Recruitment of motor neuronal persistent inward currents shapes withdrawal reflexes in the frog

The details of behaviour are determined by the interplay of synaptic connectivity within neuronal circuitry and the intrinsic membrane properties of individual neurones. One particularly dramatic intrinsic property displayed by neurones in many regions of the nervous system is membrane potential bistability, in which transient excitation of a neurone results in a persistent depolarization outlasting the initial excitation. Here we characterize the contribution of such intrinsic bistability, also referred to as plateau properties and mediated by persistent inward currents (PICs), in spinal motor neurones to the production of withdrawal behaviours in the frog. We performed experiments on the isolated frog spinal cord with attached hindlimb. This preparation allowed the simultaneous monitoring of muscle activations during motor behaviour and intracellular neuronal recordings. We found that PICs, following their potentiation by serotonin (5-HT), are recruited and contribute to the production of withdrawal behaviours. These properties conferred a voltage-dependent prolongation to the duration of motor neuronal activity. Consistent with this potentiation of motor neuronal PICs, 5-HT also increased the duration of evoked muscle activations. This behavioural potentiation, as well as the expression of PICs in individual neurones, was reduced following antagonism of L-type Ca(2+) channels. These results demonstrate that PICs in motor neurones can be recruited during the production of behaviour and play a role in specifying the temporal details of motor output.

Neural basis for the cognitive continuum in episodic memory from health to Alzheimer disease

OBJECTIVE

The authors hypothesize that, behaviorally, episodic memory in health and disease reflects a continuum.

METHODS

Subjects (N=12) with very mild Alzheimer disease (AD) and normal subjects (N=24) were investigated with functional magnetic resonance imaging (fMRI) during an episodic memory task.

RESULTS

Recruitment of a posterior medio-temporal network was correlated with memory performance across the spectrum from high- and low-performing normal subjects to patients with early AD.

CONCLUSIONS

The behavioral spectrum from health to disease in episodic memory function is mirrored neurobiologically with graded recruitment of neuronal activation in medio-temporal regions. The results call for longitudinal assessment of behavioral decline and neuronal recruitment in future studies.

The influence of normal human ageing on automatic movements

There is evidence that aged normal subjects have more difficulty in achieving automaticity than young subjects. The underlying central neural mechanism for this phenomenon is unclear. In the present study, functional magnetic resonance imaging (fMRI) was used to investigate the effect of normal ageing on automaticity. Aged healthy subjects were asked to practice self-initiated, self-paced, memorized sequential finger movements with different complexity until they could perform the tasks automatically. Automaticity was evaluated by having subjects perform a secondary task simultaneously with the sequential movements. Although it took more time, most aged subjects eventually performed the tasks automatically at the same level as the young subjects. Functional MRI results showed that, for both groups, sequential movements activated similar brain regions before and after automaticity was achieved. No additional activity was observed in the automatic condition. While performing automatic movements, aged subjects had greater activity in the bilateral anterior lobe of cerebellum, premotor area, parietal cortex, left prefrontal cortex, anterior cingulate, caudate nucleus and thalamus, and recruited more areas, including the pre-supplementary motor area and the bilateral posterior lobe of cerebellum, compared to young subjects. These results indicate that most healthy aged subjects can perform some complex motor tasks automatically. However, aged subjects appear to require more brain activity to perform automatically at the same level as young subjects. This appears to be the main reason why aged subjects have more difficulty in achieving automaticity.

Chronic measurement of the stimulation selectivity of the flat interface nerve electrode

The flat interface nerve electrode (FINE) is an attempt to improve the stimulation selectivity of extraneural electrodes. By reshaping peripheral nerves into elliptical cylinders, central fibers are moved closer to the nerve-electrode interface, and additional surface area is created for contact placement. The goals of this study were to test the hypothesis that greater nerve reshaping leads to improved selectivity and to examine the chronic recruitment properties of the FINE. Three FINEs were developed to reshape peripheral nerves to different degrees. Four electrodes of each type were implanted on the sciatic nerves of 12 cats and tested for selectivity over at least three months. There was physiologic evidence of nerve injury in two cats with the tightest cuffs, but the other animals behaved normally. All cuff types were capable of selectively activating branches of the sciatic nerve, as well as groups of fibers within branches. The electrodes that moderately reshaped the nerves demonstrated the most selectivity. Both the selectivity measurements and the recruitment curve characteristics were stable throughout the implant period. From an electrophysiological standpoint, the FINE is a viable alternative for neuroprosthetic devices. A histological analysis of the nerves is under way to evaluate the safety of the FINE.

