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In experiments on immobilized decerebrate cats, data about reorganization of efferent activity parameters of the forelimb and hindlimb locomotor generators evoked by electrical stimulation of descending systems were obtained. The generators controlling both forelimb and hindlimb locomotor movements were found to be characterized by the existence of stable states at which total influence of different descending systems on these generators was extremely limited. These data enable us to conclude that the sense of activity reorganization in locomotor generators of both forelimb and hindlimb under the influence of descending system signals is in bringing the motor program to a dynamic relation with supraspinal inflow, where a sufficient degree of limitation and balancing of the influences of corresponding descending systems on the interneuronal nets, determining time and phase characteristics of these generators, is ensured. Possible mechanisms of realization of this interaction between descending signals and locomotor activity generators are discussed.

The influences of signals in descending systems on the parameters of scratching generator activity were studied on decerebrate immobilized cats. It was shown that phasic electric stimulation of descending systems evoked certain phase-dependent reorganization of the parameters of scratching generator efferent activity. Maximum increase in scratching cycle duration during electric stimulation of Deiters' nucleus, red nucleus and pyramidal tract is observed during stimulation in the first half of aiming phase. Stimulation in the second half of aiming phase and at the beginning of scratching jerk phase virtually does not change the scratching cycle duration. Maximum increase in scratching cycle duration during electric stimulation of the nucleus reticularis gigantocellularis is observed in the second half of aiming phase. Electric activation of descending pathways during aiming phase increases its intensity and decreases the intensity of scratching jerk phase. Activation of descending pathways during scratching jerk phase increases its intensity and virtually does not change the aiming phase intensity. Influences of electric activation of descending systems on scratching generator work reveal dependence on limb position. They are increased when the limb is deflected to the rear and are decreased during over-aimed position. Decerebellation leads to a decrease of scratching generator activity parameters rearrangement under influence of electric stimulation of the red nucleus and nucleus reticularis gigantocellularis, and to its increase during Deiters' nucleus stimulation. On the basis of these results the principles of supraspinal correction of scratching generator work are discussed.

Systematic quantitative analysis of changes in the spinal scratching generator motor activity evoked by tonic and phasic peripheral afferent signals during "fictitious" scratching was carried out in the cat. Correlations between the kinematics of hindlimb scratching movement, sensory inflow, and primary afferent depolarization were investigated. Reliable correlations between the parameters of generator motor activity during fictitious scratching were revealed: they depended on tonic peripheral afferent inflow. The functional role of these dependencies consists of providing stability for aiming the hindlimb to the itch site. It was shown that scratching generator reaction to a phasic sensory signal depended significantly on afferent input, signal intensity, and its arrival phase in the cycle of motor activity. Phase correction of "scratching" rhythm was performed by inhibition of the current stage of "scratching" cycle, the inhibition magnitude depending on the intensity of a sensory signal run along high threshold afferent fibers. The moments in the scratching cycle, in which the afferent signal caused no rearrangement in scratching generator activity, were discovered for all investigated afferent inputs. These moments corresponded to the transitions from one scratching cycle phase to another. Integral afferent activity was distributed unevenly in the cycle during real scratching. The main part of it was observed just in that scratching cycle part which included the above mentioned no rearrangement phase points. The data obtained allowed us to conclude that the scratching generator should be considered as a working program for the motor optimal control system containing the intrinsic model of the controlled object dynamics (e.g. hindlimb scratching movement dynamics), which produces an inner analog of peripheral flow. This inner flow interacts with peripheral afferent inflow just as one of the latter components. Centrally originated modulation of primary afferent depolarization is a result of affecting the depolarization generating system by this inner "sensory" activity. It is the model, with the aid of which the generator can work after deafferentation. The functional organization of a central pattern generator is considered.

Rearrangement of the scratching generator activity evoked by phasic electrical stimulation of different descending systems has been investigated on decerebrated immobilized cats. This rearrangement heavily depends on the stimulation phase. Maximal increase of the scratching cycle duration by electrical stimulation of Deiters' nucleus, red nucleus and pyramidal tract takes place in the first half of an aiming phase. The electrical stimulation of structures mentioned above does not practically change duration of the scratching cycle in the second half of the aiming phase and at the beginning of the scratching phase. A maximal increase of the scratching cycle duration by electrical stimulation of reticular gigantocellular nucleus occurs in the second half of the aiming phase. The electrical activation of descending pathways during the aiming phase increases intensity of this phase and decreases intensity of the scratching phase. The electrical activation of descending pathways during the scratching phase increases its intensity and does not change practically the intensity of the aiming phase. Possible principles of the suprasegmental correction of the scratching generator activity are discussed.

