Hip viscosupplementation under ultra-sound guidance riduces NSAID consumption in symptomatic hip osteoarthritis patients in a long follow-up. Data from Italian registry

INTRODUCTION

Non-steroidal anti-inflammatory drugs (NSAIDs) consumption is strictly related to a high gastrointestinal and cardiovascular mortality and morbidity rate. Osteoarthritis Research Society International (OARSI) recommendations in patients with symptomatic hip or knee OA stated that NSAIDs should be used at the lowest effective dose but their long-term use should be avoided if possible. OARSI guidelines for the treatment of the hip OA include the use of viscosupplementation, which aims to restore physiological and rheological features of the synovial fluid.

OBJECTIVE

Aim of this multicentric, open and retrospective study is to investigate if NSAID consumption may be reduced by the use of ultrasound-guided intra-articular injection of several hyaluronic acid (HA) products in hip joint administered in patients affected by symptomatic hip OA.

MATERIALS AND METHODS

Patients affected by mono or bilateral symptomatic hip OA according to American Rheumatology Association (ARA) criteria, radiological OA graded II-IV (Kellgren and Lawrence) entered the study and were administered with ultrasound-guided intra-articular injection of hyaluronic acid products. As a primary endpoint, consumption of NSAIDs was evaluated by recording the number of days a month (range 0-30) the patient had used NSAID during the previous month, reported at each visit during the 24 months follow-up period. Secondary endpoints included further analysis for subgroups of patients categorized for Lequesne index score, Kellgren-Lawrence score, pain visual analogue scale (VAS) score, ultrasound pattern, age, hyaluronic acid used.

RESULTS

2343 patients entered the study. Regarding primary endpoint, the consumption of NSAIDs was reduced of 48.2% at the third month when compared with baseline values. This sparing effect increased at 12th and 24th month with a reduction respectively of 50% and 61% in comparison to baseline values. These differences were statistically significant.

CONCLUSIONS

These data point out that intraarticular hyaluronan preparations provide OA pain relief and reduce NSAIDs consumption in a large cohort of patients for a long period of follow-up. Multiple courses of viscosupplementation (vs) are required to maintain low dose of NSAID consumption over time. NSAIDs consumption is strictly related to an high gastrointestinal and cardiovascular mortality and morbidity rate, instead HA intra-articular treatment is well tolerated and is associated with a low incidence of adverse effects. For these reasons further studies evaluating cost-effectiveness and cost-utility of VS in the management of hip OA are required.

Viscosupplementation in the management of ankle osteoarthritis: a review

INTRODUCTION

Osteoarthritis (OA) is a disease of synovial joints and is the most common cause of chronic pain. Viscosupplementation (VS) with hyaluronic acid (HA) is largely used for knee osteoarthritis therapy but the evidence for its usefulness in ankle osteoarthritis is limited. The objective of this review is to assess the efficacy of viscosupplementation treatment of ankle osteoarthritis in the current literature.

METHODS

The following databases were searched: Medline (period 2006-2008), Database of Abstract on Reviews and Effectiveness and Cochrane Database of Systematic Reviews. Reference lists of relevant articles were controlled for additional references. The search terms Review, Viscosupplementation (VS), Osteoarthritis (OA), Hyaluronic acid (HA), Hyaluronan, Sodium hyaluronate, Ankle OA, Ankle joint were used to identify all studies relating to the use of VS therapy for the ankle OA. Methodological quality of included studies was assessed by assigning level of evidence as previously defined by the Centre for Evidence Based Medicine (CEBM).

RESULT

Seven articles concerning the efficacy of a total of 275 patients undergoing VS treatment for ankle OA were included. One European study, one Taiwanese study, one Italian study, one Turkish study and three American studies with level of evidence ranging from I to IV evaluated the following products: Hyalgan, Synvisc, Supartz, Adant.

CONCLUSION

Viscosupplementation is used widely in knee OA and is included in the professional guidelines for treatment of the disease in this joint. The potential for treating osteoarthritis of the ankle joint by viscosupplementation has been suggested in the literature, however, no dosing studies have been published to date, and dosing in the ankle joint remains an area for discussion. Viscosupplementation could potentially provide an useful alternative in treating such patients with painful ankle OA.

