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.

Division of motor units into fast and slow of the basis of profile of 20 Hz unfused tetanus.

In the medial gastrocnemius muscle of intact rats, division of motor units (MUs) into slow (S) or fast (F) types is typically based on presence of a sag phenomenon in 40 Hz unfused tetanic contraction. MUs with sag are classified as F, while those without sag as S. However, in rats one month after spinal cord injury this phenomenon almost completely disappears and cannot be used as a basis for MUs differentiation, whereas the twitch contraction time increased significantly. Analysis of myosin heavy chain (MHC) isoform composition confirmed transformational changes of muscle fibres after spinal cord transection and indicated unchanged proportion of type I MHC isoforms, disappearance of type IIa MHC isoforms, and increase of type IIb MHC isoforms. We proposed an additional method for division of MUs into types when standard criteria are not applicable. It was observed that relative effectiveness of force summation during 20 Hz tetanus, described as a ratio of the force of the last contraction of this tetanus to the force of the first contraction, did not change after spinal cord injury. This ratio for S MUs both in intact and spinal rats exceeded 2.0, whereas for F units was lower than 2.0. Calculations of this ratio made for better fused tetani, evoked by 30 Hz or 40 Hz stimulation, showed overlapping values. We conclude that this 20 Hz tetanus index appears to be an alternative method useful for division of motor units into S and F types.

Effect of precocious locomotor activity on the development of motoneurones and motor units of slow and fast muscles in rat.

We have investigated the effect of precociously increasing locomotor activity during early postnatal development by daily treatment with the monoaminergic precursor L-DOPA on the survival of motoneurones supplying the slow soleus (SOL) muscle and the fast, tibialis anterior (TA) and extensor digitorum longus (EDL) muscles as well as the contractile and histochemical properties of these muscles. L-DOPA treatment resulted in a significant loss of motoneurones to the slow SOL muscle, but not to the fast TA and EDL muscles. Moreover, motoneurones to fast muscles also die as when exposed to increased activity in early life, if their axons are repeatedly injured. The loss of normal soleus motoneurones was accompanied by an increase in force of the remaining motor units and sprouting of the surviving axons suggesting a remodelling of motor unit organisation. The time to peak contraction of both SOL and EDL muscles from L-DOPA treated rats was prolonged at 8 weeks of age. At 4 weeks the soleus muscles of the L-DOPA treated animal developed more tension than the saline treated one. This difference between the two groups did not persist and by 8 weeks of age the muscle weight and tetanic tension from either group were not significantly different from control animals. The present study shows that early transient, precocious locomotor activity induced by L-DOPA is damaging to normal soleus but not to normal EDL/TA motoneurones.

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.

Recovery of hindlimb motor functions after spinal cord transection is enhanced by grafts of the embryonic raphe nuclei.

In this study, a piece of embryonic tissue from the raphe nucleus was transplanted into the spinal cord below the lesion 1 month after transection. Two months later the recovery of hindlimb motor function in rats which had received a transplant of neural tissue (ST rats) was much better than in spinal control animals without the graft (SC rats). Analysis of the electromyographic (EMG) activity showed that the timing of muscle activity during locomotor-like movement of hindlimbs in ST rats was more regular than in SC rats. In SC rats the relationships between EMG burst duration (soleus, tibialis anterior) and step cycle duration were significantly altered. The restoration of hindlimb motor function of ST rats was also reflected in the better interlimb coordination during locomotor-like hindlimb movements. The results of several behavioural tests demonstrated that the responses to stimulation of various receptors, such as tactile or proprioceptive, in ST rats were more complex than in SC rats. Additionally, unlike in SC animals, in ST rats long-lasting spontaneous episodes of air stepping movement of hindlimbs accompanied by a relatively high amplitude of EMG activity were obtained. These results confirm that grafted embryonic raphe nuclei which contain serotoninergic cells are likely to increase the excitability of neuronal circuitry in the injured spinal cord. Moreover, transplantation of embryonic raphe nuclei encourages the recovery of hindlimb motor function in adult rats even when the grafting is carried out several weeks after spinal cord injury.

