I. N. Bogacheva

Significance of peripheral feedback in the generation of stepping movements during epidural stimulation of the spinal cord.

Acute experiments on decerebrate and spinal cats were performed to study the role of the peripheral afferent input from hindlimb receptors in forming the locomotor pattern during epidural stimulation of the spinal cord. Evoked electromyographic activity in the muscles of the hindlimbs was analyzed, along with the kinematic parameters of stepping movements. Epidural stimulation (20–100 µA, 5 Hz) of segments L4–5 of the spinal cord was found to elicit well coordinated walking in the hindlimbs on a moving treadmill band. When the support conditions were changed (non-moving treadmill, unsupported position), epidural stimulation initiated walking with an unstable rhythm. This was associated with a change in the overall nature of the locomotor pattern and the internal structure of the stepping cycle. Alteration of the direction of movement of the treadmill band led to the appearance of backward walking. An increase in the speed of movement of the treadmill band increased the stepping frequency, mainly due to decreases in the extensor phase. Epidural stimulation applied 2–4 h after complete transection of the spinal cord at the T8–T9 level could elicit stepping movements, but only when the treadmill was moving. The role of peripheral feedback in generating the locomotor pattern in conditions of complete disconnection from supraspinal control increased significantly. These data show that peripheral feedback during epidural stimulation of the spinal cord can define the properties of the motor output.

Formation of locomotor patterns in decerebrate cats in conditions of epidural stimulation of the spinal cord

Acute experiments on decerebrate cats were performed to study the mechanism of formation of the locomotor pattern in conditions of epidural stimulation of the spinal cord. These studies showed that only segments L3–L5 contributed to generating the stepping pattern in the hindlimbs. At the optimum frequency (5–10 Hz) of stimulation of these segments, formation of electromyographic burst activity in the flexor muscles was mainly due to polysynaptic reflex responses with latencies of 80–110 msec. In the extensor muscles, this process involved the interaction of a monosynaptic reflex and polysynaptic activity. In epidural stimulation, the stepping pattern was specified by spinal structures, while peripheral feedback had modulatory influences.

Effects of Spinal Cord Electrical Stimulation in Patients with Vertebrospinal Pathology

Until now, no scientific neurophysiologic methods of diagnostics and treatment of vertebrospinal pathologies were developed. Previous study showed that electrical stimulation of lumbar segments of the spinal cord in animals with complete spinal cord transection induced a well-coordinated weight-bearing locomotion. The present comparative study of motor activity triggered by electrical epidural stimulation of one or two segments of the spinal cord in spinal patients showed that stimulation of lumbar (L2-L4) or sacral (S2) segments facilitated generation of motor patterns of muscle activity. Combination of electrical stimulation with locomotor training resulted in the appearance of stepping patterns characteristic of normal walking and tonic activity of the muscles needed for body balance maintenance.

Bioelectric activity of spinal cord in patients with vertebrospinal pathologies.

A method for functional evaluation of spinal cord conducting systems in patients with vertebrospinal diseases is proposed. The method is based on the analysis of changes in electrospinoneurogram recorded with epidural electrodes below the injury during an attempt at voluntary activation of foot muscles. The degree of changes in electrospinoneurogram frequency during voluntary command addressed to motoneurons of the lumbar enlargement reflect the state of descending spinal systems.

Learning and Reproduction of Memorized Sequences of Right and Left Hand Movements

An important stage in learning, i.e., the acquisition of a new skill, is the repetitive reproduction of a sequence of movements, which plays a significant role in the formation of motor stereotypies. Two groups of right-handed subjects reproduced (6–10 repeats) sequences of movements guided by the experimenter, sequences consisting of six positions, first with the right hand (RH) and then with the left hand (LH) or vice versa. In series 1, an unfamiliar random sequence was reproduced; series 2 and 3 involved reproduction of modified sequences whose elements were in the same positions but in a different order. The processes of reproduction proceeded similarly for the RH and LH. Learning of the modified sequence was different: regardless of order of presentation, information about the positions of the elements of the sequence was used only when the LH performed the task first. This information was not used when the LH operated after the RH or when the RH performed the task. Thus, the means of encoding information activated on operation by the LH promoted learning of the position memorization task, while that activated by the RH interfered. This appears to be linked with the predominant roles of the right hemisphere in encoding positions and in motor learning processes.