Can Cyclosporine-A associated to methotrexate maintain remission induced by anti-TNF agents in rheumatoid arthritis patients? (Cynar pilot study)

Biological therapies, such as etanercept, adalimumab and infliximab, have demonstrated good efficacy in inducing rheumatoid arthritis to low disease activity levels. Nevertheless, their cost, as well as the related risk of side effects, especially in long-term therapies, are still high. Furthermore, there is a good deal of evidence proving loss of efficacy of such therapies in the long term, often necessitating the shift from one specific anti-TNF biological treatment to another. There are also other open debates on the amount of time a patient should undergo an anti-TNF therapy, on the possibility of inducing a complete remission in early arthritis and, once remission or low disease activity is obtained, on the possibility of interrupting the anti-TNF-based therapy. In this study we investigated whether A-Cyclosporin and Methotrexate association may be effective in maintaining low disease activity obtained by anti-TNF therapies. Twenty-three rheumatoid arthritis-affected patients, whose diagnosis was made according to ACR criteria, with a disease duration of less than 3 years, and DAS28<3.2 that reached a level of low disease activity within 6-8 months from beginning anti-TNF and Methotrexate therapy, were enrolled in the study. After the suspension of anti-TNF therapy, patients were started on A-Cyclosporine (2-3 mg/kg/day) and Methotrexate (15mg/week) therapy. DAS28, Pain VAS, Erythrosedimentation Rate (ESR), and C Reactive Protein (CRP) were all tested at time 0 and at 6 months, as well as liver and kidney profiles, after the interruption of the anti-TNF therapy and the beginning of A-Cyclosporine and Methotrexate therapy. Side effects were also recorded. Of 23 patients undergoing the A-Cyclosporin and Methotrexate therapy for maintaining low disease activity in rheumatoid arthritis obtained by 6-8 months of anti-TNF therapy, 21 completed the study with a 6 month follow-up. Thirteen patients maintained clinical parameters within low disease activity values, while 8 patients showed an increase in DAS28 and other parameters. Only two patients showed an increase in blood pressure that was diagnosed after two months from the beginning of the A-Cyclosporin and Methotrexate therapy. The reduction in the dosage of A-Cyclosporin from 3mg/kg/day to 2mg/kg/day caused a slow normalization of blood pressure values. Our data seem to suggest that more than half of the patients undergoing A-Cyclosporin and Methotrexate therapy seemed to maintain low disease activity parameters of rheumatoid arthritis, obtained after 6-8 months of anti-TNF therapy. Further studies on larger populations are necessary in order to confirm such results and identify predictor factors for different responses.

Eye-head coordination in monkeys: evidence for centrally patterned organization

Eye-head coordination was investigated by recording from the neck and eye muscles in monkeys. The results show that (i) during eye-head turning, neural activity reaches the neck muscles before the eye muscles, and (ii) all agonist neck muscles are activated simultaneously regardless of the initial head position. Since overt movement of the eyes precedes that of the head, it was concluded that the central neural command initiates the eye-head sequence but does not specify its serial order. Furthermore, it was determined that the compensatory eye movement is not initiated centrally but instead is dependent upon reflex activation arising from movement of the head.

A new technical contribution for ultrasound-guided injections of sacro-iliac joints

INTRODUCTION

Sacroiliac joint (SIJ) represents a difficult location for local therapies, as intra-articular injections may be hard to execute, especially in particular conditions such as chronic inflammatory diseases. However, in selected patients, local therapies may be considered. Some recent studies demonstrated the feasibility of ultrasound (US)-guided injection of SIJ, but still a complete explanation and definition of the technique is needed.

MATERIALS AND METHODS

Seven patients, four males and 3 females, affected by mono or bilateral sacroiliitis entered the study. Each patient received 40 mg of acetonide triamcinolone for each SIJ, intra articular (IA) US-guided injection. The technical originality proposed in this study consists in the spinal needle insertion in the middle of the cranial long side of the linear transducer with an orientation of about 10 degrees, determining shorter needle insertion for reaching joint space and consequently probably granting lesser pain and traumatism for patients.