The age dependent effect of partial denervation of rat fast muscles on their activity.

In 3 or 18 day old Wistar rats the hindlimb muscles were partially denervated by cutting the L4 spinal nerve. Three months later, the effects of partial denervation of the fast extensor digitorum longus (EDL) muscle on the activity of its remaining motor units were studied using electromyographic (EMG) recordings in freely moving animals. In spite of a reduced number of motor units the amount of aggregate EMG activity was greater in the partially denervated EDL muscle in all experimental conditions. This increase was more obvious at rest than during exploratory behaviour, and was significantly greater in muscles that were partially denervated at 3 days than at 18 days of age. On the other hand, the effect of partial denervation on the EMG activity pattern during locomotion was similar in animals partially denervated at 3 or 18 days of age. Unlike in intact EDL, in the partially denervated EDL muscle the duration of the bursts was influenced by the step cycle duration. Thus, we conclude that although partial denervation of EDL muscle influences the amount and pattern of activity of the remaining undamaged motor units in all animals, some of the alterations of EMG activity were more pronounced in animals denervated at younger age.

The frequency of rat's hippocampal theta rhythm is related to the speed of locomotion.

Hippocampal EEG activity was recorded in rats during locomotion, spontaneous or induced by electrical stimulation of subthalamic locomotor region (SLR) and posterior hypothalamus (PH). Spontaneous locomotion was associated with the theta frequency (6-8.5 Hz). Electrically induced locomotion was also associated with theta rhythm, frequency of which depended on the site of stimulation. Theta frequency during SLR evoked locomotion was almost the same as during spontaneous locomotion (7-9 Hz). PH induced locomotion was accompanied by higher theta frequency (9-11 Hz). Theta frequency and the speed of locomotion were positively correlated during spontaneous locomotion. During electrically induced locomotion the theta frequency was much higher than expected when related to the speed of spontaneous locomotion. The electrically evoked locomotion may result from activation of various systems. Stimulation of the SLR activates system responsible for the exploratory behavior, while stimulation of PH evokes locomotion related to aversive behavior. Thus, we conclude that the theta rhythm frequency may depend not only on the speed of locomotion but also on the rats' motivational-emotional state, associated with locomotion performed by the animal.

Time course of changes in EMG activity of fast muscles after partial denervation.

After partial denervation, the remaining motor units (MUs) of adult fast extensor digitorum longus muscle (EDL) expand their peripheral field. The time course of this event was studied using tension measurement and recordings of electromyographic (EMG) activity. The results show that after section of the L4 spinal nerve, when only 5.3 +/- 0.63 of the 40 MUs normally supplying EDL muscle remain, the force of individual motor units starts to increase between the 1st and 2nd week after the operation and continues to do so for a further week. The drastic reduction of the number of motoneurones supplying the fast EDL leads to an increase in activity of the remaining MUs. In the 1st week after partial denervation, there was a sharp increase in the EMG activity of remaining motor units. During the next 12 days, this increase became less marked, but EMG activity remained nevertheless significantly higher than that of the unoperated EDL muscle. Many MUs became tonically active during posture. The EMG activity pattern during locomotion was also altered, so that the burst duration was positively correlated with the step cycle duration. Moreover, shortly after partial denervation, the interlimb coordination was disturbed but returned to its original symmetrical use 1-2 weeks later.

Estimation of the distribution of the EMG signal amplitude.

Quantitative analysis of electromyographic (EMG) data still demands new methods. One of the methods is based on counting occurrences of signal crossings of "noise level" (per time unit). This provides an estimator of the so called "aggregate activity." These data can be obtained by a spike trigger connected to a computer. Comparison of total activity of firing motor units in intact and affected muscles may give important information about the neuromuscular system in norm and pathology. The aggregate activity method does not seem, however, to be completely objective, because its results depend on the value of noise level selected by the experimenter. Thus, we propose a modification that allows: (1) quantitative estimation of how the results obtained with the aggregate activity method depend on the value of selected level, and (2) assessment of the distribution of the EMG signal amplitude--i.e., appearance of particular height spikes in the signal. The advantages and disadvantages of the method are discussed in the context of two experimental models.