The relationship between parameters of electrical muscle activity, changes at hindlimb joint angles, intensity of integral afferent flow, and dorsal root potential during real-life locomotion was investigated in cats decerebrated at high level. Characteristics of rear limb movements before and after deafferentation were described. It was found that afferent activity during locomotion motion consists, of two components — a tonic and a periodic phasic stage. Three main waves may be distinguished in the latter, each of which gives rise to associated changes in the level of primary afferent terminal polarization. These changes in turn are summated with the effects produced by the central generator. Correlations, between the parameters of these processes were investigated and the mechanisms underlying afferent control of locomotion generator function discussed.

Pyramidal unit activity in unanesthetized cats at rest and during voluntary movement was recorded by a microelectrode technique from the motor cortex for the forelimb. Some pyramidal neurons were not spontaneously active. The conduction velocity along the axon of these neurons was sometimes high (up to 71.5 m/sec), sometimes low (up to 11.2 m/sec). The remaining pyramidal neurons had spontaneous activity with a mean frequency of 1.29 to 43 spikes/sec. Analysis of interspike interval histograms of spontaneous activity and of autocorrelation histograms showed grouping of the spikes into volleys in most pyramidal neurons (irrespective of the conduction velocity). During voluntary movements the change in the activity of many pyramidal units correlated with changes in the EMG. The firing rate of the pyramidal neurons under these circumstances began to rise at least 50 msec before the increase in amplitude of the EMG and it remained high throughout the movement. The firing rate of most neurons during movement was 40–60/sec. The results are compared with those obtained by other workers who studied pyramidal unit activity of monkeys during voluntary movement.

Using unanesthetized and decorticated (or decerebrated at level A 13) cats, it was found that spinalization leads to depolarization of the central terminals of primary afferents and an increase in the N1 component of dorsal surface potential and dorsal root potential (DRP) produced by stimulating the low-threshold cutaneous and muscle afferents. Other effects include an increase in early polysynaptic responses and DRP produced by stimulation of high-threshold muscle afferents, a reduction in the intensity of interneuron activation in the nucleus interpositus mono- and polysynaptically connected with primary afferents, and a rise in the activity of n. interpositus interneurons di- and oligo-synaptically connected with afferent terminals. Changes in the opposite direction were produced by injecting DOPA into spinal animals. The connection between changes in the state of the segmental neuronal apparatus of the lumbosacral spinal cord and the level of spinal locomotor generator activity is discussed in the light of the findings obtained.

Reorganization of the parameters of efferent activity produced in the spinal generator by electrical stimulation of the ipsilateral hindlimb muscle nerves during different limb positions were investigated in decerebrate immobilized cats. A direct relationship was found between this reordering and the stage at which stimuli were applied. The rearranged duration of the scratch cycle showed a tendency to bring motor activity into phase with stimuli so that the stimulus falls due at the onset of the motor activity phase. This phasically collated rearrangement was observed where a shift had occurred in the relationship between "aiming" and "scratch" motion tending towards intensified activity in the muscles innervated by the stimulated nerve. Rearrangement became more evident when the hindlimb deflected from the target position in accordance with the direction of muscle stretching. The physiological significance of the interposition of the "no rearrangement" phase is discussed. It is deduced that this absence of change in duration and intensity can only be produced simultaneously when a certain relationship is achieved between the phase of afferent signal reception in the scratch cycle and signal intensity.

Unit activity of the lumbar interneurons was recorded in thalamic cats during fictitious locomotion. Neurons whose activity was modulated in the rhythm of fictitious locomotion were found in the lateral parts of the intermediate zone of gray matter and ventral horn. Of these neurons, 41.2% were activated mainly in the phase of "flexion," 48.5% in the phase of extension, and 10.3% in both phases. Neurons with tonically increasing or decreasing activity during rhythmic discharges and neurons whose activity was unchanged during fictitious locomotion also were observed. During later discharges all these neurons were similarly activated, although a depth of modulation of unit activity was lower than during fictitious locomotion. Afferent inputs to the recorded interneurons also were studied. The neuronal organization of the spinal locomotor generator is discussed on the basis of these results.