RESULTS

A total of 22 injections was performed. The longer follow-up time obtained was 18 months in 3 patients. All patients reached at least a 6 month follow-up. All patients reported an amelioration in pain that lasted for at least 6 months. No systemic adverse events were reported or observed. Complete visualization of SIJ and of needle placement was performed by US imaging, while compound proper injection was also visualized by Color-Doppler US imaging.

DISCUSSION

Actually, sacroiliac joint intraarticular injections are often performed under fluoroscopy or Computerized Tomography guidance. Such techniques present several limitations, especially for repeated injections, such as the use of ionizing radiations, the need of a contrast agent and the direct and indirect costs connected. US guidance in IA SIJ injections may represent an easily repeatable imaging technique for needle placement and a precious tool for detecting inflammatory activity of the joint.

Intra-Articular Injections of Infliximab in the Treatment of Inflammatory Rheumatic Diseases: Case Reports and Review of Literature

Inflammatory chronic diseases involving joints together with other organs are usually treated with a systemic approach. In a few cases, where arthritis is not responsive to traditional treatments, an intraarticular (I.A.) therapy could be useful. Furthermore, patients not eligible for systemic therapy with anti-TNF or other DMARDs, as well as patients with an initial arthritis with the involvement of a single joint, such as the knee or hip joint, could use the I.A. injection therapy. In this article we report our experience with five patients affected by rheumatic inflammatory diseases, not responding to traditional systemic DMARDs-based therapies or not eligible for systemic use of biological response modifiers who underwent ultrasound-guided I.A. injection of Infliximab. Three of 5 patients showed a positive and long-lasting response to treatment with local Infliximab. Safety profile was good according to literature data. Moreover, in this article we review the literature on this therapeutic approach. This is the first report of I.A. use of Infliximab in the hip joint.

Comparative, double-blind, controlled study of intra-articular hyaluronic acid (Hyalubrix) injections versus local anesthetic in osteoarthritis of the hip

INTRODUCTION

Comparison of intra-articular bacterial-derived hyaluronic acid (Hyalubrix) (HA) with local analgesia (mepivacaine) for osteoarthritis (OA) of the hip.

METHODS

A pilot prospective, double-blind, 6-month randomized trial of 42 patients with hip OA. HA or mepivacaine was administered twice (once a month) under ultrasound guidance. Efficacy measurements included the Lequesne's algofunctional index, a visual analog scale for pain, concomitant use of analgesia, patient and physician global measurement, and safety.

RESULTS

Patients in the HA group exhibited a significantly reduced Lequesne's algofunctional index 3 and 6 months after treatment (P < 0.001) and significantly reduced visual analog scale pain scores 3 and 6 months after treatment (P < 0.05) compared with the local anesthetic group. All primary and secondary measures were significantly improved versus baseline, but other than the above were not different from each other at 3 or 6 months. Adverse effects were minimal.

CONCLUSIONS

This comparative study suggests a beneficial effect and safety of intra-articular HA in the management of hip OA.

TRIAL REGISTRATION NUMBER

ISRCTN39397064.

Viscosupplementation: a suitable option for hip osteoarthritis in young adults

INTRODUCTION

Young adult hip osteoarthritis (OA) is a noteworthy problem, although rarer than the elderly form of the disease, causing limitations in social and working activities and prospects. Treatment options are scarce and surgical procedures, frequently necessary, imply the major drawback of revising the prostheses periodically, whereas chronic nonsteroidal anti-inflammatory drugs (NSAID) consumption may provoke side effects. To explore alternative options to both surgery and long-term NSAID use, especially in the case of young patients, viscosupplementation seems to appear as an appropriate tool to relieve pain, ameliorate the function and delay surgery.

AIM OF THE STUDY

In this study we tackle the issue of the use of hyaluronic acid (HA) injections in young adults with symptomatic hip OA.

RESULTS AND CONCLUSIONS

These data, collected from 78 young patients, show that viscosupplementation is a safe procedure, and may provide significant relief from pain and functional recovery. Larger controlled studies are needed to establish otpimal treatment strategies and clinical factors predictive of treatment response.

