Charles J. Heckman - US grants

9419528 Heckman The research proposed in this application concerns a quantitative understanding of the dynamics of skeletal muscle. The project has two major objectives: (1) to continue the development of a model of muscle firmly based on biophysics and biochemistry, and (2) to test and validate the model in carefully controlled animal experiments. The model is a mathematical approximation to A.F. Huxley's biophysical cross-bridge theory of contraction. Current models for a most important element of the neuromuscular control system-its actuator, skeletal muscle-have tended to be rather crude viscoelastic analogies with only the most tenuous connection to modern biological concepts about how the muscle works. A state-variable model of a skeletal muscle fiber as a direct mathematical approximation to biophysical cross-bridge models of the type introduced by A.F. Huxley will continue to be developed in this project. This model is call the Distributed- Moment Model of muscle because it emphasizes the important role of the moments of the actin-myosin bond-distribution function. These moments have macroscopic interpretations as muscle stiffness, force, and elastic energy. Thus it is of particular significance that this model establishes a direct connection between the macroscopic behavior of muscle and contractile mechanisms believed to be operating at the molecular level. The project is a joint effort with Heckman at Northwestern University (grant number 9318631). ***

DESCRIPTION (Adapted from applicant's abstract) : The goal of this proposal is to test the hypothesis that monoaminergic input acts to preserve the normal threshold hierarchy among spinal motoneurons in a pool. To achieve this goal, investigations of the effects of subtype-specific monoaminergic agents on motoneuron intrinsic properties and synaptic inputs are proposed. The decerebrate cat preparation is utilized because it has both tonic activity in descending monoaminergic tracts and normal motoneuron recruitment. Reversible cold block of the spinal cord is used to eliminate endogenous monoaminergic activity. Intracellular single electrode voltage clamp (SEVC) methods provide measurements of motoneuron intrinsic properties and synaptic inputs. Our aims test the following specific hypotheses: Aim 1: that monoamines act to preserve the normal pattern of synaptic input to motoneurons. Aim 2: that monoamines generate plateau potentials and bistable behaviors in motoneurons without distorting the normal S vs. F threshold differences. Aim 3: that monoamines influence the interaction between synaptic inputs and intrinsic properties of type S and F motoneurons by allowing synaptic inputs to generate plateau potentials in dendrites even when the soma is voltage clamped. These studies have direct relevance to spinal cord injury, where loss of tonic monoaminergic input may contribute to the abnormal motor outflow seen in human patients.

DESCRIPTION (Adapted from the Applicant's Abstract): The role of muscle is to generate the forces required for movements. Thus, the long term objective of this laboratory is to understand how different muscle properties actually work together to produce the forces required for normal movements. This is an area where little is known because most studies of muscle either characterize its diverse mechanical properties or investigate their molecular mechanisms. The main thesis of this proposal is that a lack of understanding of how muscle properties behave during normal activation is the primary limit to understanding their roles in normal movements. The experiments focus on two of the most fundamental properties of muscle, the length-tension (L-T) and force-velocity (F-V) functions. Norman activation consists of recruitment and rate modulation of motor units, whereas the L-T and F-V functions are typically measured during maximal tetanic stimulation of either whole muscle or single muscle fibers. In Aim 1, the goal is to obtain the first measurements of the L-T and F-V functions during normal recruitment and rate modulation of motor units. An areflexive animal preparation has been developed for this purpose. Preliminary data show that both functions are much steeper at low recruitment and rate levels than would be expected from their tetanic behaviors. Aim 2 seeks to understand how this occurs by measuring the L-T and F-V functions of single motor units at rates that correspond to the physiological range. In Aim 3, the role of the L-T and F-V functions during a variety of dynamic changes in muscle length are investigated. Activation is again supplied by a normal pattern of recruitment and rate modulation in the areflexive preparation. In these conditions, several muscle properties can contribute to force generation, but it is expected that most of the force modulations can be accounted for by the L-T and F-V functions seen during the appropriate level of natural activation. The results of these studies are expected to show that natural activation patterns play a key role in shaping the mechanical output of muscle. This information is important for understanding how muscle is used in motor control in both normal and pathological states.