It was found during experiments on immobilized decerebrate (at intracollicular level) cats that tonic sub-threshold activation of the spinal generator of scratching action (following application of tubocurarine or bicuculline to segments C1-C2) was accompanied by depolarization of primary afferent terminals, a reduction in the N1 component of dorsal surface potential produced by stimulating the cutaneous afferents, and a reduction in the amplitude of dorsal root potentials and lead-phase polysynaptic response produced in motoneurons by stimulating the cutaneous and muscle afferents. A rise or a reduction in the activity of interneurons belonging to the interstitial nucleus connected respectively mono- and di-(oligo)synaptically with the afferents occurred in parallel with this. Spinalization produced the same changes in reverse in the animal. By administering DOPA to the spinal animal, a comparison could be made of changes occurring in the state of the segmental apparatus of the lumbar section of the spinal cord during tonic sub-threshold activation of spinal scratch generators and locomotor movements.

Activity of lumbar spinal neurons was recorded extracellularly during late long-lasting discharges in efferent nerves in immobilized thalamic cats. Of the total number of cells tested, 70% changed their activity during late discharges. The activity of 35% of neurons was increased during late discharges in nerves to flexors, but inhibited during discharges in nerves to extensors. Responses of 27% of neurons were of the opposite character. Other neurons were found whose activity was increased (5%) and reduced (3%), respectively, during later discharges in both flexor and extensor nerves. Most interneurons which changed their activity during late discharges were located in lateral parts of the intermediate zone of gray matter and the ventral horn at a depth of 2.8 mm. The character of the afferent input to a neuron was found to depend on the late efferent discharges and activity of the neurons correlated with them. Neurons whose activity was unchanged during late discharges (30%) were mainly located rather more dorsally, at a depth of about 2.0 mm. The possible mechanisms of the participation of these groups of interneurons in the generation of late discharges are discussed.

Studies on immobilized decerebrate (at intracollicular level) cats in which the scratch generator had been set up following bicuculline application to the upper cervical segments of the spinal cord, showed that the state of the segmental apparatus of the lumbosacral section of the spinal cord differs substantially from that seen in the spinal animal. Direct excitability of motoneurons of the "aiming" and "scratching" muscles rises, while recurrent and reciprocal Ia inhibition of motoneurons intensifies and the influence of Ib afferents on motoneurons declines. Afferents of the flexor reflex exert a primarily inhibitory influence on motoneurons of the "aiming" muscles. This influence becomes predominantly excitatory following spinalization, while the inhibitory effects of these afferents on motoneurons of the "scratch" muscles declines. The functional significance of the changes discovered in generation of scratch routine is discussed.

A statistical analysis of unit activity in spinal locomotor centers was undertaken on immobilized thalamic cats at rest and during generation of efferent discharges. Activation of the spinal locomotor generator was accompanied by shortening of interspike intervals in the spike sequences of neurons and a decrease in their fluctuations. Histograms of interspike intervals became more symmetrical under these circumstances and there was a considerable increase in the number of neurons whose activity showed regular fluctuations on autocorrelation histograms. Spike trains at rest were characterized by dependence of successive intervals, which increased during efferent discharge generation. The possible mechanisms of modification of the time structure of unit activity in spinal locomotor centers during their activation are discussed.

Effects of stimulation of flexor reflex afferents were studied in decerebrate immobilized cats. Stimulation of ipsilateral afferents evoked late long-lasting discharges in the nerves to the flexors, whereas stimulation of contralateral afferents led to similar discharges in nerves to both extensors and flexors. Compared with spinal animals, early segmental reflexes in thalamic cats were tonically depressed. Similar tonic inhibition of segmental reflexes took place in spinal animals after injection of dopa. Segmental reflexes were clearly modulated during late or rhythmic discharges. The possible central mechanisms of these changes in the segmental reflexes are discussed on the basis of data in the literature.

Depolarization of primary afferent terminals in the lumbosacral portion of the spinal cord evoked by selective activation of propriospinal pathways was investigated in anesthetized cats. The strongest depolarization developed as a result of activation of short (two to five segments) propriospinal pathways in the lateral funiculus; stimulation of the long propriospinal pathways of this funiculus also induced depolarization, but of lower amplitude. Stimulation of propriospinal pathways of the ventral funiculi was ineffective. Significant primary afferent depolarization developed only following the use of a series of stimuli and strong stimulation of the propriospinal pathways. Excitation of these pathways caused depolarization of afferent terminals of both cutaneous and muscular nerves, including muscular sensory fibers of group Ia, although in the latter case its intensity was low. Neuronal mechanisms involved in the generation of this depolarization and its possible functional role are dicussed.