Mixed Panniculitis Responding to Cyclosporin-A with a 12-Month Follow-up: A Case Report

Panniculitides represent a heterogeneous group of inflammatory diseases involving subcutaneous fat. Subcutaneous fat is normally organized into adipose cells, adipocytes, and septa of connective tissue. The inflammation involving such tissues can be more represented in septa (septal panniculitis) or in lobules (lobular panniculitis) or be equally distributed in both (mixed panniculitis). A bioptical study is necessary in order to discern between different forms. Vascular involvement is also different in such diseases, as it can interest arteries, or veins, or both. Different grades of fat necrosis can also be observed, such as adipocytes without nuclei, lipophagic necrosis, liquefactive fat necrosis, microcystic fat necrosis, ischaemic fat necrosis. Panniculitis can be idiopathic or secondary to other diseases such as systemic sclerosis, rheumatoid arthritis, systemic erithematous lupus and many others. Therapies usually vary on the single patient but the general orientation leads to the use of immunosuppressive drugs such as thalidomide, corticosteroids, cyclosporin-A, hydroxychloroquine and cyclophosphamide. We report a case of a 19-year-old female affected by primary mixed panniculitis, associated with fever and deep asthenia, that resolved in a few weeks and was maintained with oral cyclosporin-A.

The Safety of Anti-TNF Agents in the Elderly

Rheumatoid arthritis, ankylosing spondylitis and psoriatic arthritis are commonly thought of as inflammatory diseases that affect younger individuals. Although the initial presentation of these diseases is common in a patient's twenties or thirties, they usually persist for the duration of the patient's life. In addition, up to one-third of patients with RA have disease onset after 60 years of age. Anti-TNF-a therapies now have well-recognized safety profiles that have been demonstrated in the usual clinical trial populations for these diseases, but such populations under-represent patients > or =65 years of age. This retrospective study aims to determine the safety profiles for etanercept, infliximab and adalimumab in patients of 65 years or more, undergoing anti-TNF treatment for an active inflammatory disease such as rheumatoid arthritis, ankylosing spondylitis or psoriatic arthritis, or skin disease like psoriasis. Our data show that admitting elderly patients into anti-TNF therapeutic regimens is a safe option and that it grants these patients access to the best current therapeutic option, possibly leading to better disease outcome. Quality of life in elderly patients affected by arthritis or psoriasis, often reduced by comorbidities, is as important as quality of life in younger patients. Applying the recommended screening before using biological treatment helps to reduce adverse events related to the therapy, and the application of the same screening in elderly patients seems to lead to comparable results.

Intra-articular administration of hylan G-F 20 in patients with symptomatic hip osteoarthritis: tolerability and effectiveness in a large cohort study in clinical practice

OBJECTIVE

This prospective, observational, open study aimed to assess the efficacy and safety of hylan G-F 20 in a large cohort of patients with symptomatic hip osteoarthritis (OA), and identify predictors of clinical response.

RESEARCH DESIGN AND METHODS

Patients presenting with symptomatic hip OA received one 2 mL intra-articular (IA) injection of hylan G-F 20 under ultrasound guidance. Patients were followed-up every 3 months for a total of 12 months and were offered an optional, additional injection at each follow-up visit if symptomatically justified. At each visit, pain scores (100 mm visual analogue scale [VAS]), Lequesne index scores, NSAID intake, and physician and patient global assessments scores were recorded. Adverse events (AEs) were recorded throughout the study. MAIN OUTCOME MEASURES;

RESULTS

250 patients completed the 12 month follow-up and received a total of 734 injections. Statistically significant reductions in VAS pain scores, Lequesne index scores and NSAID usage were reported at all time-points (p < 0.05). No systemic, serious or severe side effects were observed. Fifty-two local AEs were reported (7.08% per injection) all of which were mild and transient. One predictor of clinical response was identified, with patients < 75 years of age reporting better outcomes.

CONCLUSIONS

This study supports the safety, tolerability and effectiveness of hylan G-F 20 in the treatment of symptomatic hip OA. Hylan G-F 20 may also offer economic benefits due to a reduction in NSAID usage and the resultant reduction in management costs of NSAID related side-effects. These data reflect those obtained in previous studies of hylan G-F 20 in patients with knee OA.