Corticospinal control of respiratory muscles in chronic obstructive pulmonary disease

Patients with chronic obstructive pulmonary disease (COPD) face an increased respiratory load and in consequence have an elevated respiratory drive. We used transcranial magnetic stimulation (TMS) to investigate associated changes in corticospinal excitability both at rest and during voluntary facilitation at different levels of inspiratory effort. Diaphragm and abdominal motor thresholds were significantly lower in COPD than healthy controls, but the quadriceps response was the same. In patients there was a significant increase in diaphragm response from rest during 20% inspiratory efforts but no further increase with greater efforts. In controls there was a further stepwise increase at 40% and 60% of inspiratory effort. The cortical silent period was significantly shorter in COPD. Using paired stimulation to study intracortical inhibitory and excitatory circuits we found significantly less excitability of intracortical facilitatory circuits in patients at long (>7 ms) interstimulus intervals. These results suggest that there is a ceiling effect in motor control output to the respiratory muscles of patients with COPD.

Pyramidal tract lesions and movement-associated cortical recruitment in patients with MS

Cortical functional changes, with the potential to limit the functional consequences of tissue injury, have been shown in patients with multiple sclerosis (MS). In this study, we assessed the influence of MS-related tissue damage of the brain portion of the left pyramidal tract on the corresponding movement-associated patterns of cortical recruitment in a large sample of MS patients when performing a simple motor task with their fully normal functioning right upper limbs. We investigated 76 right-handed patients with definite MS. In each subject, functional magnetic resonance imaging (fMRI) was acquired during the performance of a simple motor task with the dominant, right upper limb. During the same session, dual-echo, magnetization transfer (MT) and diffusion tensor (DT) MRI sequences were also obtained to quantify the extent and the severity of pyramidal tract damage. Lesions along the left pyramidal tract were identified in 43 patients. Compared to patients without pyramidal tract lesions, patients with such lesions had more significant activations of the contralateral primary sensorimotor cortex (SMC), secondary sensorimotor cortex (SII), inferior central sulcus, and cingulate motor area (CMA). They also showed more significant activations of several regions of the ipsilateral hemisphere, including the primary SMC and the precuneus. In these patients, T2 lesion load of left pyramidal tract was correlated with the extent of activation of the contralateral primary SMC (r2 = 0.25, P < 0.0001), whereas no correlations were found between the extent of fMRI activations and the severity of intrinsic lesion damage, as well as with left pyramidal tract normal-appearing white matter damage. This study shows that, in patients with MS, following injury of the motor pathways, there is an increased recruitment of a widespread sensorimotor network, which is likely to contribute to limit the appearance of overt clinical deficits.

Neuroethological study of status epilepticus induced by systemic pilocarpine in Wistar audiogenic rats (WAR strain)

The administration of pilocarpine (PILO) is widely recognized as resulting in an experimental model of temporal lobe epilepsy; it is characterized by induction of status epilepticus (SE) and spontaneous recurrent seizures after a latent period. We provide in this work a neuroethological description of the SE induced by PILO. Behavioral evaluations were made in Wistar Audiogenic Rats (WARs) and Wistar resistant (R) animals. The experimental group (R) and WARs were pretreated with methyl scopolamine (1mg/kg ip) and injected with PILO (R animals, 340-380 mg/kg ip; WARs, 240-280 mg/kg ip). Among R animals, 36% developed SE, and among WARs, 53%. The control group (R animals and WARs) was injected only with methyl scopolamine plus saline. The ETHOMATIC method was used for evaluation of seizures. Sequences included in the analysis were chosen using (1) fixed observation windows and (2) behavioral triggers. The R group showed that the threshold for seizure is variable, so seizure onset and behavioral evolution were better described using behavioral triggers than fixed observation windows. The observation windows selected in similar duration intervals do not characterize the seizures. Sequential analysis in the WAR group showed high mortality after SE and greater susceptibility to PILO, compared with R animals. We conclude that with neuroethological tools it is possible to better map the sequence and evolution of SE induced by PILO compared to only using behavioral and arbitrary seizure severity scales. This sequence is faster and stronger in severity when WARs are compared with R animals. Although the WARs underwent an evolution of SE in some way equivalent to that of R animals, some rats presented tonic-clonic convulsions after PILO injection, very similar to acute audiogenic seizures, a brainstem-dependent model. The current data also point to the PILO-plus-WAR combination as a suitable protocol to study the genetic-epilepsy connection in experimental temporal lobe epilepsy.

