Simon Danner | Drexel University College of Medicine (original) (raw)
Papers by Simon Danner
Electrical stimulation of the nervous system plays a major role in medicine and rehabilitation. M... more Electrical stimulation of the nervous system plays a major role in medicine and rehabilitation. Muscles can be stimulated to avoid disuse, impaired function can be improved by targeted stimulation of certain nerves, and brain and spinal cord function can be researched. To appropriately apply these techniques, it is essential to understand the underlying mechanisms involved in the artificial activation of the central and peripheral nervous system by electrical stimulation. This book elaborates on the development and comparison of nerve fiber models which describe the influence of the applied electrical field on the neurons. A computer simulation tool using Matlab is presented that enables experiments with two commonly used axon models including calculation of excitation thresholds and investigation of action potential propagation. This book addresses students as well as scientists, who either want to examine the fundamentals of modeling neural excitation pattern or are interested in ...
Interest in transcutaneous electrical stimulation of the lumbosacral spinal cord is increasing in... more Interest in transcutaneous electrical stimulation of the lumbosacral spinal cord is increasing in human electrophysiological and clinical studies. The stimulation effects on lower limb muscles depend on the depolarization of segmentally organized posterior root afferents and, thus, the rostro-caudal stimulation site. In previous studies, selective stimulation was achieved by varying the positions of single self-adhesive electrodes over the thoracolumbar spine. Here, we developed a multi-electrode surface array consisting of 3 × 8 electrode pads and tested its stimulationsite specificity. The array was placed longitudinally over the spine covering the T10-L2 vertebrae. Two different hydrogel layer configurations were utilized: a single layer adhered to all electrode pads of the array and a configuration comprised of eight separate strips attached to the three transverse electrode pads of each level. Voltage measurements demonstrated that an effectively focused field distri-
Encyclopedia of Computational Neuroscience, 2013
Finite element modeling is an important computational tool in neural engineering to simulate neur... more Finite element modeling is an important computational tool in neural engineering to simulate neural excitation with implanted electrodes or with surface electrodes. Besides its importance for analyzing artificially generated neural activities, e.g., in the spinal cord , this technique is useful to interpret recorded electrical biosignals generated by neural or muscle tissue activities.
Biomedical Engineering / Biomedizinische Technik, 2000
Epidural spinal cord stimulation can produce rhythmic motor output to the lower limbs of motor co... more Epidural spinal cord stimulation can produce rhythmic motor output to the lower limbs of motor complete spinal cord injury people. The electromyographically recorded activity consists of a series of modulated stimulus time-related compound muscle action potentials (CMAPs). Here, we investigate phase dependent modification of the CMAP latencies and present a computer model that mechanistically describes putative locomotor pattern generating circuitries of the human lumbar spinal cord. Thereby we gained insight into the organization of the human spinal pattern generating networks, revealed common control characteristics with the central pattern generators for locomotion described in animal experimental work and highlighted specifities of the studied model.
Artificial organs, 2015
In this review of neurocontrol of movement after spinal cord injury, we discuss neurophysiologica... more In this review of neurocontrol of movement after spinal cord injury, we discuss neurophysiological evidences of conducting and processing mechanisms of the spinal cord. We illustrate that external afferent inputs to the spinal cord below the level of the lesion can modify, initiate, and maintain execution of movement in absence or partial presence of brain motor control after chronic spinal cord injury. We review significant differences between spinal reflex activity elicited by single and repetitive stimulation. The spinal cord can respond with sensitization, habituation, and dis-habituation to regular repetitive stimulation. Therefore, repetitive spinal cord reflex activity can contribute to the functional configuration of the spinal network. Moreover, testing spinal reflex activity in individuals with motor complete spinal cord injury provided evidences for subclinical residual brain influence, suggesting the existence of axons traversing the injury site and influencing the activ...
