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Biomedical subjects

R B Stein

Publications and source records attributed to R B Stein.

At least 55 records · Page 3Linked to original sources
PubMed1998

Identification, localization, and modulation of neural networks for walking in the mudpuppy (Necturus maculatus) spinal cord.

We tested the hypothesis that the neural networks for walking in the mudpuppy can be divided into a flexor and an extensor center, each of which contains collections of interneurons localized in the vicinity of their motoneuron pools. Combining a battery of techniques, we identified and localized the elbow flexor center and its motoneuron pool in the C2 segment and the elbow extensor center and its motoneuron pool in the C3 segment. Rhythmic flexion or extension of the limb in isolation could be induced by continuous trains of current pulses of the C2 or C3 segments, respectively. Independent activation could also occur after application of glutamate receptor agonist NMDA. Part of segment C2 in isolation generated rhythmic elbow flexor bursts, whereas part of segment C3 in isolation generated rhythmic elbow extensor bursts. An isolated region spanning the C3 roots generated both flexor and extensor bursts. The step cycle was modulated in a phase-dependent manner by stimulation of the dorsal roots, the ventral roots, or either of the two centers. The effects of ventral root stimulation were removed by deafferentation to block reafferent input attributable to muscle contraction induced by the stimulation. We conclude that the neural networks for walking contain at least a flexor and an extensor generator that are localized in close apposition to the motoneuron pools, that the two centers can work independently despite the fact that there are reciprocal inhibitory interconnections between them, and that sensory input interacts with the spinal neural networks to reset the ongoing walking rhythm in a phase-dependent manner.

Animals↗
PubMed1998

Function of sural nerve reflexes during human walking.

1. The functions of ipsilateral cutaneous reflexes were studied with short trains of stimuli presented pseudorandomly to the sural nerve during human walking. Electromyograms (EMG) of lower (tibialis anterior (TA), soleus, lateral (LG) and medial (MG) gastrocnemius) and upper leg (vastus lateralis and biceps femoris) muscles were recorded, together with ankle, knee and hip joint angles. Net reflex EMG responses were quantified in each of the sixteen parts of the step cycle. The kinematic measurements included ankle eversion- inversion, and ankle, knee and hip flexion-extension. 2. The function of the sural reflexes depended upon the part of the step cycle in which the nerve was stimulated and the intensity of stimulation. During stance, reflexes in MG and TA muscles in response to a medium intensity of stimulation (1.9 x radiating threshold, x RT) were closely associated with ankle eversion and dorsiflexion responses, respectively. These responses could assist in accommodation to uneven terrain that applies pressure to the lateral side of the foot (sural innervation area). Non-noxious, high intensity (2.3 x RT) stimulation resulted in strong suppression of LG and MG during stance which was correlated to a small reduction in ankle plantarflexion. At this higher intensity the response would function to prevent the foot from moving more forcefully onto a potentially harmful obstacle. 3. During swing, ankle dorsiflexion increased and was significantly correlated to the net TA EMG response after both medium and high intensity stimulation. Knee flexion was increased throughout swing at both intensities of stimulation. These responses may serve in an avoidance response in which the swing limb is brought past an obstacle without destabilizing contact. 4. The net EMG and kinematic responses suggest that cutaneous reflexes stabilize human gait against external perturbations produced by an uneven surface in stance or obstacles encountered during swing.

Adult↗
PubMed1998

A simple model of force generation by skeletal muscle during dynamic isometric contractions.

The force that an isometric skeletal muscle will produce in response to time-varying stimulation ("dynamic isometric" force) is important both for understanding muscle function and for designing neuroprostheses. This paper reports a model for predicting the force produced by an isometric skeletal muscle at rest length in response to a wide range of stimulation patterns. The model consists of two linear, first-order systems separated by a static nonlinearity. The rate constant of the second first-order system varies with force level. The model was validated using three cat soleus and three cat plantaris muscles. The following whole-nerve stimulation trains were used: single pulses (twitches), 2-4 pulses, constant rates, triangularly modulated interpulse intervals, and randomly modulated interpulse intervals. The model reproduced most responses accurately. The model shows that a force-dependent rate constant is essential for model validity, and could be used in the control of neuroprostheses.

