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Biomedical subjects

M L Shik

Publications and source records attributed to M L Shik.

At least 37 records · Page 2Linked to original sources
PubMed1980

[Rhombencephalic "locomotor area" of turtles].

Electric stimulation (20-30/s, 10-20 muA) of the rhombencephalon in the decerebrated turtle may evoke cyclic coordinated movements of the limbs. The "locomotor region" is a strip oriented in the rostrocaudal direction which corresponds to the lateral reticular formation. Ipsi- and contralateral synpatic responses of single neurons were recorded extracellularly both in the medial and lateral reticular formations; the responses were evoked by stimulation of the "locomotor region" (2/s, 10-30 muA). The usual latencies of these responses were 3-12 ms (with the mode of 5-6 ms). Excitation of the "locomotor region" is followed by extensive propagation of activity in the rhombencephalon. The possible mechanisms of this propagation are considered.

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↗