Experimental Brain Research [Exp Brain Res] 2007 Sep; Vol. 182 (2), pp. 281-7. Date of Electronic Publication: 2007 Aug 24.
Adult, Analysis of Variance, Electromyography methods, Evoked Potentials, Motor radiation effects, Female, Humans, Male, Motor Cortex radiation effects, Muscle, Skeletal physiology, Pyramidal Tracts anatomy histology, Pyramidal Tracts radiation effects, Reaction Time, Time Factors, Electric Stimulation methods, Evoked Potentials, Motor physiology, Leg innervation, Motor Cortex physiology, Pyramidal Tracts physiology, and Transcranial Magnetic Stimulation
Transcranial direct current stimulation (tDCS) of the human motor cortex at an intensity of 1 mA has been shown to be efficacious in increasing (via anodal tDCS) or decreasing (via cathodal tDCS) the excitability of corticospinal projections to muscles of the hand. In this study, we examined whether tDCS at currents of 2 mA could effect similar changes in the excitability of deeper cortical structures that innervate muscles of the lower leg. Similar to the hand area, 10 min of stimulation with the anode over the leg area of the motor cortex increased the excitability of corticospinal tract projections to the tibialis anterior (TA) muscle, as reflected by an increase in the amplitude of the motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation. MEP amplitudes recorded at rest and during a background contraction were increased following anodal tDCS and remained elevated at 60 min compared to baseline values by 59 and 35%, respectively. However, in contrast to the hand, hyperpolarizing cathodal stimulation at equivalent currents had minimal effect on the amplitude of the MEPs recorded at rest or during background contraction of the TA muscle. These results suggest that it is more difficult to suppress the excitability of the leg motor cortex with cathodal tDCS than the hand area of the motor cortex.
Journal Of Neurophysiology [J Neurophysiol] 2007 Mar; Vol. 97 (3), pp. 2499-510. Date of Electronic Publication: 2007 Jan 17.
Animals, Cats, Electromyography methods, Feedback, Functional Laterality, Lower Extremity physiology, Lower Extremity radiation effects, Male, Time Factors, Weight-Bearing, Electric Stimulation, Implants, Experimental, Locomotion physiology, and Locomotion radiation effects
The physiological control of stepping is governed both by signals descending from supraspinal systems and by circuitry residing within the lumbosacral spinal cord. The goal of this study was to evaluate the capacity of physiologically based controllers to restore functional overground locomotion after neurological damage, such as spinal cord injury when used in conjunction with functional electrical stimulation. For this purpose we implemented and tested two controllers: 1) an intrinsically timed system that generated a predetermined rhythmic output and 2) a sensory-based system that used feedback signals to make appropriate transitions between the unloaded (flexion) and loaded (extension) phases of the gait cycle. A third controller, a combination of the intrinsically timed and sensory-driven controllers, was implemented and two sessions were conducted to demonstrate the functional advantages of this approach. The controllers were tested in anesthetized cats, implanted with intramuscular electrodes in six major extensor and flexor muscles of the hindlimbs. The cats were partially supported on a sliding trolley that was propelled by the hindlimbs along a 2.5-m instrumented walkway. Ground reaction forces and limb positions were measured by force plates in the walkway and by accelerometers secured to the legs of the cat, respectively. The controllers were used to generate patterns of stimulation that would elicit alternating flexor (swing) and extensor (stance) movements in the hindlimbs. Using either the intrinsically timed or sensory-driven controllers, the cats were able to travel a distance of 2.5 m, taking five to 12 steps. Functional stepping sequences were more easily achieved using the intrinsically timed controller as the result of a lower sensitivity to the selection of initial stimulation parameters. However, unlike the sensory-driven controller, the intrinsically timed controller was unable to adjust to overcome walkway resistance and muscle fatigue. Neither system was consistently able to ensure load-bearing stepping. Therefore we propose the use of a "combined controller" that relies heavily on intrinsic timing but that can be reset based on sensory signals. A combined controller such as this one may provide the best solution for restoring robust overground locomotion after spinal cord injury.
IEEE Transactions On Neural Systems And Rehabilitation Engineering: A Publication Of The IEEE Engineering In Medicine And Biology Society [IEEE Trans Neural Syst Rehabil Eng] 2006 Sep; Vol. 14 (3), pp. 266-72.
Action Potentials physiology, Adaptation, Physiological physiology, Animals, Cats, Gait physiology, Biological Clocks physiology, Electric Stimulation methods, Locomotion physiology, Lumbosacral Plexus physiology, Motor Neurons physiology, Nerve Net physiology, and Spinal Cord physiology
It is commonly accepted that locomotor-related neuronal circuitry resides in the lumbosacral spinal cord. Pharmacological agents, epidural electrical stimulation, and sensory stimulation can be used to activate these instrinsic networks in in vitro neonatal rat and in vivo cat preparations. In this study, we investigated the use of low-level tonic intraspinal microstimulation (ISMS) as a means of activating spinal locomotor networks in adult cats with complete spinal transections. Trains of low-amplitude electrical pulses were delivered to the spinal cord via groups of fine microwires implanted in the ventral horns of the lumbosacral enlargement. In contrast to published reports, tonic ISMS applied through microwires in the caudal regions of the lumbosacral enlargement (L7-S1) was more effective in eliciting alternating movements in the hindlimbs than stimulation in the rostral regions. Possible mechanisms of action of tonic ISMS include depolarization of locally oscillating networks in the lumbosacral cord, backfiring of primary afferents, or activation of propriospinal neurons.