Functional reorganization of the partially denervated hindlimb extensor and flexor muscle in rat.

In view of the neuromuscular system plasticity the functional changes induced by partial denervation are presented. The long-term effects of partial denervation of postural (soleus-SOL) or flexor (extensor digitorum longus-EDL) muscles on their EMG activity were studied in rats. The activity per motor unit was significantly higher both in the partially denervated SOL and EDL muscles. During standing or walking the EMG activity pattern of the partially denervated SOL muscle was similar to normal, while the partially denervated EDL muscle during standing exhibited abnormal tonic activity, and during locomotion its burst duration was strongly correlated to the step cycle duration. Thus, partial denervation led to an overall increase of activity of the remaining motor units in both SOL and EDL muscles, while the temporal pattern of muscle activity during locomotion was drastically altered in EDL muscle only, what indicates that partial denervation influenced the postural activity less than the phasic one.

Motor activity patterns in rat soleus muscle after neonatal partial denervation.

In normal rats the development of organized patterns of hind limb movements takes place during the first three weeks of life. After removal of a part of the rat soleus muscle's innervation in 5-day-old animals, the remaining motoneurones occupy a large peripheral field. The possibility that the development of the normal activity patterns of these motor units may be altered was studied. The EMG activity of the soleus muscles partially denervated at five days was compared to that of the contralateral unoperated muscles during spontaneous locomotion and induced reflex activity in animals at various ages. Like a normal soleus the partially denervated soleus developed with age a tonic activity pattern but the aggregate activity recorded from the partially denervated soleus was less than that in the control muscle. However, the amount of activity per motor unit was higher in the operated than in the control muscles, since these had only one-third to half of their normal complement of motor units. During locomotion both soleus muscles were activated like typical ankle extensors during the stance phase of the step cycle, but the burst duration of the operated muscle was significantly shorter. We conclude that partial denervation shortly after birth leads to an overall increase in activity of the remaining soleus motor units but does not drastically alter their temporal pattern of use during locomotion.

Theta-like rhythm in depth EEG activity of hypothalamic areas during spontaneous or electrically induced locomotion in the rat.

Spontaneous locomotion performed by an awake rat is accompanied by synchronized activity (a theta rhythm) of the hippocampus (Hipp). Locomotion can also be induced by electrical stimulation of various sites within the brain. The effectiveness of electrical stimulation of the subthalamic locomotor region (SLR) and posterior hypothalamus (PH) in inducing locomotor movements has previously shown that they are parts of the system controlling locomotion. Thus, it was of interest to determine whether their electrical activity was correlated with the motor behavior, as it is in the Hipp. The experiments done here on chronic animals with electrodes implanted into the Hipp and hypothalamus showed that rhythmic depth EEG activity (theta-like) similar to that found in the Hipp is present in motor-positive hypothalamic areas during spontaneous locomotion. In addition, our results showed that movements induced by stimulation of various hypothalamic sites are also accompanied by theta-like activity in the Hipp and some hypothalamic areas. Thus, our results support the idea that the well-known and anatomically established connections between the Hipp and hypothalamus are active during locomotion. These connections may constitute a part of the system involved in the control of the motor behavior.

Two locomotor strips in the diencephalon of thalamic cats.

Electrical stimulation of the subthalamic locomotor region (SLR) can induce locomotor movements in thalamic cats. The possibility of inducing locomotor movements from other structures in the diencephalon, using electrical stimulation, was investigated in acute thalamic cats. It was found that the area from which locomotor movements could be elicited, extended caudally to the SLR and formed two strips: (1) the dorsal strip, located at the border of the ventral thalamus and dorsal hypothalamus, and (2) the ventral strip, extending ventrally beneath the red nucleus.