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Biomedical subjects

M L Shik

Publications and source records attributed to M L Shik.

At least 19 recordsLinked to original sources
PubMed1992

[Responses of medullary and spinal neurons to simultaneous stimulation of two locomotor points].

Responses of neurons in the medulla and cervical segments to simultaneous repetitive (50 pps) stimulation of two locomotor points (LPs) by currents in the range of 1 to 2 thresholds of locomotion were studied. In most cases a neuron responded to stimulation of only one LP. Stimulation of the second LP enhanced usually the firing index of that response, if it was low, diminished it, if it was high and did not influence it, if it was about 0.2. Part of neurons increased the background activity during stimulation of one of LPs though spikes were not locked to stimuli. A sign of influence of the second LP onto that elevation depended on its value. Data about the convergence of inputs from the ipsi- and contralateral midbrain and bulbar LPs on single neurons were obtained too. Possible mechanisms of summation of subthreshold excitation of two LPs during initiation of the locomotion are discussed.

Action Potentials↗
PubMed1991

[Synaptic responses of spinal cord neurons to rhythmic microstimulation of a stepping strip].

Synaptic responses of neurons at the low thoracic level to microstimulation of a stepping strip in the dorsolateral funiculus with frequency of 20 and 40 pps were recorded in mesencephalic cats. Histograms of latencies of single neurons were usually monomodal, whereas two modes at 2-3 ms and 5-7 ms occurred in composed histograms. The data obtained show that decremental reverberation exists under these experimental conditions.

Animals↗
PubMed1991

[The convergence of influences from the locomotor points of the midbrain and medulla oblongata and from the inhibitory point of the pons onto the bulbar neurons].

Synaptic responses of medial and lateral medullary neurons to repetitive stimulation of locomotor points (LPs) of the midbrain and medulla and of an inhibitory pontine point were recorded in mesencephalic decerebellate cats. Excitatory postsynaptic potentials (PSPs) and discharges were observed usually in medial neurons as well as mixed PSPs when an inhibitory point was stimulated. Almost a half of lateral neurons and more than a quarter of medial ones changed the frequency of the background discharge giving no responses time-locked to stimuli. Medial neurons with responses time-locked to stimuli were equally susceptible to stimuli delivered to midbrain and medullary LPs and almost as often to stimuli applied to the inhibitory point. Medial neurons with responses time-unlocked to stimuli and lateral neurons were mostly susceptible to the input from the medullary LP, less affected by stimulation of the midbrain LP and responded rarely to stimulation of the inhibitory point. Convergence of influences from the midbrain and medullary LPs was the same onto neurons of all populations. Role of different neuronal populations for initiation and cessation of locomotion is discussed.

Animals↗
PubMed1990

[The pathways necessary for eliciting walking by stimulation of the locomotor area of the brain stem].

Effects of bilateral lesions of the spinal cord at the upper cervical level on the walking elicited by stimulation of the midbrain or of the spinal cord, were studied in decerebrated cats. The locomotion could be elicited after lesions of the ventral funiculus together with the dorsal part of the lateral funiculus, or of the ventrolateral funiculus only, or of the grey matter. Following large lesions in the ventral half of the spinal cord, the midbrain stimulation elicited no walking, but low-amplitude alternative rhythmic movements could be observed during 1.5-2.0 hours. Nevertheless, the stimulation of the spinal cord (mostly bilateral) elicited the walking.

Animals↗
PubMed1990

[2 types of neuronal reactions of the medulla oblongata to microstimulation of the locomotor and inhibitory points of the brain stem].

Synaptic responses (postsynaptic potentials and action potentials) of medial and lateral bulbar neurons were evoked by stimulation of the medullary locomotor point (LP) and pontine inhibitory point (IP) by current up to 20 microA in mesencephalic decerebellated cats. Some neurons responded even to single (2-5/s) stimuli. Other neurons responded only to rhythmic (30-60/s) stimulation. Both groups of medial neurons were more susceptible to the input from IP. Lateral neurons which responded even to single stimuli were more reactive to the input from LP, whereas those which responded only to rhythmic stimulation were under the predominant influence from IP. Many neurons (both lateral and medial) with the background activity did not generate responses which were time-locked to a stimulus, however the background activity was either enhanced or inhibited under the rhythmic stimulation. These reactions were encountered more often during stimulation of LP. Partial redistribution of target neurons which accompanied the elevation of the frequency of the presynaptic input may be very significant in the motor control.

Action Potentials↗
PubMed1988

[Frequency potentiation of neuronal synaptic reactions in the medial tegmentum of the medulla oblongata to microstimulation of the inhibitory point of the pons Varolii].