Artificial organs, 2015
The level of sustainable excitability within lumbar spinal cord circuitries is one of the factors... more The level of sustainable excitability within lumbar spinal cord circuitries is one of the factors determining the functional outcome of locomotor therapy after motor-incomplete spinal cord injury. Here, we present initial data using noninvasive transcutaneous lumbar spinal cord stimulation (tSCS) to modulate this central state of excitability during voluntary treadmill stepping in three motor-incomplete spinal cord-injured individuals. Stimulation was applied at 30 Hz with an intensity that generated tingling sensations in the lower limb dermatomes, yet without producing muscle reflex activity. This stimulation changed muscle activation, gait kinematics, and the amount of manual assistance required from the therapists to maintain stepping with some interindividual differences. The effect on motor outputs during treadmill-stepping was essentially augmentative and step-phase dependent despite the invariant tonic stimulation. The most consistent modification was found in the gait kinem...
Artificial organs, 2015
Interest in transcutaneous electrical stimulation of the lumbosacral spinal cord is increasing in... more Interest in transcutaneous electrical stimulation of the lumbosacral spinal cord is increasing in human electrophysiological and clinical studies. The stimulation effects on lower limb muscles depend on the depolarization of segmentally organized posterior root afferents and, thus, the rostro-caudal stimulation site. In previous studies, selective stimulation was achieved by varying the positions of single self-adhesive electrodes over the thoracolumbar spine. Here, we developed a multi-electrode surface array consisting of 3 × 8 electrode pads and tested its stimulation-site specificity. The array was placed longitudinally over the spine covering the T10-L2 vertebrae. Two different hydrogel layer configurations were utilized: a single layer adhered to all electrode pads of the array and a configuration comprised of eight separate strips attached to the three transverse electrode pads of each level. Voltage measurements demonstrated that an effectively focused field distribution alo...
Neurorehabilitation and neural repair, Jan 18, 2015
The human lumbosacral spinal circuitry can generate rhythmic motor output in response to differen... more The human lumbosacral spinal circuitry can generate rhythmic motor output in response to different types of inputs after motor-complete spinal cord injury. To explore spinal rhythm generating mechanisms recruited by phasic step-related sensory feedback and tonic posterior root stimulation when provided alone or in combination. We studied stepping in 4 individuals with chronic, clinically complete spinal cord injury using a robotic-driven gait orthosis with body weight support over a treadmill. Electromyographic data were collected from thigh and lower leg muscles during stepping with 2 hip-movement conditions and 2 step frequencies, first without and then with tonic 30-Hz transcutaneous spinal cord stimulation (tSCS) over the lumbar posterior roots. Robotic-driven stepping alone generated rhythmic activity in a small number of muscles, mostly in hamstrings, coinciding with the stretch applied to the muscle, and in tibialis anterior as stance-phase synchronized clonus. Adding tonic 3...
Journal of neurophysiology, Jan 22, 2015
In individuals with motor-complete spinal cord injury, epidural stimulation of the lumbosacral sp... more In individuals with motor-complete spinal cord injury, epidural stimulation of the lumbosacral spinal cord at 2 Hz evokes unmodulated reflexes in the lower limbs, while stimulation at 22-60 Hz can generate rhythmic burst-like activity. Here, we elaborated on an output pattern emerging at transitional stimulation frequencies with consecutively elicited reflexes alternating between large and small. We analyzed responses concomitantly elicited in thigh and leg muscle groups bilaterally by epidural stimulation in eight motor-complete spinal cord injured individuals. Periodic amplitude-modulation of at least 20 successive responses occurred in 31.4% of all available data sets with stimulation frequency set at 5-26 Hz, with highest prevalence at 16 Hz. It could be evoked in a single muscle group only, but was more strongly expressed and consistent when occurring in pairs of antagonists or in the same muscle group bilaterally. Latencies and waveforms of the modulated reflexes corresponded ...
Brain : a journal of neurology, 2015
Constant drive provided to the human lumbar spinal cord by epidural electrical stimulation can ca... more Constant drive provided to the human lumbar spinal cord by epidural electrical stimulation can cause local neural circuits to generate rhythmic motor outputs to lower limb muscles in people paralysed by spinal cord injury. Epidural spinal cord stimulation thus allows the study of spinal rhythm and pattern generating circuits without their configuration by volitional motor tasks or task-specific peripheral feedback. To reveal spinal locomotor control principles, we studied the repertoire of rhythmic patterns that can be generated by the functionally isolated human lumbar spinal cord, detected as electromyographic activity from the legs, and investigated basic temporal components shared across these patterns. Ten subjects with chronic, motor-complete spinal cord injury were studied. Surface electromyographic responses to lumbar spinal cord stimulation were collected from quadriceps, hamstrings, tibialis anterior, and triceps surae in the supine position. From these data, 10-s segments...