Relevance of 99mTc-HYNIC-tir-octreotide scintigraphy in a patient affected by sarcoidosis with lung and joints involvement and secondary Sjogren's syndrome treated with infliximab: case report

We report the case of a 59 years old woman affected by lung and joint sarcoidosis, secondary Sjogren's syndrome refractory to common disease-modifying antirheumatic drugs (DMARDs) that regressed with infliximab and methotrexate. 99mTc-HYNIC-TOC scintigraphy was useful in diagnosis, choice of treatment and follow-up.

Combining modules for movement

We review experiments supporting the hypothesis that the vertebrate motor system produces movements by combining a small number of units of motor output. Using a variety of approaches such as microstimulation of the spinal cord, NMDA iontophoresis, and an examination of natural behaviors in intact and deafferented animals we have provided evidence for a modular organization of the spinal cord. A module is a functional unit in the spinal cord that generates a specific motor output by imposing a specific pattern of muscle activation. Such an organization might help to simplify the production of movements by reducing the degrees of freedom that need to be specified.

High levels of NK cells in the peripheral blood of patients affected with anti-phospholipid syndrome and recurrent spontaneous abortion: a potential new hypothesis

OBJECTIVES

No data regarding phenotypic assets of circulating lymphocytes in anti-phospholipid syndrome (APS) are reported in the literature. Role of anti-phospholipid antibodies (aPL) in recurrent spontaneous abortion (RSA) remains uncertain, while natural killer (NK)-cells are involved in RSA pathogenesis. In this study, patients affected with APS without RSA, APS with RSA and RSA without aPL were studied for NK-cell subpopulation to evaluate its role in abortive events typical of APS.

METHODS

NK-cell levels in peripheral blood of APS patients without RSA (n = 28) and in APS-RSA patients (n = 25) were evaluated by means of flow cytofluorimetry. NK-cells levels were evaluated also in RSA without aPL associated with either endocrine (n = 86), anatomic (n = 30) or idiopathic (n = 77) conditions and in 42 healthy women.

RESULTS

High NK levels were found in 14/25 (56%) APS-RSA patients. Among these patients, all except one aborted before the 10th gestational week (GW), while among the remaining patients all except one aborted after the 10th GW. NK mean levels were significantly higher in APS-RSA than in all the other conditions studied, including healthy subjects, except idiopathic RSA.

CONCLUSIONS

Our results demonstrate that the numbers and proportions of NK-cells are significantly higher in patients with RSA with APS than in APS without RSA. Increased numbers of NK-cells correlate with reduced gestational age at abortion in patients with APS-RSA. These data lead to a hypothesis that NK-cells contribute to the development of RSA in patients with APS. NK-cells might precipitate damage initiated by aPL or they might cause pathology in RSA independent of aPL.

Multiple chemical sensitivity syndrome in Sjögren's syndrome patients: casual association or related diseases?

Multiple chemical sensitivity (MCS) is defined by multiple symptoms, affecting multiple organs, that wax and wane in response to varying chemical exposures at or below previously tolerated levels. Sjögren's syndrome (SS) is a common autoimmune disease affecting 3% of women aged over 55 years. Except for keratoconjunctivitis sicca (which is associated with SS not MCS), systemic features are common between the 2 diseases, leading to considerable morbidity and, occasionally, mortality. The authors report 3 cases of association between SS and MCS. Three women who were diagnosed with SS showed MCS symptoms and also were diagnosed with MCS. Further studies are needed to understand physiopathogenic mechanisms that eventually may be revealed as common to the 2 syndromes.

Modular organization of spinal motor systems

The vertebrate nervous system produces a wide range of movement flexibly and efficiently, even though the simplest of these movements is potentially highly complex. The strategies by which the nervous system overcomes these complexities have therefore been of interest to motor physiologists for decades. In this review, the authors present a number of recent experiments that propose one strategy by which the nervous system might simplify the production of movement. These experiments suggest that spinal motor systems are organized in terms of a small number of distinct motor responses, or "modules." These distinct modules can be combined together simply to produce a wide range of different movements. Such a modular organization of spinal motor systems can potentially allow the nervous system to produce a wide range of natural behaviors in a simple and flexible manner.