Task-specific recruitment of dorsal and ventral visual areas during tactile perception

Many studies have found that visual cortical areas are active during tactile perception. Here we used positron emission tomographic (PET) scanning in normally sighted humans to show that extrastriate cortical regions are recruited in a task-specific manner during perceptual processing of tactile stimuli varying in two dimensions. Mental rotation of tactile Forms activated a focus around the anterior part of the left intraparietal sulcus. Since prior studies have reported activity nearby during mental rotation of visual stimuli, this focus appears to be associated with the dorsal visual (visuospatial) pathway. Discrimination between tactile Forms activated the right lateral occipital complex, an object-selective region in the ventral visual (visual Form) pathway. Thus, tactile tasks appear to recruit cortical regions that are active during corresponding visual tasks. Activation of these areas in both visual and tactile tasks could reflect visual imagery during tactile perception, activity in multisensory representations, or both.

Depression of diaphragm motor cortex excitability during mechanical ventilation

The effect of mechanical ventilation on the diaphragm motor cortex remains unknown. We assessed the effect of mechanical ventilation on diaphragm motor cortex excitability by measuring the costal and crural diaphragm motor-evoked potential (MEP) elicited by single and paired transcranial magnetic stimulation. In six healthy subjects, MEP recruitment curves of the costal and crural diaphragms were assessed at relaxed end expiration during spontaneous breathing [baseline tidal volume (Vtbaseline)] and isocapnic volume cycled ventilation delivered noninvasively (NIV) at three different levels of tidal volume (Vtbaseline, Vtbaseline + 5 ml/kg liters, and Vtbaseline + 10 ml/kg liters). The costal and crural diaphragm response to peripheral stimulation of the right phrenic nerve was not reduced by NIV. NIV reduced the costal and crural MEP amplitude during NIV ( P < 0.0001) with the maximal reduction at Vtbaseline + 5 ml/kg. Response to paired TMS showed that NIV (Vtbaseline + 5 ml/kg) significantly increased the sensitivity of the cortical motoneurons to facilitatory (>9 ms) interstimulus intervals ( P = 0.002), suggesting that the diaphragm MEP amplitude depression during NIV is related to neuromechanical inhibition at the level of motor cortex. Our results demonstrate that mechanical ventilation directly inhibits central projections to the diaphragm.

Surface electromyogram signal modelling

The paper reviews the fundamental components of stochastic and motor-unit-based models of the surface electromyogram (SEMG). Stochastic models used in ergonomics and kinesiology consider the SEMG to be a stochastic process whose amplitude is related to the level of muscle activation and whose power spectral density reflects muscle conduction velocity. Motor-unit-based models for describing the spatio-temporal distribution of individual motor-unit action potentials throughout the limb are quite robust, making it possible to extract precise information about motor-unit architecture from SEMG signals recorded by multi-electrode arrays. Motor-unit-based models have not yet been proven as successful, however, for extracting information about recruitment and firing rates throughout the full range of contraction. The relationship between SEMG and force during natural dynamic movements is much too complex to model in terms of single motor units.

Investigation of the facilitation of the nociceptive withdrawal reflex evoked by repeated transcutaneous electrical stimulations as a measure of temporal summation in conscious horses

OBJECTIVE

To investigate whether facilitation of the nociceptive withdrawal reflex (NWR) can be evoked and quantified as a measure of temporal summation from the distal aspect of the left forelimb and hind limb in standing nonsedated horses via repeated stimulations of various subthreshold intensities and frequencies.

ANIMALS

10 adult horses.

