Clinical Neurophysiology: Official Journal Of The International Federation Of Clinical Neurophysiology [Clin Neurophysiol] 2008 Dec; Vol. 119 (12), pp. 2675-6. Date of Electronic Publication: 2008 Oct 05.
Humans, Central Nervous System cytology, Motor Neurons physiology, and Walking physiology
Norton JA, Bennett DJ, Knash ME, Murray KC, and Gorassini MA
Brain: A Journal Of Neurology [Brain] 2008 Jun; Vol. 131 (Pt 6), pp. 1478-91. Date of Electronic Publication: 2008 Mar 15.
Adult, Animals, Case-Control Studies, Electric Stimulation, Electromyography, Excitatory Postsynaptic Potentials physiology, Foot, Humans, Rats, Reflex, Spasm etiology, Spasm physiopathology, Motor Neurons physiology, Muscle Spasticity physiopathology, and Spinal Cord Injuries physiopathology
Following spinal cord injury (SCI), prolonged muscle spasms are readily triggered by brief sensory stimuli. Animal and indirect human studies have shown that a substantial portion of the depolarization of motoneurons during a muscle spasm comes from the activation of persistent inward currents (PICs). The brief (single pulse) sensory stimuli that trigger the PICs and muscle spasms in chronically spinalized animals evoke excitatory post-synaptic potentials (EPSPs) that are broadened to more than 500 ms, the duration of depolarization required to activate a PIC in the motoneuron. Thus, in humans, we investigated if post-synaptic potentials (PSPs) evoked from brief (<20 ms) sensory stimulation are changed after SCI and if they are broadened to > or =500 ms to more readily activate motoneuron PICs and muscle spasms. To estimate both the shape and duration of PSPs in human subjects we used peristimulus frequencygrams (PSFs), which are plots of the instantaneous firing frequency of tonically active single motor units that are time-locked to the occurrence of the sensory stimulus. PSFs in response to cutaneomuscular stimulation of the medial arch or toe of the foot, a sensory stimulus that readily triggers muscle spasms, were compared between non-injured control subjects and in spastic subjects with chronic (>1 year), incomplete SCI. In non-injured controls, a single shock or brief (<20 ms) train of cutaneomuscular stimulation produced PSFs consisting of a 300 ms increase in firing rate above baseline with an interposed period of reduced firing. Parallel intracellular experiments in motoneurons of adult rats revealed that a 300 ms EPSP with a fast intervening inhibitory PSP (IPSP) reproduced the PSF recorded in non-injured subjects. In contrast, the same brief sensory stimulation in subjects with chronic SCI produced PSFs of comparatively long duration (1200 ms) with no evidence for IPSP activation, as reflected by a lack of reduced firing rates after the onset of the PSF. Thus, unlike non-injured controls, the motoneurons of subjects with chronic SCI are activated by very long periods of pure depolarization from brief sensory activation. It is likely that these second-long EPSPs securely recruit slowly activating PICs in motoneurons that are known to mediate, in large part, the many seconds-long activation of motoneurons during involuntary muscle spasms.
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.