Hearing research, 2014
Early neural responses to acoustic signals can be electrically recorded as a series of waves, ter... more Early neural responses to acoustic signals can be electrically recorded as a series of waves, termed the auditory brainstem response (ABR). The latencies of the ABR waves are important for clinical and neurophysiological evaluations. Using a biophysical model of transmembrane currents along spiral ganglion cells, we show that in human (i) the non-myelinated somatic regions of type I cells, which innervate inner hair cells, predominantly contribute to peak I, (ii) the supra-strong postsynaptic stimulating current (400 pA) and transmembrane currents of the myelinated peripheral axons of type I cells are an order smaller; such postsynaptic currents correspond to the short latencies of a small recordable ABR peak I', (iii) the ABR signal involvement of the central axon of bipolar type I cells is more effective than their peripheral counterpart as the doubled diameter causes larger transmembrane currents and a larger spike dipole-length, (iv) non-myelinated fibers of type II cells wh...
Biomedical Engineering-Biomedizinische Technik, 2012
Stimulation of different spinal cord segments in humans is a widely developed clinical practice f... more Stimulation of different spinal cord segments in humans is a widely developed clinical practice for modification of pain, altered sensation and movement. The human lumbar cord has become a target for modification of motor control by epidural and more recently by transcutaneous spinal cord stimulation. Posterior columns of the lumbar spinal cord represent a vertical system of axons and when activated can add other inputs to the motor control of the spinal cord than stimulated posterior roots. We used a detailed three-dimensional volume conductor model of the torso and the McIntyre-Richard-Grill axon model to calculate the thresholds of axons within the posterior columns in response to transcutaneous lumbar spinal cord stimulation. Superficially located large diameter posterior column fibers with multiple collaterals have a threshold of 45.4 V, three times higher than posterior root fibers (14.1 V). With the stimulation strength needed to activate posterior column axons, posterior root fibers of large and small diameters as well as anterior root fibers are co-activated. The reported results inform on these threshold differences, when stimulation is applied to the posterior structures of the lumbar cord at intensities above the threshold of large-diameter posterior root fibers.
PLoS ONE, 2014
Electrical stimulation (ES) devices interact with excitable neural tissue toward eliciting action... more Electrical stimulation (ES) devices interact with excitable neural tissue toward eliciting action potentials (AP's) by specific current patterns. Low-energy ES prevents tissue damage and loss of specificity. Hence to identify optimal stimulationcurrent waveforms is a relevant problem, whose solution may have significant impact on the related medical (e.g. minimized side-effects) and engineering (e.g. maximized battery-life) efficiency. This has typically been addressed by simulation (of a given excitable-tissue model) and iterative numerical optimization with hard discontinuous constraints -e.g. AP's are all-or-none phenomena. Such approach is computationally expensive, while the solution is uncertain -e.g. may converge to local-only energy-minima and be model-specific. We exploit the Least-Action Principle (LAP). First, we derive in closed form the general template of the membrane-potential's temporal trajectory, which minimizes the ES energy integral over time and over any space-clamp ionic current model. From the given model we then obtain the specific energy-efficient current waveform, which is demonstrated to be globally optimal. The solution is model-independent by construction. We illustrate the approach by a broad set of example situations with some of the most popular ionic current models from the literature. The proposed approach may result in the significant improvement of solution efficiency: cumbersome and uncertain iteration is replaced by a single quadrature of a system of ordinary differential equations. The approach is further validated by enabling a general comparison to the conventional simulation and optimization results from the literature, including one of our own, based on finite-horizon optimal control. Applying the LAP also resulted in a number of general ES optimality principles. One such succinct observation is that ES with long pulse durations is much more sensitive to the pulse's shape whereas a rectangular pulse is most frequently optimal for short pulse durations. Citation: Krouchev NI, Danner SM, Vinet A, Rattay F, Sawan M (2014) Energy-Optimal Electrical-Stimulation Pulses Shaped by the Least-Action Principle. PLoS ONE 9(3): e90480.