Segmental reflex responses of the lumbosacral region of the spinal cord to stimulation of peripheral afferents were studied in immobilized decerebrate cats before and after application of D-tubocurarine or bicuculline to the superior cervical segments, potentiating the scratch reflex, and also during fictitious scratching evoked by mechanical stimulation of the ear. Application of these substances led to inhibition of the N1-component of the dorsal cord potential, the dorsal root potential, and polysynaptic responses in efferent nerves. The appearance of fictitious scratching was accompanied by additional tonic inhibition of these responses, against the background of which modulation of the amplitudes of the responses was observed depending on the phase of fictitious scratching. Modulation of amplitudes of monosynaptic reflexes also developed during fictitious scratching. Against the background of these results the mechanisms and physiological role of reorganization of segmental responses during activation of the spinal scratching generator are discussed.

Research was performed on spinal cats injected with DOPA and decorticate (decerebrated at level A 13) and spinal cats. It was found that formation (activation) of the spinal locomotor generator is accompanied by heightened excitability in the extensor and the reverse trend in the flexor motoneurons, by an increase in the efficacy of recurrent and reciprocal Ia inhibition of α-motoneurons, and by a weakening of the influence of Ib afferents and extensor reflex afferents on these same motoneurons. The likely functional role of these changes in tuning of the spinal segmental apparatus in the generation of locomotor rhythm is discussed.

Activity of lumbosacral spinal interneurons was studied during fictitious scratching in decerebrate, immobilized cats. Neurons whose activity changed during fictitious scratching were located in the substantia intermedia lateralis and ventral horn. Among these neurons cells were distinguished whose activity was modulated in rhythm with motor discharges to different muscles (61.6%) and cells which were activated tonically (21.4%) or inhibited tonically (17%). By correlation of activity with discharges to corresponding muscles the rhythmically activated neurons were divided into "aiming" (36.6%) and "scratching" (25%). Neurons whose activity was unchanged during fictitious scratching also were observed. These cells were located mainly in the more dorsal regions of gray matter. Neurons to which wide convergence of excitatory influences from high-threshold cutaneous and muscular afferents was observed were mainly placed in the "aiming" group. "Scratching" neurons, compared with "aiming," more often received inputs only from low-threshold cutaneous or high-threshold muscular afferents. Group Ia interneurons were activated in phase with the corresponding motoneurons. Passive displacement of the limb in a forward direction predominantly inhibited spike activity of the "aiming" and potentiated activity of the "scratching" neurons. The neuronal organization of the spinal scratch generator is discussed on the basis of the results.

Segmental responses of the lumbosacral region of the spinal cord to peripheral afferent stimulation were studied in decorticated, immobilized cats before and during fictitious locomotion. The appearance of fictitious locomotion was accompanied by a tonic increase in the N1-component of the dorsal cord potential and dorsal root potential. Against the background of this tonic increase, modulation of these responses depending on the phase of fictitious locomotion was observed. When the N1-component and dorsal root potential were evoked at the end of the "extension" phase and at the beginning of the "flexion" phase their amplitude was greater, but when they were evoked at the end of the "flexion" phase and the beginning of the "extension" phase it was smaller. Polysynaptic and monosynaptic reflex response of motoneurons exhibited the same phase dependence during fictitious locomotion. The mechanisms and physiological importance of this retuning of segmental responses are discussed.

The kinetics of primary afferent depolarization (PAD) in the lumbar spinal cord was studied in immobilized decerebrate and decapitated cats during fictitious scratching. Fictitious scratching was accompanied by tonic and periodic PAD. Periodic PAD was cophased in different segments of the lumbosacral enlargement. Tonic depolarization was observed in terminals of all groups of afferent fibers studied (low-threshold cutaneous and of groups Ia and Ib). Periodic changes were observed only in endings of low-threshold cutaneous fibers and group Ib fibers. The physiological role of modulation of PAD by the spinal scratch generator is discussed.

