A theory of geometric constraints on neural activity for natural three-dimensional movement.
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Although the orientation of an arm in space or the static view of an object may be represented by a population of neurons in complex ways, how these variables change with movement often follows simple linear rules, reflecting the underlying geometric constraints in the physical world. A theoretical analysis is presented for how such constraints affect the average firing rates of sensory and motor neurons during natural movements with low degrees of freedom, such as a limb movement and rigid object motion. When applied to nonrigid reaching arm movements, the linear theory accounts for cosine directional tuning with linear speed modulation, predicts a curl-free spatial distribution of preferred directions, and also explains why the instantaneous motion of the hand can be recovered from the neural population activity. For three-dimensional motion of a rigid object, the theory predicts that, to a first approximation, the response of a sensory neuron should have a preferred translational direction and a preferred rotation axis in space, both with cosine tuning functions modulated multiplicatively by speed and angular speed, respectively. Some known tuning properties of motion-sensitive neurons follow as special cases. Acceleration tuning and nonlinear speed modulation are considered in an extension of the linear theory. This general approach provides a principled method to derive mechanism-insensitive neuronal properties by exploiting the inherently low dimensionality of natural movements. (+info)
Rupture of the intestinal epithelial barrier and mucosal invasion by Shigella flexneri.
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Invasion of the intestinal barrier by Shigella flexneri involves complex interactions with epithelial and phagocytic cells. Major perturbation of the signals that maintain epithelial integrity permits mucosal invasion, leading to tissue destruction. Expression of this invasive phenotype depends on the secretion of Ipa proteins (invasins), which can trigger entry of the pathogen into epithelial cells by causing massive rearrangement of the host cell cytoskeleton and cause macrophage apoptotic death by direct interaction of IpaB with interleukin-1beta (IL-1beta)-converting enzyme. This results in the killing of defense cells and in the release of IL-1beta. In vivo, bacteria translocate through the epithelial barrier, essentially via M cells of the follicle-associated epithelium in the colonic and rectal mucosa. Apoptotic death of macrophages in subepithelial tissues allows bacterial survival and triggers inflammation, which destabilizes epithelial structures and facilitates further bacterial entry. Once they are intracellular, bacteria multiply within the cytoplasm and move from cell to cell by an actin-dependent process. (+info)
Transport of colloidal particles in lymphatics and vasculature after subcutaneous injection.
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This study was designed to determine the transport of subcutaneously injected viral-size colloid particles into the lymph and the vascular system in the hind leg of the dog. Transport of two colloid particles, with average size approximately 1 and 0.41 microm, respectively, and with and without leg rotation, was tested. Leg rotation serves to enhance the lymph flow rates. The right femoral vein, lymph vessel, and left femoral artery were cannulated while the animal was under anesthesia, and samples were collected at regular intervals after subcutaneous injection of the particles at the right knee level. The number of particles in the samples were counted under fluorescence microscopy by using a hemocytometer. With and without leg rotation, both particle sets were rapidly taken up into the venous blood and into the lymph fluid. The number of particles carried away from the injection site within the first 5 min was <5% of the injected pool. Particles were also seen in arterial blood samples; this suggests reflow and a prolonged residence time in the blood. These results show that particles the size of viruses are rapidly taken up into the lymphatics and blood vessels after subcutaneous deposition. (+info)
Physical activity assessment in American Indian schoolchildren in the Pathways study.
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The objective of the Pathways physical activity feasibility study was to develop methods for comparing type and amount of activity between intervention and control schools participating in a school-based obesity prevention program. Two methods proved feasible: 1) a specially designed 24-h physical activity recall questionnaire for assessing the frequency and type of activities and 2) use of a triaxial accelerometer for assessing amount of activity. Results from pilot studies supporting the use of these methods are described. Analyses of activity during different segments of the day showed that children were most active after school. The activities reported most frequently (e.g., basketball and mixed walking and running) were also the ones found to be most popular in the study population on the basis of formative assessment surveys. Both the physical activity recall questionnaire and the triaxial accelerometer methods will be used to assess the effects of the full-scale intervention on physical activity. (+info)
Multijoint muscle regulation mechanisms examined by measured human arm stiffness and EMG signals.
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Stiffness properties of the musculo-skeletal system can be controlled by regulating muscle activation and neural feedback gain. To understand the regulation of multijoint stiffness, we examined the relationship between human arm joint stiffness and muscle activation during static force control in the horizontal plane by means of surface electromyographic (EMG) studies. Subjects were asked to produce a specified force in a specified direction without cocontraction or they were asked to keep different cocontractions while producing or not producing an external force. The stiffness components of shoulder, elbow, and their cross-term and the EMG of six related muscles were measured during the tasks. Assuming that the EMG reflects the corresponding muscle stiffness, the joint stiffness was predicted from the EMG by using a two-link six-muscle arm model and a constrained least-square-error regression method. Using the parameters estimated in this regression, single-joint stiffness (diagonal terms of the joint-stiffness matrix) was decomposed successfully into biarticular and monoarticular muscle components. Although biarticular muscles act on both shoulder and elbow, they were found to covary strongly with elbow monoarticular muscles. The preferred force directions of biarticular muscles were biased to the directions of elbow monoarticular muscles. Namely, the elbow joint is regulated by the simultaneous activation of monoarticular and biarticular muscles, whereas the shoulder joint is regulated dominantly by monoarticular muscles. These results suggest that biarticular muscles are innervated mainly to control the elbow joint during static force-regulation tasks. In addition, muscle regulation mechanisms for static force control tasks were found to be quite different from those during movements previously reported. The elbow single-joint stiffness was always higher than cross-joint stiffness (off-diagonal terms of the matrix) in static tasks while elbow single-joint stiffness is reported to be sometimes as small as cross-joint stiffness during movement. That is, during movements, the elbow monoarticular muscles were occasionally not activated when biarticular muscles were activated. In static tasks, however, monoarticular muscle components in single-joint stiffness were increased considerably whenever biarticular muscle components in single- and cross-joint stiffness increased. These observations suggest that biarticular muscles are not simply coupled with the innervation of elbow monoarticular muscles but also are regulated independently according to the required task. During static force-regulation tasks, covariation between biarticular and elbow monoarticular muscles may be required to increase stability and/or controllability or to distribute effort among the appropriate muscles. (+info)
Transformations in embryonic motility in chick: kinematic correlates of type I and II motility at E9 and E12.