Electrical stimulation of the nervous system plays a major role in medicine and rehabilitation. M... more Electrical stimulation of the nervous system plays a major role in medicine and rehabilitation. Muscles can be stimulated to avoid disuse, impaired function can be improved by targeted stimulation of certain nerves, and brain and spinal cord function can be researched. To appropriately apply these techniques, it is essential to understand the underlying mechanisms involved in the artificial activation of the central and peripheral nervous system by electrical stimulation. This book elaborates on the development and comparison of nerve fiber models which describe the influence of the applied electrical field on the neurons. A computer simulation tool using Matlab is presented that enables experiments with two commonly used axon models including calculation of excitation thresholds and investigation of action potential propagation. This book addresses students as well as scientists, who either want to examine the fundamentals of modeling neural excitation pattern or are interested in ...
Interest in transcutaneous electrical stimulation of the lumbosacral spinal cord is increasing in... more Interest in transcutaneous electrical stimulation of the lumbosacral spinal cord is increasing in human electrophysiological and clinical studies. The stimulation effects on lower limb muscles depend on the depolarization of segmentally organized posterior root afferents and, thus, the rostro-caudal stimulation site. In previous studies, selective stimulation was achieved by varying the positions of single self-adhesive electrodes over the thoracolumbar spine. Here, we developed a multi-electrode surface array consisting of 3 × 8 electrode pads and tested its stimulationsite specificity. The array was placed longitudinally over the spine covering the T10-L2 vertebrae. Two different hydrogel layer configurations were utilized: a single layer adhered to all electrode pads of the array and a configuration comprised of eight separate strips attached to the three transverse electrode pads of each level. Voltage measurements demonstrated that an effectively focused field distri-
Encyclopedia of Computational Neuroscience, 2013
Finite element modeling is an important computational tool in neural engineering to simulate neur... more Finite element modeling is an important computational tool in neural engineering to simulate neural excitation with implanted electrodes or with surface electrodes. Besides its importance for analyzing artificially generated neural activities, e.g., in the spinal cord , this technique is useful to interpret recorded electrical biosignals generated by neural or muscle tissue activities.
Biomedical Engineering / Biomedizinische Technik, 2000
Epidural spinal cord stimulation can produce rhythmic motor output to the lower limbs of motor co... more Epidural spinal cord stimulation can produce rhythmic motor output to the lower limbs of motor complete spinal cord injury people. The electromyographically recorded activity consists of a series of modulated stimulus time-related compound muscle action potentials (CMAPs). Here, we investigate phase dependent modification of the CMAP latencies and present a computer model that mechanistically describes putative locomotor pattern generating circuitries of the human lumbar spinal cord. Thereby we gained insight into the organization of the human spinal pattern generating networks, revealed common control characteristics with the central pattern generators for locomotion described in animal experimental work and highlighted specifities of the studied model.
Artificial organs, 2015
In this review of neurocontrol of movement after spinal cord injury, we discuss neurophysiologica... more In this review of neurocontrol of movement after spinal cord injury, we discuss neurophysiological evidences of conducting and processing mechanisms of the spinal cord. We illustrate that external afferent inputs to the spinal cord below the level of the lesion can modify, initiate, and maintain execution of movement in absence or partial presence of brain motor control after chronic spinal cord injury. We review significant differences between spinal reflex activity elicited by single and repetitive stimulation. The spinal cord can respond with sensitization, habituation, and dis-habituation to regular repetitive stimulation. Therefore, repetitive spinal cord reflex activity can contribute to the functional configuration of the spinal network. Moreover, testing spinal reflex activity in individuals with motor complete spinal cord injury provided evidences for subclinical residual brain influence, suggesting the existence of axons traversing the injury site and influencing the activ...