Initiation of Locomotor Activity in Decerebrate and Spinal Cats Using Noninvasive Transcutaneous Electrical Stimulation of the Spinal Cord

Spinal neural networks activated by epidural electrical stimulation of the spinal cord (ESSC) are known to be able to take part in generating the stepping EMG pattern and controlling locomotor behavior. We show here that noninvasive transcutaneous stimulation of the spinal cord (TES) in the lumbosacral enlargement area can initiate locomotor activity in decerebrate and spinal animals. Comparison of motor responses in ESSC and TES showed them to have similar reflex mechanisms, as well as similarities in the properties of the locomotor patterns. Our data support the view that TES is an effective approach for further studies of locomotor control in acute and chronic experiments. Considering the noninvasive nature and relative simplicity of using TES, this method may be suitable for further use in clinical practice in the rehabilitation of patients with vertebrospinal pathology.

Memorization of sequences of movements of the right and left hand by right- and left-handers

The errors of right- and left-handers were analyzed when they performed memorized sequences by right or the left hand during the task that activates positional coding: after six to ten times, the order of movements changed (the positions remained the same during the task). The task was at first performed by one (initial) hand, right or left, and then by the contralateral (continuing) hand; there were two groups of right- handers and two groups of left-handers. It was found that the pattern of errors during the task performance by the initial hand was similar in right- and left-handers both for the dominant and subdominant hands. Information about the previous positions after changing the order of elements is used in the sequences for subdominant hands and not used in the sequences for dominant hands. After changing the hand, right- and left-handers exhibited different patterns of errors. Thus, the errors of right- and left-handers are symmetrical at the early stages of task performance, while the transfer of this motor skill in right and left-handers occurs in different ways.

Memorization of sequences of right and left hand movements in right- and left-handers: Vector coding

Using studies of the right and left hemisphere’s specialization for positional and vector coding, we analyzed the errors made by right- and left-handers while reproducing sequences of right and left hand movements in a task that activates vector coding by changing the order of movements in memorized sequences. The task was performed first with one hand (starting) and then with the other (continuing). Both right- and lefthanders were found to use information about previous movements of the starting hand only when the dominant hand was starting. After changing the hand, right-handers used information about previous movements of the continuing hand, while left-handers did not. The results were compared with data from earlier experiments wherein positional coding was activated. The comparison showed that vector coding was predominantly involved in memorizing sequences of movements made by the dominant hand, while positional coding was used in the case of the opposite hand in both right- and left-handers. Patterns of errors after changing the hand differed between right- and left-handers, and the conclusion was made that skills are transferred in different ways in right- and left-handers, depending on the type of coding.

Effects of transcutaneous electrical spinal cord stimulation on stepping patterns during walking

Effects of transcutaneous electrical spinal cord stimulation (tESCS) on the parameters of stepping movements in healthy subjects were investigated during two kinds of activity: walking on a moving treadmill belt (active treadmill) as well as pushing the treadmill belt by effort of the legs (passive treadmill). It was found that the total interference electromyogram (EMG) activity during stepping performance on a passive treadmill was 1.5–2 times higher than during stepping on an active treadmill. In addition, the amplitude of angular displacement of the hip joint and ankle was 2.5 times and 1.7 times higher, respectively, during passive vs. active treadmill, while the duration of stepping cycle decreased by 19%. Although the muscles were exposed to different load and the parameters of motion on the active and passive treadmill were different, tESCS caused an increase in the total EMG activity in 96% of cases both on the active and on the passive treadmill. In both cases, the stepping cycle period decreased by 4–43% in all subjects. These results suggest that tESCS can affect voluntary stepping patterns under conditions of different afferent control.

Mechanisms of stepping rhythm formation in decerebrated and chronic spinal cats under epidural stimulation

A study was made of the stepping pattern formation in decerebrated and in chronic spinal cats during epidural stimulation (ES). The hindlimb stepping performance depended on the parameters of ES and afferent input. At non-optimal ES parameters, no stepping was induced, only muscle reflexes followed the stimulation rhythm. Optimized ES (3–5 Hz, 50–100 µA for decerebrated and 20–30 Hz, 150–250 µA for spinal cats) evoked coordinated stepping movements at a natural rate (0.8–1 Hz) accompanied by electromyographic burst activity of the corresponding muscles. In decerebrated cats, the bursts are formed owing to modulation of early responses and the late polysynaptic activity. In chronic spinal cats, this process is mainly due to amplitude modulation of the early responses. Formation of the stepping pattern in decerebrated cats involves spinal interneurons responsible for the polysynaptic activity, which allows its correction based on processing the afferent signals. Activation of this system in chronic spinal cats can be realized by afferent stimulation alone, without ES.