The characteristics of conduction of the excitation wave along propriospinal fibers of the dorsolateral tract of the spinal cord were studied in cats anesthetized with pentobarbital. At a preliminary operation, 10–18 days beforehand, lateral hemisection of the spinal cord was performed, cranially in the lumbar division and caudally and cranially in the cervical division to the segments to be studied, leading to degeneration of the long descending and ascending fibers. During stimulation, the dorsolateral tract developed a composite response consisting of a positive-negative wave recorded up to 60–65 mm (4 or 5 segments) from the point of stimulation. The mean conduction velocity of this wave in the lumbar division was 37.9 m/sec compared with 44.5 m/sec in the cervical division. From its properties as a whole this wave can be regarded as the result of excitation of relatively fast-conducting propriospinal fibers of the dorsolateral tract. If the strength of stimulation was increased, late components began to appear in the response. These were evidently connected with excitation of thinner propriospinal fibers and synaptic activation of other other groups of spinal neurons.

Synaptic responses of 121 identified cervical motoneurons to stimulation of the pyramidal tract and red nucleus were investigated by intracellular recording in cats. Responses of EPSP or EPSP-IPSP type were predominant in motoneurons of distal groups of muscles and proximal flexors, while responses of IPSP type were predominant in motoneurons of the proximal extensors. The minimal effective number of stimuli for most motoneurons was 2 or 3. The mean latent period, counted from the first stimulus in the series, was 7.86 msec for EPSPs for stimulation of the pyramidal tract and 7.91 msec for stimulation of the red nucleus, while the corresponding periods for IPSPs were 8.68 and 8.75 msec. The segmental delay of 1.3–2 msec for EPSPs and IPSPs generated in certain motoneurons in response to stimulation of both structures indicates that the shortest pathway for transmission of activity from the fibers of these tracts to the motoneurons may be disynaptic. At the same time, the possible presence of an additional neuron for most inhibitory pathways cannot be ruled out. Analysis of the results also suggests the presence of a common interneuronal apparatus for both systems.

The distribution of focal potentials over the cross section of the 7th cervical segment of the spinal cord was studied during stimulation of the pyramids, the red nucleus, and a peripheral nerve (ulnar) in adult cats anesthetized with chloralose and Nembutal. The earliest focal potentials in the fasciculus dorsolateralis were recorded 1.4–1.5 msec after stimulation of the pyramids and 0.8–0.9 msec after stimulation of the red nucleus. These times correspond to maximal condution velocities of 56–68 and 105–124 m/sec respectively. The earliest post-synaptic activity in response to pyramidal stimulation was found in the lateral areas of laminae V and VI, and in response to stimulation of the red nucleus in laminae VI and VII in Rexed's classification. The pyramidal wave also evoked considerable postsynaptic activity in medial areas of the dorsal horn. In response to stimulation of peripheral afferents activity was evoked in neurons in the central and medial parts of laminae V and VI. It is postulated on the basis of these results that corticospinal and rubrospinal fibers may be connected monosynaptically with specialized interneurons, free from peripheral influences, in the lateral areas of laminae V and VII respectively; in the lateral part of lamina VI convergence of both types of influences on the same cells is possible. Interaction between descending and afferent influences possibly takes place on more medially located neurons.

The dorsal cord and dorsal root potentials were recorded in immobilized thalamic cats during fictitious scratching evoked by mechanical stimulation of the ear. Depolarization of primary afferents was shown to be simulated by the central scratching generator. Antidromic spike discharges appeared at the peak of the primary afferent depolarization waves in certain afferent fibers. Similar discharges arise in the resting state in response to stimulation of limb mechanoreceptors. It is suggested that during real scratching primary afferent depolarization and antidromic spikes evoked by it may effectively modulate the level of the afferent flow to spinal neurons.

The cord dorsum and dorsal root potentials were recorded during fictious scratching in L6 segment of the thalamic cats. It is shown that primary afferent depolrization (PAD) can be modulated by the central generator of scratching. In some afferent fibres antidromic spikes appear on the top of PAD-waves. Antidromic spikes can be also evoked in the period of "rest" by mechanical stimulation of hindlimb receptors. It is supposed that PAD and resulting antidromic spikes which appear during real scratching may effectively control the level of afferent inflow to the spinal neurons.

Impulse activity of lumbosacral interneurons was recorded extracellularly during fictitious scratching in decerebrated immobilized cats. Most neurons whose activity was modulated during fictitious scratching were revealed in the lateral intermediate zone and in the lateral ventral horn. These neurons were divided into some groups differing in the type of activity modulation during fictitious scratching. Neurons whose activity did not change during fictitious scratching were mainly located dorsally in the gray matter. Cells showing wide convergence of excitatory influences from high threshold cutaneous and muscle afferents referred mainly to "aiming" neurons. In contrast to "aiming" neurons, most cells with inputs from only low threshold cutaneous or high threshold muscle afferents belonged to "scratching" neurons. The neuronal activity in different groups is shown to depend on the limb position. The neuronal organization of the spinal scratching generator is discussed on the basis of the obtained data.