Synaptic responses of medial medullary neurons to single (2 pps) and repetitive (30-50 pps) stimuli delivered to the pontine inhibitory point were recorded in decerebrated cats. Firing index, inhibitory to the less extent, excitatory postsynaptic potentials usually increased when repetitive stimulation was applied. Suppression of the background impulse activity was observed in some neurons. Frequency potentiation makes a substantial contribution to functional effect of stimulation of the inhibitory point (termination of elicited locomotion).

Animals↗
PubMed1988

[Propagation of the activity along the "stepping strip" of the spinal cord in the cat].

Three stepping points (SPs) were found in one dorsolateral funiculus of the spinal cord at the low thoracic level in mesencephalic cats. The SPs were at a distance of about 8 mm from one another, and stimulation of each SP elicited stepping of the ipsilateral hind limb. Synaptic responses of single neurons to stimulation of the caudal and rostral SPs before and after electrolytic lesion of the intermediate SP were recorded 5-17 mm caudal to the caudal SP. Neurons excited by caudal SP stimulation encountered after the lesion as often as before it, whereas stimulation of the rostral SP (4 pps) evoked responses 5 times more seldom than before lesion. Even stimulation of the rostral SP with the frequency of 40-60 pps which increased essentially a lesion firing index before coagulation, excited only a few neurons. Thus, synaptic excitation of neurons becomes significantly more difficult after damage of the stepping strip between stimulation and recording sites.

Action Potentials↗
PubMed1987

[Activity of spinal cord neurons and their responses to stimulation of the "stepping" strip during spontaneous locomotor rhythmicity].

Spike activity of single neurons in the upper cervical segments C2-C3 or at the brachial enlargement C5-C6 was recorded in decerebrated cats during spontaneous locomotor rhythm after transection of the spinal cord at the low thoracic level. The shortest interspike intervals in 66% of cyclically active neurons at C5 were observed during electrical activity of extensor muscles of the ipsilateral forelimb. Only 8% of these neurons responded to stimulation of the stepping strip. The shortest interspike intervals in 65% of cyclically active neurons at C2 were observed during a silent period in extensor activity. 45% of C2 neurons responded to stimulation of the stepping strip. 87% of non-cyclic neurons responded to stimulation of the strip, were located at the C2 level.

Animals↗
PubMed1986

[Responses of medullary neurons to stimulation of locomotor and inhibitory loci in the brain stem of the cat].

The medullary locomotor point (L) and the pontine inhibitory point (I) were found in mesencephalic decerebellate cats. Repetitive (60/s) microstimulation of L elicited stepping of forelimbs which terminated during repetitive microstimulation of I. Responses of neurons were evoked applying 1.5 s-1 single or paired stimuli to L or a train of 2-4 stimuli to I, the interstimulus interval being 2 ms. Medial neurons (N-301) gave PSPs or action potentials of 1 stimulation three times as often as to L stimulation. Contrary, lateral neurons (N-166) responded two times as often as to L stimulation. IPSPs were recorded in both groups of neurons two times as seldom as EPSPs. In medial neurons IPSPs were produced mainly by I stimulation. I stimulation did not evoke usually IPSPs in neurons excited from L. Possible mechanisms of termination of stepping due to I stimulation are discussed.

Animals↗
PubMed1985

[Synaptic responses of propriospinal neurons on stimulation of the locomotor strip of the dorsolateral funiculus of the cat].

Stepping points in the dorsolateral funiculus of thoracic and cervical divisions of the spinal cord were found in mesencephalic cats. Synaptic responses of single neurons at level Th12-Th13 to microstimulation of these points were recorded extracellularly. Latencies of these responses increase when the distance between the stepping point and the site of the recording is over 20 mm, and paired stimuli have now to be delivered to evoke responses. Neurons responding orthodromically to stimulation of the stepping strip send their axons into the ventrolateral funiculus near the grey matter. Occurrence of antidromic responses among neurons sending their axons in the caudal direction is 37% in the average till the distance between the recording and stimulation sites is up to 40 mm. Half of neurons with synaptic responses to stimulation of the stepping point can be also excited by a train of 3-6 stimuli applied to the midbrain locomotor point. Revealed neurons may contribute to propagation of the activity to the stepping generator of the hindlimb.

Animals↗
PubMed1979

[Neurons of the superior cervical segments responding to stimulation of the bulbar locomotor strip].

Synaptic responses of single neurons of upper cervical segments to stimulation of the bulbar "locomotor strip" were recorded extracellularly in mesencephalic cats. The stimulating current being about 30 muA, these responses usually had 2-7 ms latencies and appeared in neurons located at a depth of 2-4 mm from the dorsal surface (Rexed's laminae V-VIII). These neurons cannot be excited antidromically either from lumbar or lower cervical segments. However antidromic responses, could be evoked by stimuli applied 3-5 mm caudally of the recording electrode. It is assumed that neurons in C2, C3 excited from the "locomotor strip" are elements of the cell column which is responsible for the polysynaptic propagation of activity to the spinal generators of stepping.