Analog-Digital Conversion↗
PubMed1998

Reflexes from the superficial peroneal nerve during walking in stroke subjects.

The function of ipsilateral cutaneous reflexes was studied with short trains of stimuli presented pseudorandomly to the superficial peroneal nerve (SP; innervates the top of the foot) during treadmill walking in neurologically intact (NI) subjects and subjects who had had a stroke. Ankle and knee joint angles together with electromyograms (EMG) of tibialis anterior (TA), soleus (SOL), medial gastrocnemius (MG), vastus lateralis (VL), and biceps femoris (BF) muscles were recorded. Net reflex EMG and kinematic responses to stimulation were quantified in each of the 16 parts of the step cycle and responses compared between the stroke and NI subjects. Stimulation strongly suppressed extensor muscles throughout stance in the stroke subjects. TA muscle showed a significant suppression during swing phase that was correlated with reduced ankle dorsiflexion in both stroke and NI subjects. BF reflexes were facilitatory during parts of swing and VL reflexes were suppressive throughout stance in the stroke subjects. There was a significant correlation between BF facilitation and knee flexion during swing, which was stronger in NI subjects. We conclude that only part of the stumble correction to foot dorsum electrical stimulation observed in NI subjects is maintained after stroke, and that new, suppressive responses are seen.

Adult↗
PubMed1998

Analysis of rapid stopping during human walking.

The mechanisms involved in rapidly terminating human gait were studied. Subjects were asked to walk at a comfortable speed and to stop walking as soon as they felt an electrical stimulus to the superficial peroneal nerve. This simulated hitting an obstacle with the top of the foot. Stimuli were presented repeatedly at random during a 20-min period of walking. Electromyograms and joint angular movements of the right leg and forces under both feet were recorded. The step cycle was divided into 16 parts, and the responses to stimuli in each part were analyzed separately. Subjects generally stopped with the right foot in front of the left or vice-versa, depending on when the stimulus was applied in the step cycle. There was also a transition region in which subjects would rise up on their toes and either back down or take one more quick, short forward step. Three different mechanisms were used to produce a stop. 1) An extension synergy in the swing leg was initiated just before this leg hit the ground to brake the forward momentum of the body. 2) The push-off phase of the stance leg was inhibited to reduce the forward thrust and maintain the stance leg on the ground behind the body. 3) If these mechanisms were insufficient, the body rose up onto the toes of the extended forward leg and thereby converted more kinetic energy to potential energy. A decision to take an additional step depends on whether the momentum of the body is sufficient to carry the center of mass in front of its support on the forward leg. If so, an additional step is taken. Despite the complexity of the decisions that must be made, changes in electromyographic activity are seen throughout the legs and trunk in 150-200 ms.

Adult↗
PubMed1998

Medical management of Crohn's disease.

Crohn's disease is a chronic inflammatory bowel disease of unknown etiology. Sulfasalazine and the newer 5-aminosalicylates remain the first agents of choice to treat mild to moderate disease and often are effective at high doses as maintenance therapies. Corticosteroids are often required to treat more moderate to severe disease activity, although approximately one-third of patients become steroid-dependent after a steroid-induced remission. Corticosteroids have proven ineffective in maintaining remission and side effects resulting from prolonged exposure preclude their long-term use. Azathioprine and 6-mercaptopurine are effective in the setting of steroid dependence and steroid resistance, as well as for the treatment of perianal and fistulizing complications unresponsive to antibiotics. Crohn's disease commonly recurs following surgical resection, and there is expanding evidence that postoperative prophylaxis with certain antibiotics (e.g., metronidazole), aminosalicylates or immunomodulators may be beneficial in the prevention of disease recurrence following resection. Cyclosporin may benefit patients with severe Crohn's disease or refractory fistulas. Recent trials of newer immunosuppressive agents commonly employed in transplant recipients and ongoing development of a new class of "biologic" agents targeting specific sites in the immunoinflammatory cascade, are expanding the pharmacological armamentarium available to treat certain patients.

Journal Article↗
PubMed1997

Identification of intrinsic and reflex contributions to human ankle stiffness dynamics.