Primary afferent depolarization (PAD) in the lumbosacral enlargement of the spinal cord evoked by selective activation of propriospinal (PS) pathways was studied in anesthetized cats. Activation of PS pathways in the lateral funiculus was more effective for PAD generation than that in the ventral funiculi. Excitation of PS pathways evoked PAD in terminals of cutaneous and muscle fibres including Ia muscle fibres. Neuronal mechanisms of intersegmental distribution of PAD and its possible functions are discussed.

Segmental reflex responses of the lumbosacral spinal cord to peripheral afferent stimulation were investigated in immobilized decorticated cats before and during fictitious locomotion. Appearance of fictitious locomotion was accompanied by an increase of the N1 component of the dorsal cord potential and of the dorsal root potential. Modulation of these responses depending on the phase of fictitious locomotion was observed against a background of their tonic increase. The N1 component and dorsal root potential had larger amplitudes when they appear at the end of the "extensor" phase and at the beginning of the "flexor" phase and smaller amplitudes when they were evoked at the end of the "flexor" phase and at the beginning of the "extensor" phase. Poly- and monosynaptic reflex responses of motoneurons revealed the same phase dependence during fictitious locomotion. Mechanisms and physiological significance of the revealed retuning of segmental reflex responses are discussed on the basis of the obtained data.

The sequence of discharges in nerves to 22 hindlimb muscles during fictitious locomotion was investigated on immobilized thalamic cats. It is shown that for most hindlimb muscles the central locomotor program is rather simple and consists of alternating activation of flexors and extensors. A conclusion is made on the existence of interneuronal flexor and extensor centres in each half of the spinal cord, working in turns during locomotion. More complex discharges were recorded from the nerves to Edb and Per. tert. and also from the nerves to biarticular muscles (St, P. bic., Ten.). It is suggested that motoneurons of these muscles are connected to the flexor and extensor centres in a more complex way.

Changes in depolarization of primary afferents and their correlation with afferent impulsation and limb movement were studied in the lumbar spinal cord during real scratching of decerebrated cats. Two components in rhythmic dorsal root potential were observed. First--centrally evoked, retained during fictitive scratching after immobilization; second--evoked by afferent discharge, coming to the spinal cord during the scratching phase of the limb movement.

Impulse activity of lumbar interneurons was recorded extracellularly during fictious locomotion of thalamic cats. Neurons whose activity was modulated in the rhythm of fictitious locomotion were revealed in the lateral intermediate zone and in the lateral ventral born. 41.2% of these neurons were mainly activated at the phase of "flexion", 48.5%--at the phase of "extension" and 10.3%--at both phases. Neurons with tonically increasing or decreasing activity during rhythmic efferent discharges and neurons whose activity did not change during fictitious locomotion were also observed. All these types of neurons were similarly activated during late discharges in efferent nerves. But in this case the depth of modulation of neuronal activity was lower than during fictitious locomotion. Afferent inputs to the recorded interneurons were also investigated. The neuronal organization of the spinal locomotor generator is discussed on the basis of the obtained data.

Segmental reflex responses of the lumbosacral spinal cord to peripheral afferent stimulation were investigated in immobilized decerebrated cats before and after d-tubocurarine or bicuculline application to the upper cervical segments and during fictitious scratching evoked by mechanical stimulation of the pinna. Application of these drugs led to a decrease of N1 component of the cord dorsum potential, dorsal root potential and polysynaptic responses in efferent nerves. The appearance of fictitious scratching was accompanied by additional tonic inhibition of these reactions and against its background the modulation of amplitudes of segmental reactions depending on the fictitious scratching phase was observed. Modulation of monosynaptic reflexes was also observed during fictitious scratching. Mechanisms and physiological significance of retuning of segmental reflex responses during activation of a spinal scratching generator are discussed on the basis of the data obtained.

The influence of tonic afferent inflow conditioned by ipsilateral hindlimb position on the spinal generator of scratching efferent activity parameters was investigated in immobilized decerebrated cats. When the afferent inflow was absent (after the hindlimb deafferentation) strong correlation between the motor activity parameters of successive "scratch" cycles was observed. In case of the intact afferentation this correlation was somewhat weaker and decreased with shifting the hindlimb from the aimed position to overaimed or deflecting backward. The discovered statistically significant correlations between the parameters of different motor activity phases and their dependence on the hindlimb position are organized in such a way that they can provide stability of the aimed position of the hindlimb carrying out oscillatory movements. According to the parameters of the efferent activity and their correlation type, the hindlimb deafferentation is most similar to the aimed position of the hindlimb.