Neuronal correlates of motor performance and motor learning in the primary motor cortex of monkeys adapting to an external force field

The primary motor cortex (M1) is known to control motor performance. Recent findings have also implicated M1 in motor learning, as neurons in this area show learning-related plasticity. In the present study, we analyzed the neuronal activity recorded in M1 in a force field adaptation task. Our goal was to investigate the neuronal reorganization across behavioral epochs (before, during, and after adaptation). Here we report two main findings. First, memory cells were present in two classes. With respect to the changes of preferred direction (Pd), these two classes complemented each other after readaptation. Second, for the entire neuronal population, the shift of Pd matched the shift observed for muscles. These results provide a framework whereby the activity of distinct neuronal subpopulations combines to subserve both functions of motor performance and motor learning.

Modulation and vectorial summation of the spinalized frog's hindlimb end-point force produced by intraspinal electrical stimulation of the cord

The ability to produce various force patterns at the ankle by microstimulation of the gray matter of the spinal cord was investigated in spinalized frogs. We evaluated the recruitment properties of individual spinal sites and found that forces increase linearly with activation level in the low-force range studied, while the structure of the force pattern remains invariant. We also measured the responses produced by coactivation of two spinal sites activated at two pairs of stimulation levels. Responses were measured at the mechanical level by recording forces at the ankle; and, at the muscular level by recording the electromyographic (EMG) activity of 11 hindlimb muscles. We found that for both pairs of activation, the forces under coactivation were the scaled vectorial summation of the individual responses. At the muscular level, rectified and integrated EMGs also summated during coactivation. Numerous force patterns could, thus, be created by the activation of a few individual sites. These results suggest that microstimulation of the circuitry of the spinal cord (higher order neurons than the motoneurons) holds promise as a new functional neuromuscular stimulation (FNS) technique for the restoration of multi-joint movements.

Muscle synergies encoded within the spinal cord: evidence from focal intraspinal NMDA iontophoresis in the frog

This paper relates to the problem of the existence of muscle synergies, that is whether the CNS command to muscles is simplified by controlling their activity in subgroups or synergies, rather than individually. We approach this problem with two methods that have been recently introduced: intraspinal N-methyl-D-aspartate (NMDA) microstimulation and a synergy-extracting algorithm. To search for a common set of synergies encoded for by the spinal cord whose combinations would account for a large range of electromyographic (EMG) patterns, we chose, rather than examining a large range of natural behaviors, to chemically microstimulate a large number of spinal cord interneuronal sites in different frogs. A possible advantage of this complementary method is that it is task-independent. Visual inspection suggested that the NMDA-elicited EMG patterns recorded from 12 leg muscles might indeed be constructed from smaller subgroups of muscles whose activity co-varied, suggestive of synergies. We used a modification of our extracting computational algorithm whereby a nonnegative least-squares method was employed to iteratively update both the synergies and their coefficients of activation in reconstructing the EMG patterns. Using this algorithm, a limited set of seven synergies was found whose linear combinations accounted for more than 91% of the variance in the pooled EMG data from 10 frogs, and more than 96% in individual frogs. The extracted synergies were similar among frogs. Further, preferred combinations of these synergies were clearly identified. This was found by extracting smaller sets of four, five, or six synergies and fitting each synergy from those sets as a combination from the set of seven synergies extracted from the same frog; the synergy combinations from each frog were then pooled together to examine their frequency of occurrence. Concordance with this method of identifying synergy combinations was found by examining how the synergies from the set of seven correlated pair-wise as they reconstructed the EMG data. These results support the existence of muscle synergies encoded within the spinal cord, which through preferred combinations, account for a large repertoire of spinal cord motor output. These findings are contrasted with previous approaches to the problem of synergies.