We have examined dynamic stiffness at the human ankle using position perturbations which were designed to provide a wide-bandwidth input with low average velocity. A parallel-cascade, nonlinear system identification technique was used to separate overall stiffness into intrinsic and reflex components. Intrinsic stiffness was described by a linear, second-order system similar to that demonstrated previously. Reflex stiffness dynamics were more complex, comprising a delay, a unidirectional rate-sensitive element and then lowpass dynamics. Reflex mechanisms were found to be most important at frequencies of 5-10 Hz. The gain and dynamics of reflex stiffness varied strongly with the parameters of the perturbation, the gain decreasing as the mean velocity of the perturbation increased. Under some conditions, torques generated by reflex mechanisms were of the same magnitude as those from intrinsic mechanisms. It is concluded that reflex stiffness can be large enough to be important functionally, but that its effects will depend strongly upon the particular conditions.

Ankle Joint↗
PubMed1997

Cutaneous reflexes during human gait: electromyographic and kinematic responses to electrical stimulation.

The functions of ipsilateral cutaneous reflexes were studied with short trains of stimuli presented pseudorandomly to the superficial peroneal (SP) and tibial nerves during human gait. Electromyograms (EMGs) of tibialis anterior (TA), soleus, lateral and medial gastrocnemius, vastus lateralis (VL), and biceps femoris (BF) muscle were recorded, together with ankle and knee joint angles. Net reflex EMG responses were quantified in each of the 16 parts of the step cycle according to a recently developed technique. After SP nerve stimulation, TA muscle showed a significant suppression during swing phase that was highly correlated to ankle plantarflexion. BF and VL muscles were both excited throughout swing and significantly correlated to knee flexion during early swing. Tibial nerve stimulation caused dorsiflexion during late stance, but plantarflexion during late swing. We argue that SP nerve reflexes are indicative of a stumbling corrective response to nonnoxious electrical stimulation in humans. The correlated kinematic responses after tibial nerve stimulation may allow smooth movement of the swing leg so as to prevent tripping during swing and to assist placing and weight acceptance at the beginning of stance.

Adult↗
PubMed1997

Increased G-CSF responsiveness of bone marrow cells from hematopoietic cell phosphatase deficient viable motheaten mice.

The mouse mutation viable motheaten (me(v)) results in defects in the expression and catalytic activity of the cytoplasmic protein tyrosine phosphatase known as hematopoietic cell phosphatase (HCP). This reduction in HCP activity leads to the aberrant regulation of several myeloid and lymphoid cell lineages, including substantial increases in numbers of granulocytes. The differentiation, proliferation, and survival of cells in this lineage are normally supported by granulocyte-colony stimulating factor (G-CSF). In this study we have determined the consequences of the loss of HCP activity in me(v)/me(v) mice on the response of bone marrow cells to G-CSF. Bone marrow from these mice exhibited substantial increases in clonogenic and proliferative responses to G-CSF. These enhanced activities of G-CSF correlated with an increase in the level of immature granulocytic, G-CSF receptor positive cells in the bone marrow. These results suggested the possibility that HCP may regulate the G-CSF receptor by a direct interaction. However, under conditions where the previously described interaction between the erythropoietin receptor and HCP was readily observed, HCP did not detectably associate with the G-CSF receptor.

Animals↗
PubMed1996

Multiple signaling pathways induced by granulocyte colony-stimulating factor involving activation of JAKs, STAT5, and/or STAT3 are required for regulation of three distinct classes of immediate early genes.