Animals↗
PubMed1978

[Reactions of cat hindbrain "locomotor strip" neurons to microstimulation].

Synaptic responses of single neurons in the locomotor strip" were recorded extracellularly. Neurons of the rostral part of the strip produced short-latency responses to stimulation of the mesencephalic "locomotor region". Neurons of the caudal part of the strip responded to microstimulation of other sites of the strip, rostral ones included. When the distance between the site of stimulation and a neuron along the strip was less than 2--3 mm, short-latency (1.2--1.6 ms) responses were recorded. Thresholds and latencies grew with the distance. Polysynaptic responses with a 3--4 ms latency could be potentiated when repetitive (30--40 pulses per s) stimulation was used instead of a single stimulus. The results suggest that axons in the "locomotor strip" are oriented in a rostrocaudal direction and give off collaterals to neighbour neurons. The "locomotor strip" can be an integrative centre "intercalated" between the rostral centers of the brain and the spinal cord.

Animals↗
PubMed1977

Role of pontine tegmentum for locomotor control in mesencephalic cat.

1. An attempt has been made to elucidate how direct stimulation of the mesencephalic locomotor region (MLR, with Horsley-Clarke coordinates P2, L4, and H0) is transmitted through the pons to the spinal cord where a stepping generator is presumed to exist. 2. A longitudinal strip, termed the "pontine locomotor region" (PLR), was identified. It extends ventrocaudally throughout the lateral pontine tegmentum (P3-P9, L4 and about 2 mm beneath the floor of the IVth ventricle). 3. Stimulation of this locomotor strip at P4-5 and P8-9 levels generated hindlimb stepping or four-legged locomotion on a treadmill similar to that elicited by MLR stimulation. However, "PLR stepping" was more often accompanied by spasticity of the hindlimbs. Stimulation of the pontine strip at the P6-7 level produced stepping accompanied by an opening of the mouth. 4. Subthreshold MLR stimulation together with subthreshold PLR stimulation generated locomotion. Ipsilateral and contralateral MLR-PLR stimulations were of equal effectiveness for the generation of locomotion. 5. Stimulation of rostral (P3-6), but not caudal (P6-9), parts of the PLR evoked field potentials in the MLR with two negative components. The points at which these potentials were evoked with minimum current were usually coincident with the best points for eliciting locomotion. Short-latency monophasic negative potentials were evoked in the rostral part of the PLR by MLR stimulation. 6. Locomotion elicited by stimulation of either the MLR or the PLR was suppressed by stimulation within a midpontine region, 1.5-2.0 mm beneath the floor of the IVth ventricle (P6-7, L0-0.5, H-5 to -6). Stimulation applied to the close vicinity of this "inhibitory" region did not evoke field potentials in the MLR. 7. In some animals stimulation between the inhibitory region and the underlying PLR could facilitate locomotion elicited by MLR stimulation, although no stepping was produced by such stimulation alone.

Animals↗
PubMed1976

Neurophysiology of locomotor automatism.

It had long been known that the decapitated cock can cross a yard. During the last century an automatic mechanism controlling stepping movements has also been found in other vertebrates. The system controlling locomotion has many features similar to these systems controlling other natural movements: respiration (28), micturition (98), scratching (154), mastication (33), etc. Today we know that there are spinal automatisms for each limb generating its stepping movements. Activity of these automatisms depends essentially on the afferent inflow from the moving limbs. There also is interaction of the limbs during locomotion that promotes their coordination. The existence of two descending systems with different functions in the control of locomotion (Fig. 1) also can be considered as an established fact. Activity of a number of neurons involved in the control of locomotion has been studied directly during locomotion in decorticate, thalamic, and mesencephalic cats. To explain the experimental data at hand, several hypotheses of organization of the spinal automatism of stepping have been forwarded: a chain-reflex hypothesis, a hypothesis of two reciprocal half-centers, and a ring hypothesis (Fig. 2). Although general features of the system controlling locomotion are more or less clear, many questions are not yet answered. It is unknown what relative contributions to motoneuronal activity are made by proprioceptive reflexes versus influences from the automatism of stepping. Furthermore the structure of the spinal stepping automatism is not known. It is not clear if the spinal stepping automatisms of the forelimbs are as potent as those of the hindlimbs. The descending system responsible for activation of the spinal automatism of stepping has not yet been identified in direct experiments. The inputs and outputs of the subthalamic and midbrain "locomotor" regions have not been found, and we know almost nothing about intrinsic interaction of neurons in these regions. The role of inhibitory thalamic influences is scarcely known. Finally, we have no data concerning the influence of either cortical (42, 186) or visual mechanisms in locomotor control.

Afferent Pathways↗