Retraining movement in patients with acquired brain injury using a virtual environment

We report preliminary results of an ongoing study in which a virtual environment (VE) system is used to facilitate motor relearning of a pouring task in patients with Acquired Brain Injury (ABI). Four subjects were evaluated pre- and post-VE training using virtual-world and real-world tests in which subjects performed a pouring motion while holding a cup. Standard clinical tests of motor and functional ability were also used. Three of four subjects demonstrated improvement in end-point trajectories (cup path) performed during the virtual and real world tests. Clinical test scores also improved. Results indicate that subjects with ABI were able to learn a movement in VE, and generalize this ability to real-world performance of similar and unrelated tasks. VE training appears to be a feasible and promising approach to the rehabilitation of subjects with ABI.

Motor learning through the combination of primitives

In this paper we discuss a new perspective on how the central nervous system (CNS) represents and solves some of the most fundamental computational problems of motor control. In particular, we consider the task of transforming a planned limb movement into an adequate set of motor commands. To carry out this task the CNS must solve a complex inverse dynamic problem. This problem involves the transformation from a desired motion to the forces that are needed to drive the limb. The inverse dynamic problem is a hard computational challenge because of the need to coordinate multiple limb segments and because of the continuous changes in the mechanical properties of the limbs and of the environment with which they come in contact. A number of studies of motor learning have provided support for the idea that the CNS creates, updates and exploits internal representations of limb dynamics in order to deal with the complexity of inverse dynamics. Here we discuss how such internal representations are likely to be built by combining the modular primitives in the spinal cord as well as other building blocks found in higher brain structures. Experimental studies on spinalized frogs and rats have led to the conclusion that the premotor circuits within the spinal cord are organized into a set of discrete modules. Each module, when activated, induces a specific force field and the simultaneous activation of multiple modules leads to the vectorial combination of the corresponding fields. We regard these force fields as computational primitives that are used by the CNS for generating a rich grammar of motor behaviours.

New perspectives on spinal motor systems

The production and control of complex motor functions are usually attributed to central brain structures such as cortex, basal ganglia and cerebellum. In traditional schemes the spinal cord is assigned a subservient function during the production of movement, playing a predominantly passive role by relaying the commands dictated to it by supraspinal systems. This review challenges this idea by presenting evidence that the spinal motor system is an active participant in several aspects of the production of movement, contributing to functions normally ascribed to 'higher' brain regions.

Cortical correlates of learning in monkeys adapting to a new dynamical environment

In this paper, we describe the neural changes observed in the primary motor cortex of two monkeys while they learned a new motor skill. The monkeys had to adapt their reaching movements to external forces that interfered with the execution of their arm movements. We found a sizable population of cells that changed their tuning properties during exposure to the force field. These cells took on the properties of neurons that are involved in the control of movement. Furthermore, the cells maintained the acquired activity as the monkey readapted to the no-force condition. Recent imaging studies in humans have reported the effects of motor learning in the primary motor cortex. Our results are consistent with the findings of these studies and provide evidence for single-cell plasticity in the primary motor cortex of primates.

Output units of motor behavior: an experimental and modeling study

Cognitive approaches to motor control typically concern sequences of discrete actions without taking into account the stunning complexity of the geometry and dynamics of the muscles. This begs the question: Does the brain convert the intricate, continuous-time dynamics of the muscles into simpler discrete units of actions, and if so, how? One way for the brain to form discrete units of behavior from muscles is through the synergistic co-activation of muscles. While this possibility has long been known, the composition of potential muscle synergies has remained elusive. In this paper, we have focused on a method that allowed us to examine and compare the limb stabilization properties of all possible muscle combinations. We found that a small set (as few as 23 out of 65,536) of all possible combinations of 16 limb muscles are robust with respect to activation noise: these muscle combinations could stabilize the limb at predictable, restricted portions of the workspace in spite of broad variations in the force output of their component muscles. The locations at which the robust synergies stabilize the limb are not uniformly distributed throughout the leg's workspace, but rather, they cluster at four workspace areas. The simulated robust synergies are similar to the actual synergies we have previously found to be generated by activation of the spinal cord. Thus, we have developed a new analytical method that enabled us to select a few muscle synergies with interesting properties out of the set of possible muscle combinations. Beyond this, the identification of robustness as a common property of the synergies in simple motor behaviors will open the way to the study of dynamic stability, which is an important and distinct property of the vertebrate motor-control system.