Granulocyte colony-stimulating factor (G-CSF) is the major regulator of proliferation and differentiation of neutrophilic granulocyte precursor cells. G-CSF activates multiple signaling molecules, including the JAK1 and JAK2 kinases and the STAT transcription factors. To investigate G-CSF signaling events regulated by the JAK-STAT pathway, we have generated UT7-epo cells stably expressing either wild-type (wt) G-CSF receptor or a series of C-terminal deletion mutants. Gel mobility shift and immunoprecipitation/Western analysis showed that STAT5 is rapidly activated by G-CSF in cells expressing the wt G-CSF receptor, in addition to the previously reported STAT3 and STAT1. Mutants lacking any tyrosine residues in the cytoplasmic domain maintain their ability to activate STAT5 and STAT1 but cannot activate STAT3, implying that STAT5 and STAT1 activation does not require receptor tyrosine phosphorylation. We also observed significant changes in the ratio of STAT1:STAT3:STAT5 activated by various G-CSF receptor C-terminal deletion mutants. These mutant receptors were further used to investigate the role of JAKs and STATs in G-CSF-mediated responses in these cells. We found that JAK activation correlates with G-CSF-induced cell proliferation, whereas STAT activation is not required. We have also identified three classes of G-CSF immediate early genes, whose activation correlates with the activation of distinct JAK-STAT pathways. Our data show that, whereas c-fos is regulated through a pathway independent of STAT activation, oncostatin M, IRF-1, and egr-1 are regulated by an STAT5-dependent pathway and fibrinogen is regulated by an STAT3-dependent pathway. In conclusion, our results suggest that G-CSF regulates its complex biologic activities by selectively activating distinct early response genes through different JAK-STAT signaling molecules.

Animals↗
PubMed1996

Gain of the triceps surae stretch reflex in decerebrate and spinal cats during postural and locomotor activities.

1. The triceps surae (TS) stretch reflex was measured in decerebrate cats during crossed extensor stimulation and after spinalization during rhythmic locomotor activity induced by clonidine and manual perineal stimulation. The TS force in response to sinusoidal stretch was measured at a given contraction level before and after deafferentation, and the 'reflex force' was computed by subtracting these two responses. Reflex 'gain' was computed as the ratio of the reflex and deafferented force responses (a unitless estimate of the open loop feedback gain). 2. Prior to locomotion the spontaneous muscle activity was low (less than 15% of maximum), but the reflex gain was relatively high (close to 1.0 with a 5 Hz stretch). When locomotion commenced the reflex gain was markedly lowered when measured at the same contraction level as before locomotion (25% of the gain prior to locomotion). At higher contraction levels the reflex gain was not significantly increased. The reflex force and EMG responses to stretch increased with the contraction level, but their effect on the total reflex gain was cancelled by an associated increase in the intrinsic muscle stiffness. 3. In the decerebrate cat, during weak tonic contractions (spontaneous), the reflex gain was high and comparable with the gain in the resting spinal cat. However, with increased tonic contractions produced by crossed extensor stimulation the reflex gain dropped. At higher contraction levels the gain was not significantly different from the gain during spinal locomotion. 4. When the frequency of stretch was increased from 3 to 20 Hz, EMG responses to stretch increased, but the reflex force decreased, since a more fused contraction developed with the more frequent reflex activations. Overall, the reflex gain decreased with frequency in both spinal and decerebrate cats. The phase lag of the reflex force, relative to the intrinsic muscle force, increased with increasing frequency, due to reflex delays, with a 180 deg lag occurring between 12 and 18 Hz (tremor frequencies). The mean gain was significantly lower and the phase lag was significantly greater during locomotion than during tonic crossed extensor contractions, suggesting different reflex mechanisms. 5. In conclusion, during locomotion in spinal cats afferent feedback from low frequency ankle movements, similar to those occurring during the normal step cycle, reflexly produces a small but significant fraction of the extensor force (about a quarter of the stretch-related force modulation). This fraction is remarkably constant at the different contraction levels of the step cycle. Afferent feedback during higher frequency movement is less effective, minimizing the chance of instability and tremor. In contrast during tonic contractions afferent feedback produces half of the total muscle force during perturbations, clearly contributing to the maintenance of posture.

Afferent Pathways↗
PubMed1996

Regulation of soleus muscle spindle sensitivity in decerebrate and spinal cats during postural and locomotor activities.