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Soon after hatching, chicks exhibit an array of adaptive, coordinated behaviors. Chick embryos also acquire nearly 18 days of movement experience, referred to as embryonic motility, before hatching. The chick expresses three forms of motility, types I, II, and III, and each emerges at a different stage of embryonic development. Although much is known about the mechanisms associated with motility at early embryonic stages and at the onset of hatching, the transformations in behavior and underlying mechanisms are not fully understood. Thus the purpose of this study was to determine how motility is modified during the first expected transformation, from type I to type II. It was hypothesized that kinematic features for motility at embryonic day 12 (E12) would differ significantly from features at E9 because type II motility emerges during E11. Embryos were video taped for extended intervals in ovo at E9 or E12 and entire sequences of motility were computer digitized for kinematic analyses. Results reported here indicate that several of the kinematic features characteristic of motility at E9 are also reliable features at E12. On the basis of these findings, a kinematic definition of type I motility is posed for use in subsequent behavioral studies. Several parameters distinguished motility at E12 from E9. The most notable difference between ages was the less regular timing of repetitive limb movements at E12, a finding consistent with recent reports suggesting early motility is an emergent product of a transient neural network rather than a specialized pattern generator. As predicted from established definitions for type II motility, startle-like movements were common at E12; however, they also were present in many kinematic plots at E9, suggesting the discreet age-dependent boundaries in the established definition for type II motility may require modification. Some age-related differences, such as increased intralimb coordination and excursion velocity, may be prerequisites for adaptive behavior after hatching. (+info)
Role of proprioceptive signals from an insect femur-tibia joint in patterning motoneuronal activity of an adjacent leg joint.
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Interjoint reflex function of the insect leg contributes to postural control at rest or to movement control during locomotor movements. In the stick insect (Carausius morosus), we investigated the role that sensory signals from the femoral chordotonal organ (fCO), the transducer of the femur-tibia (FT) joint, play in patterning motoneuronal activity in the adjacent coxa-trochanteral (CT) joint when the joint control networks are in the movement control mode of the active behavioral state. In the active behavioral state, sensory signals from the fCO induced transitions of activity between antagonistic motoneuron pools, i.e., the levator trochanteris and the depressor trochanteris motoneurons. As such, elongation of the fCO, signaling flexion of the FT joint, terminated depressor motoneuron activity and initiated activity in levator motoneurons. Relaxation of the fCO, signaling extension of the FT joint, induced the opposite transition by initiating depressor motoneuron activity and terminating levator motoneuron activity. This interjoint influence of sensory signals from the fCO was independent of the generation of the intrajoint reflex reversal in the FT joint, i.e., the "active reaction," which is released by elongation signals from the fCO. The generation of these transitions in activity of trochanteral motoneurons barely depended on position or velocity signals from the fCO. This contrasts with the situation in the resting behavioral state when interjoint reflex action markedly depends on actual fCO stimulus parameters, i.e., position and velocity signals. In the active behavioral state, movement signals from the fCO obviously trigger or release centrally generated transitions in motoneuron activity, e.g., by affecting central rhythm generating networks driving trochanteral motoneuron pools. This conclusion was tested by stimulating the fCO in "fictive rhythmic" preparations, activated by the muscarinic agonist pilocarpine in the otherwise isolated and deafferented mesothoracic ganglion. In this situation, sensory signals from the fCO did in fact reset and entrain rhythmic activity in trochanteral motoneurons. The results indicate for the first time that when the stick insect locomotor system is active, sensory signals from the proprioceptor of one leg joint, i.e., the fCO, pattern motor activity in an adjacent leg joint, i.e., the CT joint, by affecting the central rhythm generating network driving the motoneurons of the adjacent joint. (+info)
Correlation of primate superior colliculus and reticular formation discharge with proximal limb muscle activity.
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We studied the discharge of neurons from both the superior colliculus (SC) and the underlying mesencephalic reticular formation (MRF) and its relation to the simultaneously recorded activity of 11 arm muscles. The 242 neurons tested with a center-out reach task yielded 2,586 pairs of neuron/muscle cross-correlations (normalized, such that perfect correlations are +/-1.0). Of these, 43% had peaks with magnitude as large as 0.15, a value that corresponds to the 5% level of significance, and 16% were as large as 0.25. The great majority of peaks in this latter group was positive. The median lag time within this group was 52 ms, indicating that the neuronal discharge tended to precede the correlated muscle activity. We found a small but significantly higher proportion of cells with these relatively strong correlations in the MRF than in the SC. For both areas, these occurred most frequently with muscles of the shoulder girdle and became less frequent for axial as well as for increasingly distal arm musculature. The results support a role for the SC and MRF in guiding the arm during reach movements via the control of proximal limb musculature. (+info)