DESCRIPTION (provided by applicant): Neural processing in the vertebrate spinal cord is critically dependent on neuromodulatory input from the brainstem, which is dominated by the monoamines serotonin (5HT) and norepinephrine (NE). In motoneurons, monoamines act via G-protein coupled pathways to facilitate a very large persistent inward current (PIC) that is generated primarily in the dendrites. This PIC dominates the motoneuron's electrical behavior, amplifying synaptic input as much as 5 to 10 fold and allowing generation of long lasting behaviors like plateau potentials. In essence, the cell is switched from a state of passive dendritic integration to having its behavior dominated by highly active dendrites. Monoaminergic input to the cord is very diffuse, affecting many motor pools simultaneously. This highly excitable state has been considered to be very stable. Does such a generalized state of high excitability cause widespread co-contraction? We propose instead that the net effect of monoaminergic input is to dramatically increase the sensitivity of motoneurons to a very specific input, the reciprocal inhibition evoked by antagonist muscle length changes. Because reciprocal inhibition suppresses the PIC, the degree to which dendrites integrate actively or passively becomes highly sensitive to joint rotation. As a result, the descending monoaminergic systems should promote not co-contraction but reciprocal movement patterns. All studies are carried out using voltage clamp techniques in an in vivo preparation with a natural level of brainstem monoaminergic input. In Aim 1, we consider what types of inhibition can control the PIC. Aim 2 focuses on how excitatory and inhibitory synaptic inputs interact in the control of the PIC. In Aims 3 and 4, we combine natural 3D movements of the hindlimb generated by a robotic arm with voltage clamp of motoneurons. This novel approach allows us to directly measure the coupling between muscle length and active dendritic integration via the PIC. The results will be directly relevant to spinal injury, in which monoaminergic input is severely disrupted. Drugs that mimic monoaminergic actions may restore the delicate balance required to control the interaction between movement and motoneuron electrical properties.

Normal function within spinal circuits depends on precise regulation of baseline levels for neuron excitability. Perhaps the most important sources of this regulation are the monoaminergic tracts that originate in the brainstem and release either serotonin or norepinephrine in the cord. These two neuromodulators, which act via intracellular second messenger systems, have potent effects that can be either excitatory or inhibitory, depending on the receptor subtype of the target neuron. Our recent data suggest a new organizing principle for monoaminergic actions on spinal interneurons. We postulate that interneurons with narrow receptive fields receive monoaminergic excitation while those with broad receptive fields receive strong inhibition. We test this hypothesis for the interneurons that process information on muscle length, which is generated by muscle spindle group Ia and II afferents. The Ia interneurons mediate reciprocal inhibition between antagonist muscles and we hypothesize that these cells have narrow receptive fields and receive monoaminergic excitation. In contrast, the receptive fields of group II interneurons are hypothesized to be broad and their monoaminergic input to be inhibitory. Interneuron discharge patterns are recorded extracellularly in an in vivo preparation with tonic activity in monoaminergic fibers. In aim I, systematic measurements of Ia and II interneuron receptive fields are obtained from controlled length changes of many different muscles and from precise joint rotations. These studies may show that II interneurons calculate net length changes for the whole limb, while Ia interneurons specify length only for rotation of individual joints. Because length feedback via these interneurons plays a fundamental role in controlling muscles, the monoaminergic system may exert differential control on mechanical properties of single joints versus the whole limb. In aim 2, the effect of altering tonic monoaminergic drive is assessed, with increases achieved via stimulation of the brainstem and decreases via a reversible cold block. The results are especially important for understanding the spasticity and other deficits accompanying spinal injury. Injury mediated loss of excitation to Ia interneurons would suppress the normal reciprocal relations between antagonist muscles. At the same time, the debilitating spasms that afflict spinal cord injured patients may be caused by the loss of inhibition of group II interneurons. This mechanism would account for the tendency of these spasms to rapidly spread throughout an entire limb. In aim 3, the specific receptor subtypes that mediate monoaminergic excitation of Ia interneurons and inhibition of II interneurons are evaluated. These studies are likely to lead to novel pharmacological strategies to help restore normal function in spinal cord injury and stroke.