Artificial organs, 2015
The level of sustainable excitability within lumbar spinal cord circuitries is one of the factors... more The level of sustainable excitability within lumbar spinal cord circuitries is one of the factors determining the functional outcome of locomotor therapy after motor-incomplete spinal cord injury. Here, we present initial data using noninvasive transcutaneous lumbar spinal cord stimulation (tSCS) to modulate this central state of excitability during voluntary treadmill stepping in three motor-incomplete spinal cord-injured individuals. Stimulation was applied at 30 Hz with an intensity that generated tingling sensations in the lower limb dermatomes, yet without producing muscle reflex activity. This stimulation changed muscle activation, gait kinematics, and the amount of manual assistance required from the therapists to maintain stepping with some interindividual differences. The effect on motor outputs during treadmill-stepping was essentially augmentative and step-phase dependent despite the invariant tonic stimulation. The most consistent modification was found in the gait kinem...
Artificial organs, 2015
Interest in transcutaneous electrical stimulation of the lumbosacral spinal cord is increasing in... more Interest in transcutaneous electrical stimulation of the lumbosacral spinal cord is increasing in human electrophysiological and clinical studies. The stimulation effects on lower limb muscles depend on the depolarization of segmentally organized posterior root afferents and, thus, the rostro-caudal stimulation site. In previous studies, selective stimulation was achieved by varying the positions of single self-adhesive electrodes over the thoracolumbar spine. Here, we developed a multi-electrode surface array consisting of 3 × 8 electrode pads and tested its stimulation-site specificity. The array was placed longitudinally over the spine covering the T10-L2 vertebrae. Two different hydrogel layer configurations were utilized: a single layer adhered to all electrode pads of the array and a configuration comprised of eight separate strips attached to the three transverse electrode pads of each level. Voltage measurements demonstrated that an effectively focused field distribution alo...
Neurorehabilitation and neural repair, Jan 18, 2015
The human lumbosacral spinal circuitry can generate rhythmic motor output in response to differen... more The human lumbosacral spinal circuitry can generate rhythmic motor output in response to different types of inputs after motor-complete spinal cord injury. To explore spinal rhythm generating mechanisms recruited by phasic step-related sensory feedback and tonic posterior root stimulation when provided alone or in combination. We studied stepping in 4 individuals with chronic, clinically complete spinal cord injury using a robotic-driven gait orthosis with body weight support over a treadmill. Electromyographic data were collected from thigh and lower leg muscles during stepping with 2 hip-movement conditions and 2 step frequencies, first without and then with tonic 30-Hz transcutaneous spinal cord stimulation (tSCS) over the lumbar posterior roots. Robotic-driven stepping alone generated rhythmic activity in a small number of muscles, mostly in hamstrings, coinciding with the stretch applied to the muscle, and in tibialis anterior as stance-phase synchronized clonus. Adding tonic 3...
Journal of neurophysiology, Jan 22, 2015
In individuals with motor-complete spinal cord injury, epidural stimulation of the lumbosacral sp... more In individuals with motor-complete spinal cord injury, epidural stimulation of the lumbosacral spinal cord at 2 Hz evokes unmodulated reflexes in the lower limbs, while stimulation at 22-60 Hz can generate rhythmic burst-like activity. Here, we elaborated on an output pattern emerging at transitional stimulation frequencies with consecutively elicited reflexes alternating between large and small. We analyzed responses concomitantly elicited in thigh and leg muscle groups bilaterally by epidural stimulation in eight motor-complete spinal cord injured individuals. Periodic amplitude-modulation of at least 20 successive responses occurred in 31.4% of all available data sets with stimulation frequency set at 5-26 Hz, with highest prevalence at 16 Hz. It could be evoked in a single muscle group only, but was more strongly expressed and consistent when occurring in pairs of antagonists or in the same muscle group bilaterally. Latencies and waveforms of the modulated reflexes corresponded ...
Brain : a journal of neurology, 2015
Constant drive provided to the human lumbar spinal cord by epidural electrical stimulation can ca... more Constant drive provided to the human lumbar spinal cord by epidural electrical stimulation can cause local neural circuits to generate rhythmic motor outputs to lower limb muscles in people paralysed by spinal cord injury. Epidural spinal cord stimulation thus allows the study of spinal rhythm and pattern generating circuits without their configuration by volitional motor tasks or task-specific peripheral feedback. To reveal spinal locomotor control principles, we studied the repertoire of rhythmic patterns that can be generated by the functionally isolated human lumbar spinal cord, detected as electromyographic activity from the legs, and investigated basic temporal components shared across these patterns. Ten subjects with chronic, motor-complete spinal cord injury were studied. Surface electromyographic responses to lumbar spinal cord stimulation were collected from quadriceps, hamstrings, tibialis anterior, and triceps surae in the supine position. From these data, 10-s segments...