1. In order to study fusimotor control in reduced preparations, soleus muscle spindle afferents were recorded in premammillary decerebrate cats (n = 15) during crossed extensor reflexes and, after spinalization, during locomotion produced by either clonidine or L-beta-3,4-dihydroxyphenylalanine (L-DOPA). The soleus muscle was oscillated sinusoidally (0.25 mm, 4 Hz) and the afferent mean firing rate and modulation were calculated. An increase in firing rate was assumed to arise from activity in dynamic gamma-motoneurones (dynamic gamma-drive) when associated with an increase in modulation to stretching, and in static gamma-motoneurones (static gamma-drive) when modulation decreased. 2. At rest in all preparations the firing rate and modulation in primary muscle spindle afferents were generally much higher than after de-efferentation (ventral root section), suggesting a predominant dynamic gamma-drive. Clonidine decreased and even eliminated this presumed resting gamma-drive in many afferents, both in the decerebrate (7 of 8) and the spinal (6 of 18) state. This effect on gamma-drive may account, at least in part, for its suppressive effect on spasticity in humans. 3. When locomotion commenced in clonidine-treated spinal cats, primary afferents generally fired with much higher mean rates (+121%) and lower sensitivities (-32%), suggesting a large increase in static gamma-drive (possibly accompanied by a small decrease in dynamic gamma-drive). These high rates were usually maintained tonically throughout the step cycle. However, a third of the afferents were silenced during locomotor contractions, and de-efferentation had no significant effect on their firing rates. Thus, for some spindles alpha-activity can occur without significant gamma-drive. 4. During locomotion in L-DOPA-treated spinal cats the inferred static gamma-drive only occurred phasically, coactivated with the EMG, though it could precede the EMG by 100-500 ms. In the flexion phase both the afferent rate and modulation were lower than before locomotion, suggesting a lack of effective gamma-drive. 5. Crossed extensor reflexes in decerebrate cats also produced a substantial increase in primary afferent firing rate (+187%) and decrease in sensitivity (-37%), again suggesting increased static gamma-drive (n = 18). This gamma-drive was largely independent of EMG activity and often occurred without alpha-activity. The mean firing rate of secondary muscle spindle afferents increased significantly during locomotion (with L-DOPA) and crossed extensor reflexes, again indicating increased static gamma-drive. Clonidine reduced or eliminated the gamma-drive in seven of eight afferents during crossed extensor reflexes. 6. In conclusion, although there are some common features, such as a predominant static gamma-drive in all walking preparations, the pattern of static and dynamic gamma-drive is not closely linked to alpha-activity under the conditions studied. As well as gamma-drive without alpha-activity, we have shown for the first time that alpha-motoneurones can be activated without significant gamma-drive to many spindles during behavioural tasks.

Action Potentials↗
PubMed1996

Serotonergic modulation of the mudpuppy (Necturus maculatus) locomotor pattern in vitro.

The aims of the present study were to: (1) study the role of serotonin (5-HT) in modulating the central pattern generator (CPG) underlying locomotion in the mudpuppy (Necturus maculatus); (2) investigate whether there is an intrinsic spinal serotonergic system. These aims were achieved by the use of pharmacological and immunocytochemical methods. To study modulation of the locomotor pattern and rhythm, we applied 5-HT, its uptake blocker zimelidine, and a variety of 5-HT receptor agonists and antagonists to an in vitro brainstem-spinal cord preparation isolated from the mudpuppy. The preparation consisted of the first five segments of the spinal cord and the right forelimb attached by the brachial plexus. The spinal CPG for locomotion was activated chemically by adding NMDA to the superfusing solution. During locomotion, bipolar electromyographic (EMG) recordings were made unilaterally from flexor and extensor ulnae muscles. 5-HT on its own did not induce locomotion, but it did have a profound modulatory effect on NMDA-induced locomotion. 5-HT produced a dose-dependent increase in the overall cycle duration and enhanced the EMG burst duration. Use of zimelidine indicated that there is an endogenous release of 5-HT which modulated the locomotor rhythm. The endogenous release was antagonized by 5-HT1/5-HT2 receptor antagonist methiothepin. Immunocytochemical analysis, in which the entire spinal cord of the mudpuppy was used, revealed that there were more than one type of spinal serotonergic neuron. They were differentiated according to the cell diameter, shape, and arborization pattern of their processes. These neurons were located within the central gray matter ventrolateral to the central canal. Our results suggest that 5-HT plays an important role in modulating the locomotor CPG in the mudpuppy, by acting through a well-developed spinal serotonergic system. This is in contrast to what has been reported in higher vertebrates, where serotonergic innervation is derived from supraspinal structures.

Animals↗
PubMed1996

Discovery of novel intracellular receptor modulating drugs.