[unreadable] DESCRIPTION (provided by applicant): One of the leading hypotheses for motoneuron degeneration in ALS is excitotoxicity, in which excessive calcium entry leads to cell death. Most work on excitotoxicity has focused on ligand-gated channels activated by the excitatory neurotransmitter glutamate. In contrast, this proposal focuses on the possibility that ALS alters voltage-gated channels and thus alters the intrinsic excitability of the motoneuron. We have shown that a specific type of sodium (Na+) channel is markedly elevated in motoneurons cultured from a transgenic mouse model of ALS (the SOD1 model). This current is NaP, the persistent component of the total Na+ current generating the action potential. NaP is a major factor controlling the number of action potentials per time generated in response to a given amount of synaptic input. Because each action potential allows calcium to enter the cell, elevated NaP in ALS motoneurons could play a major role in excitotoxic death. The goals of this proposal are to investigate mechanisms of the aberrant upregulation of NaP in mutant SOD1 motoneurons and to assess how drugs that change NaP influence motoneuron survival. Studies are carried out in culture, in vitro in a slice preparation, and in vivo in the intact animal, all using the mutant SOD1 mouse. Aim 1 considers the issue of whether molecular subtypes or densities of the Na channels themselves change. Aim 2 focuses on potential changes in the regulation of NaP in response to acute and chronic drug administration. The monoamines serotonin and norepinephrine enhance NaP and likely play a particularly important role in its normal regulation. In Aim 3, the effects of NaP-specific drugs studied in aim 2 are evaluated for their effect on motoneuron survival both in culture and in the intact mouse. A key question addressed by Aims 2 and 3 is whether the monoamines further increase NaP above its already high levels in mutant SOD1 motoneurons. If so, then standard anti-depressant drugs may actually exacerbate motoneuron degeneration. In Aim 4, we evaluate whether, as predicted from our cell culture work, NaP is upregulated at a very early stage in life. Presence of enhanced NaP in very young animals would indicate that this aberrant property may play a significant role in the disease onset. These studies will play an essential role in determining if NaP makes an important contribution to motoneuron degeneration in ALS. Moreover, the results may prove invaluable in establishing new therapeutic strategies [unreadable] [unreadable]

DESCRIPTION (provided by applicant): The motor unit is the fundamental element of motor output and consists of a motoneuron and the muscle fibers that its axon innervates. Muscle fiber twitches are normally 1-to-1 with motoneuron action potentials and thus the motor unit is a single functional entity. Despite this, most studies, both experimental and simulation, tend to focus either on motoneurons or on muscle. This separation of focus has sharply limited understanding of motor outflow in both normal and pathological states. To bridge this gap, this proposal seeks to develop a highly realistic and thoroughly validated computer simulation of the set of motor units for a single muscle. We focus on hindlimb extensors in the cat, for which the most complete experimental database is available. The key issue limiting previous efforts at simulating motor units is the lack of understanding of the effects of neuromodulators on conversion of synaptic input to spiking outputs in motoneurons. Systematic studies in our lab and many others have now identified these neuromodulator effects, and found them to be remarkably strong in influencing motoneuron excitability. The most potent of all are serotonin (5HT) and norepinephrine (NE), which are released in the spinal cord by axons originating in the brainstem. 5HT and NE facilitate persistent inward currents (PICs) in the dendrites of motoneurons, which then amplify synaptic input by as much as 5-fold. We have successfully developed an initial model of the motoneuron with PICs. Moreover, we have successfully developed a good muscle model for representing muscle units. In Aim 1 of the proposed work, these initial models are further developed, carefully validated against experimental data and expanded into the set of more the 200 members needed to accurately represent the full motor pool and muscle. In Aim 2, we use the simulated pool/muscle to investigate the structure of motor outflow, focusing on how neuromodulatory inputs alter overall system gain as well as influence details like motoneuron firing patterns and noise fluctuations in force. This model has great potential for use in a wide range of simulations of motor control, but simulations that involved multiple sets of neurons and multiple muscles require computational efficiency. Thus in Aim 3, we investigate several different approaches for simplifying the full set of 100s of motor units to achieve great increases in computational speed. Successful completion of these aims will provide a biologically realistic model of motor output that can be used in a wide range of computational studies of the neural control of movement. These simulations can be used to generate deep insights into the structures of motor commands and to identify deficits in motor systems in disease states like spinal injury. In the long term, we hope to develop a user interface to allow widespread use by the motor control community.