Hearing research, 2014
Early neural responses to acoustic signals can be electrically recorded as a series of waves, ter... more Early neural responses to acoustic signals can be electrically recorded as a series of waves, termed the auditory brainstem response (ABR). The latencies of the ABR waves are important for clinical and neurophysiological evaluations. Using a biophysical model of transmembrane currents along spiral ganglion cells, we show that in human (i) the non-myelinated somatic regions of type I cells, which innervate inner hair cells, predominantly contribute to peak I, (ii) the supra-strong postsynaptic stimulating current (400 pA) and transmembrane currents of the myelinated peripheral axons of type I cells are an order smaller; such postsynaptic currents correspond to the short latencies of a small recordable ABR peak I', (iii) the ABR signal involvement of the central axon of bipolar type I cells is more effective than their peripheral counterpart as the doubled diameter causes larger transmembrane currents and a larger spike dipole-length, (iv) non-myelinated fibers of type II cells wh...
Biomedical Engineering-Biomedizinische Technik, 2012
Stimulation of different spinal cord segments in humans is a widely developed clinical practice f... more Stimulation of different spinal cord segments in humans is a widely developed clinical practice for modification of pain, altered sensation and movement. The human lumbar cord has become a target for modification of motor control by epidural and more recently by transcutaneous spinal cord stimulation. Posterior columns of the lumbar spinal cord represent a vertical system of axons and when activated can add other inputs to the motor control of the spinal cord than stimulated posterior roots. We used a detailed three-dimensional volume conductor model of the torso and the McIntyre-Richard-Grill axon model to calculate the thresholds of axons within the posterior columns in response to transcutaneous lumbar spinal cord stimulation. Superficially located large diameter posterior column fibers with multiple collaterals have a threshold of 45.4 V, three times higher than posterior root fibers (14.1 V). With the stimulation strength needed to activate posterior column axons, posterior root fibers of large and small diameters as well as anterior root fibers are co-activated. The reported results inform on these threshold differences, when stimulation is applied to the posterior structures of the lumbar cord at intensities above the threshold of large-diameter posterior root fibers.
PLoS ONE, 2014
Electrical stimulation (ES) devices interact with excitable neural tissue toward eliciting action... more Electrical stimulation (ES) devices interact with excitable neural tissue toward eliciting action potentials (AP's) by specific current patterns. Low-energy ES prevents tissue damage and loss of specificity. Hence to identify optimal stimulationcurrent waveforms is a relevant problem, whose solution may have significant impact on the related medical (e.g. minimized side-effects) and engineering (e.g. maximized battery-life) efficiency. This has typically been addressed by simulation (of a given excitable-tissue model) and iterative numerical optimization with hard discontinuous constraints -e.g. AP's are all-or-none phenomena. Such approach is computationally expensive, while the solution is uncertain -e.g. may converge to local-only energy-minima and be model-specific. We exploit the Least-Action Principle (LAP). First, we derive in closed form the general template of the membrane-potential's temporal trajectory, which minimizes the ES energy integral over time and over any space-clamp ionic current model. From the given model we then obtain the specific energy-efficient current waveform, which is demonstrated to be globally optimal. The solution is model-independent by construction. We illustrate the approach by a broad set of example situations with some of the most popular ionic current models from the literature. The proposed approach may result in the significant improvement of solution efficiency: cumbersome and uncertain iteration is replaced by a single quadrature of a system of ordinary differential equations. The approach is further validated by enabling a general comparison to the conventional simulation and optimization results from the literature, including one of our own, based on finite-horizon optimal control. Applying the LAP also resulted in a number of general ES optimality principles. One such succinct observation is that ES with long pulse durations is much more sensitive to the pulse's shape whereas a rectangular pulse is most frequently optimal for short pulse durations. Citation: Krouchev NI, Danner SM, Vinet A, Rattay F, Sawan M (2014) Energy-Optimal Electrical-Stimulation Pulses Shaped by the Least-Action Principle. PLoS ONE 9(3): e90480.