Spontaneous rhythmic discharges (fictitious locomotion) were recorded in motor hindlimb nerves of decerebrated animals with intact caudal hypothalamic region. Stimulation of this region led to the appearance of fictitious locomotion. Spontaneous discharges did no-appear after decerebration at lower levels and in spinalized cats. A conclusion is made on the presence of structures in the caudal hypothalamus which are necessary for the activation of lower locomotor centres and the production of spontaneous rhythmic discharges in motor nerves.

Impulse activity of lumbar interneurons was recorded extracellularly during the late long-lasting discharges in efferent nerves of immobilized thalamic animals. 70% of investigated neurons changed their activity during the late discharges. The activity of 35% of neurons increased, during late discharges in flexor nerves, while during activity in extensor nerves it was inhibited. 27% of neurons had the opposite reaction. Neurons, whose activity correspondingly increased (5%) or decreased (3%) during the late discharges both in flexor and extensor nerves were also observed. Most interneurons, changing their activity during late discharges, were located in the lateral parts of intermediate zone and in the lateral ventral horn at the depth of 2.8 mm. Certain dependence between the character of afferent inputs to the neuron and correlation of its activity with late efferent discharges was revealed. Neurons whose activity did not change during late discharges (40%) were mainly located somewhat dorsally, at the depth of about 2.0 mm. Possible role of described interneurons in generation of late discharges is discussed.

Correlations between parameters of electrical muscle activity, changes of joint angles of the hindlimb, intensity of afferent inflow and dorsal root potential during real locomotion were studied in highly decerebrate cats. Characteristics of the hindlimb movements before and after deafferentation were described. It has been shown that during locomotion the afferent activity consisted of two components: tonic and periodic phase ones. In the latter one three principal waves can be distinguished, each inducing appropriate changes in primary afferent terminals polarization level. These changes in their turn, were summated with those elicited by central generator. Correlation dependences between parameters of the above mentioned processes were investigated. Mechanisms of afferent control of the locomotor generator function are discussed.

Rebuildings of the scratching generator activity caused by phasic electrical stimulation of ipsilateral hindlimb muscle nerves during different hindlimb positions were studied in decerebrated immobilized cats. Strong dependence of these rebuildings on the stimulation phase was observed. The character of the "scratch" cycle duration rebuilding was formed by the scratching generator tendency to bring efferent activity into such correlation with the stimulus that the stimulation moment coincided with the moment of efferent activity phase triggering. Phasic altering of the efferent activity intensity rebuilding was observed against a background of "aiming" and "scratching" activity correlation shift in the direction of strengthening activation of muscles innervated by the stimulated nerve. This rebuilding was intensified when the hindlimb deflects from the aimed position in the direction of corresponding muscles stretching. Physiological sense of "rebuilding absence phases" is discussed. It is postulated that absence of the duration and intensity changes can be achieved simultaneously only with definite correlation between phase and intensity of the afferent impulsation burst.

The statistical analysis of the neuronal impulse activity of spinal locomotor centres was performed at rest and during generation of efferent discharges in thalamic immobilized cats. The activation of the spinal locomotor generator was accompanied by shortening of interspike intervals in neuronal spike trains and reduction of their fluctuations. During this process the interspike interval histograms became more symmetrical and the quantity of neurons which had regular oscillations in autocorrelation histograms increased considerably. At rest neuronal spike trains were characterized by dependence of successive intervals. This dependence increased during generation of efferent discharges. Possible mechanisms of the reconstruction of time organization of the neuronal impulse activity of spinal locomotor centres during their activation are discussed.

The quantity of impulses which is necessary to be registered for detecting a reliable correlation between spike trains of synaptically connected neurons was estimated using a mathematical model. This quantity of impulses was studied as dependent on the weight of connection between neurons (the amplitude of EPSP evoked by a single impulse of an input neuron in an output one), on the intensity of total background excitatory influences upon the output neuron and on its own parameters. The necessary quantity of impulses was 10(7)-10(8) for cells with connection weights, parameters and characteristics of background activity similar to those of real spinal neurons. An increase in the amplitude of unitary EPSP and a decrease in the intensity of the input background impulse train and in duration and amplitude of interspike hyperpolarization of the postsynaptic neuron led to a decrease in the quantity of impulses necessary for detecting a reliable correlation. The possible principles of the construction of the spinal locomotor generator are discussed on the basis of those data.