DESCRIPTION (provided by applicant): Spinal motoneurons provide the output for all motor commands and thus receive inputs from many different motor systems in the CNS, both supraspinal and sensory. Motoneurons connect directly to muscle fibers and motoneuron action potentials are one to one with their muscle fiber action potentials. Because muscle fiber action potentials are easy to assess in human subjects, motoneurons are the only neurons in the CNS whose firing patterns can be readily measured in human subjects. These motoneuron firing patterns have the potential to provide deep insights into function in CNS motor systems in humans, in both normal and disease states. Over that past 9 years, a series of international meetings on motoneurons have been implemented with the goal of bringing together researchers who study cellular mechanism of motoneuron firing patterns in animal preparations with researchers who study motoneuron firing patterns in human subjects. This proposal is for travel support for trainees (post-docs and students) to attend the 6th meeting in this series, entitled "Towards Translational Research in Motoneurons". These motoneuron meetings have alternated between the United States and Europe from their inception in 2000. The interactions at these meeting have been essential in a number of advances that now bring the field to the threshold of true translational work to develop new therapies. This is the primary focus of the meeting that will occur at Universit[unreadable] Paris Descartes, Paris, France in the summer of 2010 (July 9-13). The meeting has become very popular and offers many opportunities for presentations and discussions for trainees. PUBLIC HEALTH RELEVANCE: Motoneurons are the only cells in the CNS whose firing pattern can be measured in human subjects. The 6th international meeting on motoneuron, which will take place in the summer of 2010 at the University of Paris, provides trainees with many opportunities for presentation and discussion motoneurons in normal and disease states. This proposal seeks travel support for this purpose.

DESCRIPTION (provided by applicant): This proposal focuses on identifying functional deficits in motoneurons as they degenerate in a mouse model of ALS, based on the recently published and preliminary results indicating that motoneuron properties that normally specify their activation patterns may play a key role in their degeneration. We focus especially on motoneuron size. The motoneuron normally functions as the central component of a motor unit, which consists of the motoneuron, its axon and the muscle fibers innervated. Thus size involves not just the cell body but also the dendrites (which reflect number of inputs) and axon terminal branches (which is proportional to number of innervated muscle fibers). Normally, motoneurons are activated from small to large: type S motor units are small in terms of motoneuron anatomy and number of muscle fibers, all of which are slow. Progressive larger and faster motor units follow (type FR and FFs). Yet studies of the denervation of muscle fibers in the periphery in a standard animal model of ALS, the mutant SOD1 mouse, indicate that initial failure to generate force occurs in the opposite sequence: FF > FR >S, i.e. from large to small. This reverse sequence suggests excess size is a deficit that contributes to degeneration and indeed we have recently been surprised to find that mutant SOD1 motoneurons began to grow excessively at a very young age, before 10 days of birth. This is long before the first FF motor units begin to fail in force generation (about 50 days) and even longer before classic symptom onset (90 days). Remarkably, the intrinsic electrical properties of these larger cells are also distorted, potentially leading to a combination of metabolic and excitotoxic stress. In addition, changes in the structure of input could occur. To investigate the relations between size, intrinsic excitability and synaptic input requires intracellular study of mouse motoneurons in the adult state. We have developed 3 new preparations that allow the first intracellular studies of motoneuron in the adult state for sacral lumbar and brainstem motoneurons. Two are in vitro, allowing systematic drug studies while one is in situ, allowing direct comparison of motoneuron electrical properties to its mechanical properties. Thus the in situ prep studies will identify the properties of the motoneuron as it undergoes force failure. Aim 1 uses the in situ preparation to test the hypothesis that excess size predicts the pattern of force failure. Aim 2 uses an in vitro sacral cord preparation to asses whether there is parallel upregulation in intrinsic electrical properties and inputs to match the distortion in size, while Aim 3 uses brainstem slice to see if these smaller motoneurons undergo the same pattern. In Aim 4, chronic drug administration is used to determine if alterations in electrical properties cause changes in size. Overall, this work constitutes a new approach to study of mechanisms of ALS.

