed in a parallel study.