Utilizing the co-transfection assay as a guide to determining structure activity relationships, we have been pursuing the discovery of non-steroidal hPR modulators. Small molecule, non-steroidal lead structures have been identified. Optimization of these structures has yielded more potent hPR modulators. Improved cross-reactivity profiles with other intracellular receptors are a feature of these compounds owing to their non-steroidal nature.

Animals↗
PubMed1996

Application of tilt sensors in functional electrical stimulation.

Tilt sensors, or inclinometers have been investigated for the control of Functional Electrical Stimulation (FES) to improve the gait of persons who had a stroke or incomplete spinal cord injury (SCI). Different types of tilt sensors were studied for their characteristics and their performance in measuring the angular displacement of leg segments during gait. Signal patterns of the lower leg with inertial tilt sensors were identified with control subjects and subjects with footdrop who are being stimulated during level walking. To minimize acceleration responses when the foot swings or hits the ground, we use low-pass filtering (1.5-2 Hz). A finite state approach allows the sensor fixed on the shank to effectively detect the step intention in a population of stroke and incomplete SCI subjects and to control the FES. When the lower leg tilts backward, the common peroneal nerve is stimulated to bring the foot up and forward. We have designed a miniature footdrop stimulator with a magnetoresistive tilt sensor built in, so no external sensor cables are required. The thresholds to turn the stimulator on and off can be adjusted, as well as the maximum period of stimulation and the minimum interval between periods of stimulation. This device features several important advantages over traditional AFO's or stimulators controlled by foot switches. Initial trials with stroke and SCI subjects have demonstrated substantial gait improvement for some subjects, while most liked the good cosmesis and ease of using the device with a tilt sensor.

Biomechanical Phenomena↗
PubMed1996

Estimating mechanical parameters of leg segments in individuals with and without physical disabilities.

Methods are described for estimating the inertia, viscosity, and stiffness of the lower leg around the knee and of the whole leg around the hip that are applicable even to persons with considerable spasticity. These involve: 1) a "pull" test in which the limb is slowly moved throughout its range of motion while measuring angles (with an electrogoniometer) and torques (with a hand-held dynamometer) to determine passive stiffness and 2) a "pendulum" test in which the limb is moved against gravity and then dropped, while again measuring angles and torques. By limiting the extent of the movement and choosing a direction (flexion or extension) that minimizes reflex responses, the mechanical parameters can be determined accurately and efficiently using computer programs. In the sample of subjects studied (nine with disability related to spinal cord injury, head injury, or stroke, and nine with no neurological disability), the inertia of the lower leg was significantly reduced in the subjects with disability (p < 0.05) as a result of atrophy, but the stiffness and viscosity were within normal limits. The values of inertia were also compared with anthropometric data in the literature. The identification of these passive parameters is particularly important in designing systems for functional electrical stimulation of paralyzed muscles, but the methods may be widely applicable in rehabilitation medicine.

Adult↗
PubMed1996

Determinants of promoter-specific activity by glucocorticoid receptor.

Glucocorticoid receptor (GR) is expressed at essentially equal levels in almost all tissues and cell types. Remarkably, glucocorticoids themselves regulate transcription in vivo in both a promoter- and tissue-specific manner. Thus, specific systems must be in place to regulate receptor action within certain cells and at certain promoters. To address two specific aspects of these systems, we have analyzed promoter-specific activity of GR using two different, well studied promoters (termed simple and composite promoters) from which GR activates transcription. The simple promoter depends only on the receptor for glucocorticoid-responsive transcriptional activation, while GR activity at the composite promoter depends on additional transcription factors. We have compared the action of several GR ligands at these promoters and demonstrate fundamental differences in the activities of these ligands on receptor activity. Furthermore, these compounds induce unique conformational changes in receptor, resulting in promoter-specific receptor function. We have identified critical amino acid residues within GR which, when mutated, genetically distinguish the action of GR at these promoters. Taken together, the data indicate that the presence of only the receptor and the ligand is not sufficient to allow activation of transcription. An additional system of regulation influences receptor action in both a tissue- and promoter-selective fashion, suggesting that multiple, regulated surfaces of the receptor respond to the cellular environment and determine the spectrum of GR activities. These functional surfaces may be induced or regulated by ligand binding, by the DNA sequence to which receptor is bound, or by the nonreceptor factors resident at the promoter or in the tissue.

Animals↗