DESCRIPTION (provided by applicant): In spinal injury, the loss of descending input not only impairs motor commands but also damages descending control of spinal excitability. The goal of this proposal is to understand the cellular and synaptic mechanisms of the resulting distortions in the behavior of spinal circuits, which can generate aberrant muscle activation patterns and unintended movements like spasms. To achieve this goal, we use our chronic spinal cat preparation to develop a novel translational approach, in which we link cellular mechanisms to system behaviors. Our results potentially provide clear predictions for experiments to evaluate whether these same mechanisms occur in human spinal injured subjects. In the intact state, considerable data show that the effects of sensory inputs on motor outputs are focused, reciprocal and consistent. These effects are focused in the sense that sensory input from one joint primarily induces motor output at that same joint, reciprocal in that sensory inputs generate opposite actions on antagonists and consistent in that sensory inputs generate stable responses to repeated activations. The basic concept underlying this proposal is that the loss of descending control of spinal excitability induces precisely the opposite sensory processing state, one that is diffuse, co- active and inconsistent. Although likely overly simple this concept provides clearly testable hypotheses and moreover is supported by significant data, including our recent preliminary studies. The translational nature of our experimental design arises from two novel techniques that link intracellular measurements to the real world: 1) synaptic currents are measured during voltage clamp in response to precise movements of the entire hindlimb via a 6 degree of freedom robotic arm and 2) firing patterns from intracellular recordings are compared to firing patterns of populations of motor units recorded by a newly developed electrode array placed on muscle. Both the robotic and the array techniques are already in use in human subjects, thus providing the basis for our proposed predictions for human experiments. Aim 1 seeks to identify the mechanisms of expanded receptive fields by identifying the types of sensory afferents involved. Aim 2 examines the balance of excitation versus inhibition, primarily relying on intracellular recordings. Aim 3 investigates wind-up, both n terms of quantifying its strength and in terms of determining its source in motoneurons versus interneurons. The proposed experiments will provide a new depth of understanding of the distortions in spinal processing of sensory input that emerge in spinal injury. Each aim will provide information critical for developing specific therapeutic interventions, focusing on restoration of normal functional connections and minimization of wind- up.

DESCRIPTION (provided by applicant): All movements are controlled by the graded activation of different muscles. Muscle activation is controlled by the activation of motoneurons in the brainstem and spinal cord. Each motoneuron drives the muscle fibers it innervates in a one-to-one fashion. Muscle fiber action potentials are relatively easy to record in human subjects and it has long been appreciated that the firing patterns of motor units provide a unique window on the properties of motoneurons and their inputs. The goal of this proposal is to deduce the pattern of inputs to motoneurons based on an analysis of their output. The success of this reverse engineering approach depends upon two factors: (1) the existence of accurate models of the input-output properties of a population of motoneurons, and (2) the ability to extract the maximum amount of information from motoneuron output, based upon recording the activity of multiple, simultaneously-active motor units. We have recently developed a set of motoneuron models that can replicate several key features of the input-output properties of motoneurons with different intrinsic excitability. When these models are driven with different patterns of excitatory and inhibitory inputs they reproduce a number of the features of the discharge of human motor units during voluntary contractions. Previously, recording the behavior of multiple units required combining recordings taken across multiple sessions and subjects. New developments in recording and analysis techniques now make it possible to record from 10 or more motor units in each trial. This provides a rich data set that can be used to constrain the set of input patterns that give rise to a given pattern of output. The overall hypothesis of this proposal is that the firing patterns of a population of motoneurons are determined by the patterns of their synaptic inputs and the level of neuromodulatory drive, making it possible to deduce input patterns from the firing patterns of multiple motor units. We will test this hypothesi by comparing our estimates of input based on the reverse engineering approach with direct voltage-clamp measurements of the inputs in the same experimental preparation. There are three specific aims (1) to improve techniques for recording the simultaneous activity of multiple motor units, (2) to validate our reverse engineering approach by comparing synaptic input patterns that are predicted from recordings of motor unit discharge to those that are directly recorded from motoneurons, and (3) to determine the role of synaptic inhibition in shaping motor unit discharge patterns, and the ability of our reverse engineering approach to detect different patterns of inhibition. Once successfully developed in our animal preparation, the reverse engineering methods can be adapted for use in human subjects, allowing maximal use of the rich information available in motor unit firing patterns for understanding the structure of motor commands in humans in both normal and pathological states.