The experiments performed on decerebrated cats have shown that afferent activity during scratching consisted of two components--tonic and periodic phasic ones. The first component was determined by the limb position, the second was closely related to the amplitude and velocity of joint angle changes. Maximum of integral afferent activity in the cycle coincided with the scratching jerk phase. These two components of afferent activity evoked corresponding depolarization changes in primary afferent terminals and these changes added to those elicited by central generator. Statistical correlations between the aforementioned parameters were studied. The afferent control mechanisms of scratching generator are under discussion.

In immobilized intercollicularly decerebrated cats tonic underthreshold activation of the spinal scratching generator (after application of tubocurarine or bicuculline on C1-C2 segments) is accompanied by an increase in primary different terminal depolarization, decrease in N1-component of cord dorsum potential evoked by stimulation of cutaneous afferents, decrease in the amplitude of DRP and early polysynaptic responses of motoneurons evoked by stimulation of cutaneous and muscle afferents; a respective rise and reduction in activity of intermediate nucleus interneurons which are mono- and di (oligo)-synaptically connected with afferent terminals. Spinalization of animal led to reverse changes. Injection of DOPA into spinal animals allowed comparing changes in the state of lumbar segmental apparatus during tonic underthreshold activation of spinal scratching and locomotor generators.

Central program of interaction of hindlimbs during fictitious locomotion expressed in the time structure of motor discharges in nerves to different muscles was investigated in decerebrated and spinal immobilized cats. It was shown that in the decerebrated animals the program of interaction of hindlimbs (alternative or synphase) is determined by interaction of the flexor half-centres. The latter work either in-phase or out-of-phase. The character of activity of extensor half-centres is determined secondarily by alternating interaction of each of them with homolateral flexor half-centre. After injection of DOPA the central program of interaction of hindlimbs can be determined by synchronous work of extensor half-centres.

Changes in electric polarization of lumbosacral primary afferent terminals were studied during fictious locomotion of immobilized decorticated and spinal cats. Fictitious locomotion was accompanied by a stable hyperpolarization of afferent terminals. In the presence of hyperpolarization a periodic depolarization took place with the rhythm of efferent activity. These tonic and phasic changes were observed in the terminals of all the studied groups of afferent fibres (cutaneous and muscle Ia and Ib). Periodic depolarization was in-phase indifferent ipsilateral lumbosacral segments. During fictitious gallop the changes in depolarization of primary afferent terminals were in-phase on different sides; during fictitious walking these periodic changes were of out-of-phase character. Physiological significance of changes in electric polarization of primary afferent terminals by the spinal locomotor generator is discussed on the basis of the data obtained.

It is shown on unanesthetized immobilized decorticated cats that spinalization of the animal leads to depolarization of the central afferent terminals, decrease of early polysynaptic responses in motoneurons and dorsal root potentials (DRP) evoked by stimulation of the low-threshold cutaneous and muscle afferents, increase of early polysynaptic responses and DRP evoked by stimulation of high-threshold muscle afferents, reduction in the activity of intermediate nucleus interneurons mono- and polysynaptically connected with primary afferents, rise of the activity of interneurons di- and oligosynaptically connected with primary afferents. Injection of DOPA into spinal animals leads to opposite changes. Dependence between changes in the state of segmental neuronal apparatus and the level of spinal locomotor generator activity are discussed on the basis of the data obtained.

Descending projections of brain cortex and basal ganglia to the hypothalamic locomotor region were demonstrated in cat with horseradish peroxidase technique. Neurons projecting directly to the functionally identified locomotor region are dispersed in different gyri with predominance in areas 4 and 6 of the motor cortex. Such neurons are also located in entopeduncular nucleus. Massive cortico-and pallido-hypothalamic projections as a rule do not reach the most caudal hypothalamic divisions where the locomotor region is localized.

It is shown on decorticated (high-decerebrated) and spinal cats and on spinal cats injected with DOPA that the activation of the spinal locomotor generator is accompanied by an increase of the extensor and a decrease of the flexor motoneurons' excitability, an increase of the recurrent and reciprocal Ia inhibition, a decrease of Ib and flexor reflex afferent influences on motoneurons. The possible physiological role of changes in tuning of the spinal segmental apparatus in the generation of the locomotor rhythm is discussed.