N. A. Shcherbakova

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.

The role of cutaneous afferents in controlling locomotion evoked by epidural stimulation of the spinal cord in decerebrate cats

The effects of the cutaneous input on the formation of the locomotor pattern in conditions of epidural stimulation of the spinal cord in decerebrate cats were studied. Locomotor activity was induced by rhythmic stimulation of the dorsal surface of spinal cord segments L4–L5 at a frequency of 3–5 Hz. Electromyograms (EMG) recorded from the antagonist muscles quadriceps, semitendinosus, tibialis anterior, and gastrocnemius lateralis were recorded, along with the kinematics of stepping movements during locomotion on a moving treadmill and reflex responses to single stimuli. Changes in the pattern of reactions observed before and after exclusion of cutaneous receptors (infiltration of lidocaine solution at the base of the paw or irrigation of the paw pads with chlorothane solution) were assessed. This treatment led to impairment of the locomotor cycle: the paw was placed with the rear surface downward and was dragged along in the swing phase, and the duration of the stance phase decreased. Exclusion of cutaneous afferents suppressed the polysynaptic activity of the extensor muscles and the distal flexor muscle of the ipsilateral hindlimb during locomotion evoked by epidural stimulation of the spinal cord. The effects of exclusion of cutaneous afferents on the monosynaptic component of the EMG response were insignificant.

Initiation of Locomotion in Decerebrate Cats by Pulsed Magnetic Fields Projected onto Spinal Cord Segments

The motor effects induced by pulsed magnetic fields (PMF) projected onto the lumbar and cervical spinal cord were studied in decerebrate cats. A magnetic coil (inductor) of diameter 8 cm was positioned 1–2 cm above the surface of the spinal cord. Stimulation of the spinal cord with PMF was performed in two regimes: with single impulses with an intensity of 0.5–1 T and with continuous rhythmic stimulation at a frequency of 1 Hz and an intensity of 0.5 T. Application of single stimuli to the lumbar enlargement evoked reflex responses in the proximal and distal hindlimb muscles. Rhythmic stimulation initiated locomotor activity of the limb on a running treadmill, i.e., activated the neural locomotor network of the spinal cord (stepping movement generator). Magnetic stimulation of the lumbar enlargement evoked coordinated stepping movements of the hindlimbs only. Application of PMF to the cervical enlargement induced coordinated stepping movements of all four limbs, hindlimb movements starting before forelimb movements. After cessation of magnetic stimulation, the limbs completed several further coordinated movement cycles. This is the first report of the triggering of limb stepping movement generators with PMF in decerebrate cats. The results obtained here demonstrate that the neural locomotor networks of the spinal cord can be activated noninvasively and open new perspectives for the clinical use of PMF.

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.

Effect of locomotor training and functional electrical stimulation on postural function in children with severe cerebral palsy

Cerebral palsy (CP) considerably impairs the ability to maintain upright stance. The effects of locomotor training and functional electrical stimulation (FES) on postural control were determined in 27 children aged 6–12 years with severe CP. The severity level of the clinical manifestations of CP was classified as 3 according to the Gross Motor Function Classification System (GMFCS). All patients participated in 15 30-min mechanical therapy sessions using robot-assisted passive stepping. In 12 out of 27 children, the locomotion therapy was accompanied by FES. Stabilometry and plantography tests were performed in 23 healthy age-matched children. Postural control in children with CP differed from the stabilograms of healthy children in a forward shift of the center of pressure (COP) projection; higher values of the COP trajectory area and length, the mean amplitude of the COP oscillations, and the absence of COP response to the eyes closed condition. After treatment, the posturographic characteristics tended to normalize in relation to the values obtained in neurologically intact children. The improvement was observed in 43% of children without FES and in 75% of children in the group with FES. Analysis of plantograms revealed normalization of footprints in children who received FES. Thus, it was demonstrated that FES combined with locomotor training resulted in the improvement in vertical posture control in children with severe CP.

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.

Effect of transcutaneous electrical spinal cord stimulation on the blood flow in the skin of lower limbs

Changes in the blood flow in the skin of the plantar surface of the hallux were investigated by laser Doppler flowmetry in eight healthy subjects during transcutaneous electrical spinal cord stimulation (tESCS) with the pulse parameters used to activate locomotion. Continuous tESCS in the area of C5–C6 vertebrae did not cause significant changes in the blood flow, while electrical stimulation at T12–T1 and L1–L2 levels resulted in an increase in skin perfusion by 22–27%. Wavelet analysis of microcirculatory fluctuations showed that tESCS induced flaxomotions in the range of sensory peptidergic fibers and enhanced the amplitude of fluctuations of microcirculation in the endothelium-dependent range. These results suggest that tESCS stimulates microcirculation in the skin mainly due to antidromic stimulation of sensory peptidergic nerve fibers, which promotes activity of microvascular endothelium, vasodilator secretion, a decrease in vascular resistance, and an increase in microcirculation.

Noninvasive method to control the human spinal locomotor systems

The mechanism of interactions between receptor activation in the musculoskeletal system and stimulation of the spinal cord in the regulation of locomotor behavior was studied in healthy subjects. Afferent stimulation was tested for effect on the patterns of stepping movements induced by percutaneous stimulation of the spinal cord. A combination of percutaneous spinal cord stimulation and vibratory stimulation was shown to increase the amplitude of leg movements. It was demonstrated that vibratory stimulation of limb muscles at a frequency of less than 30 Hz can be used to control involuntary movements elicited by noninvasive stimulation of the spinal cord.