PROJECT SUMMARY/ABSTRACT All motor output is generated by motoneurons and consequently their firing patterns contain detailed information about the structure of motor commands. Remarkably, this information is accessible in humans, because motoneuron spikes are 1 to 1 with those of their muscle fibers. Our overall concept is that motoneuronal firing patterns vary systematically across the muscles of the human body and that this variation reflects fundamental connections between the synaptic organization of motor commands, the structure of the musculoskeletal system and the diversity of motor tasks. To understand these connections, our overall goal is to create a detailed ?motor output map? of the human body, using newly developed array electrodes. The array electrodes are placed on the skin and are capable of measuring the firing patterns of up to 30 motor units simultaneously in the underlying muscle. We plan further technical development of these arrays to expand the range of motor tasks they can be used for. The human motor output map will be created from the analysis of firing patterns of populations of motor units in muscles throughout the body for matched motor tasks. We will interpret the resulting map in light of our recent advances in understanding of how firing patterns of motoneurons are determined by the organization of their synaptic inputs. The effects of excitatory and inhibitory inputs on firing patterns are fundamental for generating firing patterns, but neuromodulatory inputs from the brainstem are equally if not more important. These neuromodulatory inputs release serotonin and norepinephrine, which have a profound influence on the intrinsic excitability of motoneurons and thus control how motoneurons process their excitatory and inhibitory inputs. In Aim 1, we create a basic version of the human motor output map by asking subjects to generate slow linear increases and decreases in torque for more than 20 muscles across the body. The protocol is kept exactly the same across muscles to allow comparisons of the resulting firing patterns. Our primary hypothesis is that muscles involved in stabilization of the body, such as proximal muscles, will generate motor unit firing patterns consistent with high levels of neuromodulatory drive, while muscles involved more in precision tasks will generate patterns consistent with low levels of neuromodulation. These experiments are essentially function anatomy, the proximal-distal variations in firing patterns probably arise from differences in the anatomical projections of synaptic inputs to motor pools. In Aim 2, we assess whether there are task dependent changes in firing patterns, such as increases in neuromodulation drive with increased effort and increases in sensory inhibition with movement. Taken together, these experiments will define the fundamental structure of motor output for the human musculoskeletal structure and provide a quantitative basis for understanding the distortions that occur in disease states like spinal cord injury.

Multiple mechanisms has been proposed for the selective vulnerability of motoneurons in neurodegenerative diseases. In reflecting on the prior work from our laboratories, as well as that of our colleagues around the world, we have developed a synthetic hypothesis that accounts for a vast majority of the reported findings. We propose that the net response of mouse motoneurons to the presence of mutant proteins is a disregulation of homeostatic plasticity. This manifests as an increased `gain' of both the up- and down-regulation of compensatory mechanisms designed to control the level of motoneuronal activity. The toxic increase of gain function leads to overcompensation and a dramatic cascade of homeostatic oscillations that increases motoneuron morbidity. Further, we propose that size-scaling of these compensatory mechanisms leads to the observed greater vulnerability of the largest motoneurons. The goal of this project is to provide rigorous testing of this novel disregulation hypothesis using mutant SOD1 mice as a model system for neurodegenerative diseases that disproportionately target motoneurons. The proposed experiments rest heavily on our recent technical breakthroughs that enable us to perform intracellular recordings of mouse motoneurons throughout disease progression, from neonate through adult, using both in vivo and in vitro preparations, as well as our expertise in assessing the density and spatial distributions of membrane channels in motoneurons. Our approach entails presenting a series of `homeostatic challenges' to motoneuron excitability and comparing the compensatory responses of mSOD1 motoneurons to those of wild-type controls. If our hypothesis is correct, we expect to observe that mSOD1 motoneurons exhibit consistently greater responses to each of the challenges than do wild-types and that these mSOD1 responses scale with motoneuron size. There are three specific aims: to assess the responses of mSOD1 and control motoneurons to drug perturbations that alter the intrinsic electrical properties of motoneurons (Aim 1), the synaptic inputs to motoneurons (Aim 2) and the neuromodulatory inputs to motoneurons (Aim 3). The resulting data will provide a strong impetus for pursuing radical, novel therapeutic strategies as well as for elucidating the specific signal transduction cascades underlying the different homeostatic mechanisms.

The neural circuitry of the spinal cord has a unique, repetitive structure that forms an especially promising target for control via electrical stimulation. Furthermore, this structure allows the essential circuits for generation of movements to be preserved below the level of a spinal cord injury (SCI). Electrical stimulation techniques targeted at these remaining sensorimotor circuits are thus becoming highly promising therapies. These approaches usually take advantage of another basic aspect of spinal anatomy, that all sensory axons enter the cord via a highly accessible location, its dorsal surface. Thus dorsal electrical stimulation (DES) via surface electrodes provides effective activation of sensory axons without the need for penetrating electrodes. The spinal connections of these sensory axons mediate potent effects on spinal motor circuits. In this proposal, we examine the neural mechanisms of DES to clearly define its potential for controlling motor output and to create a rational basis for improving its therapeutic implementation. The basic goal of DES is to recreate key functions of the descending inputs from the brain to the cord, which are of course damaged or lost in SCI. Thus a fundamental question is, how well can DES of sensory axons replicate the effects of descending inputs on spinal neurons. To address this question, we focus on the canonical motor microcircuit (CMM), which comprises a single set of antagonist motor pools and the local circuits that process their sensory feedback about muscle length and velocity. The group Ia axons conveying this information are large and likely to be more sensitive to DES than any other type of sensory input. We apply multiple techniques, including intra-axonal recording in sensory axons, extracellular recording of interneurons and voltage clamp in motoneurons. Our Aims are to map the distribution of excitatory and inhibitory synaptic input generated in the CMM by DES, identify the roles of the intrinsic electrical properties of spinal neurons in processing these inputs, assess whether DES activation of sensory axons interferes with their normal function and probe the mechanism that underlie the stability and focus of the CMM when driven by DES or normal sensory inputs. The proposed studies will provide a fundamental underpinning for DES of the spinal cord and are likely to identify new opportunities for improvement its therapeutic effectiveness.

Project Summary The goal of this proposal is to implement a novel Research Training in Sensorimotor Neurorehabilitation program to prepare the next generation of scientists that can address the growing need for neuroscience- based rehabilitation solutions. Northwestern University has a long and recognized history in the study of motor control, motor disability and recovery, and neural reorganization; and in training the next generation of movement and rehabilitation scientists (MRS) through the Northwestern University Interdepartmental Neuroscience Program (NUIN) PhD program since 2009, with demonstrated success reflected in more than 20 graduates currently leading successful research careers in academia and industry. The proposed program is built on this research and training record with the mission to train students with clinical and life/applied science backgrounds and post-doctoral fellows with clinical backgrounds to become rehabilitation scientists in basic, translational or clinical research. These scientists will have the ability to integrate knowledge from the various disciplines involved in sensorimotor neurorehabilitation, including neuroscience, engineering and clinical sciences. The training program will focus on four training goals: 1) to provide a thorough grounding in the basic neurobiology of sensorimotor function and control, including translation of insights between animal and human research models; 2) to learn fundamental engineering experimental methods for measurement and analysis of neuronal and musculoskeletal sensory and motor function; 3) to facilitate knowledge sharing in the clinical management of individuals with movement disorders between the post-doctoral and pre-doctoral trainees; and 4) to provide professional development training to support future rehabilitation research career success. The unique structure of the program, couples training of post-doctoral trainees with clinical expertise with pre-doctoral trainees focusing on neuroscience. The program is the first of its kind to provide post-doctoral research training of Doctors of Physical Therapy (DPT), an untapped cohort of prospective scientists that has emerged in the contemporary age of science-based medicine and evidence-based practice. These DPT candidates for post-doctoral slots will be drawn from research intensive Physical Therapy Programs across the United States. Pre-doctoral trainees will be drawn from the NUIN-MRS program. The translational interdisciplinary nature of the program will allow close interaction between clinical investigators and basic and applied sciences investigators, providing a unique opportunity for training in translational research. We intend to support a total of 12 pre-doctoral trainees and 5 post-doctoral trainees with NIH funding for up to two years each. Additional institutional support is available in the form of tuition and stipend supplements for trainees and diversity fellowships to support diversity recruiting efforts.