PROCEDURE

Surface electromyographic activity evoked by stimulation of the digital palmar and plantar nerves was recorded from the common digital extensor and cranial tibial muscles. For each horse, the NWR threshold intensity to a single stimulus was determined for the forelimb and hind limb. Repeated stimulations were performed at subthreshold intensities and at frequencies of 2, 5, and 10 Hz. The reflex amplitude was quantified, and the behavioral responses accompanying the stimulations were scored.

RESULTS

Repeated stimulations at subthreshold intensities were able to summate and facilitate the NWR in conscious horses. The reflex facilitation was significantly related to the intensity of the repeated stimuli, whereas no effect of stimulation frequency was found. Reaction scores increased significantly for increasing stimulation intensities.

CONCLUSIONS AND CLINICAL RELEVANCE

Temporal summation obtained by repeated stimulations of subthreshold intensity appears to represent a new tool for investigating nociceptive pathophysiologic processes in horses; this experimental model may be useful to examine the mode of action and efficacy of analgesic and anesthetic interventions and possibly to assess sensory dysfunction in clinical settings.

Electrophysiology and morphometry of the Aalpha- and Abeta-fiber populations in the normal and regenerating rat sciatic nerve

We studied electrophysiological and morphological properties of the Aalpha- and Abeta-fibers in the regenerating sciatic nerve to establish whether these fiber types regenerate in numerical proportion and whether and how the electrophysiological properties of these fiber types are adjusted during regeneration. Compound action potentials were evoked from isolated sciatic nerves 12 weeks after autografting. Nerve fibers were gradually recruited either by increasing the stimulus voltage from subthreshold to supramaximal levels or by increasing the interval between two supramaximal stimuli to obtain the cumulative distribution of the extracellular firing thresholds and refractory periods, respectively. Thus, the mean conduction velocity (MCV), the maximal charge displaced during the compound action potential (Q(max)), the mean firing threshold (V(50)), and the mean refractory period (t(50)) were determined. The number of myelinated nerve fibers and their fiber diameter frequency distributions were determined in the peroneal nerve. Mathematical modeling applied to fiber recruitment and diameter distributions allowed discrimination of the Aalpha- and Abeta-fiber populations. In regenerating nerves, the number of Aalpha-fibers increased fourfold while the number of Abeta-fibers did not change. In regenerating Aalpha- and Abeta-fibers, the fiber diameter decreased and V(50) and t(50) increased. The regenerating Aalpha-fibers' contribution to Q(max) decreased considerably while that of the Abeta-fibers remained the same. Correlation of the electrophysiological data to the morphological data provided indications that the ion channel composition of both the Aalpha- and Abeta-fibers are altered during regeneration. This demonstrates that combining morphometric and electrophysiological analysis provides better insight in the changes that occur during regeneration.

Motor unit recruitment and derecruitment induced by brief increase in contraction amplitude of the human trapezius muscle

The activity pattern of low-threshold human trapezius motor units was examined in response to brief, voluntary increases in contraction amplitude ('EMG pulse') superimposed on a constant contraction at 4-7 % of the surface electromyographic (EMG) response at maximal voluntary contraction (4-7 % EMGmax). EMG pulses at 15-20 % EMGmax were superimposed every minute on contractions of 5, 10, or 30 min duration. A quadrifilar fine-wire electrode recorded single motor unit activity and a surface electrode recorded simultaneously the surface EMG signal. Low-threshold motor units recruited at the start of the contraction were observed to stop firing while motor units of higher recruitment threshold stayed active. Derecruitment of a motor unit coincided with the end of an EMG pulse. The lowest-threshold motor units showed only brief silent periods. Some motor units with recruitment threshold up to 5 % EMGmax higher than the constant contraction level were recruited during an EMG pulse and kept firing throughout the contraction. Following an EMG pulse, there was a marked reduction in motor unit firing rates upon return of the surface EMG signal to the constant contraction level, outlasting the EMG pulse by 4 s on average. The reduction in firing rates may serve as a trigger to induce derecruitment. We speculate that the silent periods following derecruitment may be due to deactivation of non-inactivating inward current ('plateau potentials'). The firing behaviour of trapezius motor units in these experiments may thus illustrate a mechanism and a control strategy to reduce fatigue of motor units with sustained activity patterns.

Recruitment of single human low-threshold motor units with increasing loads at different muscle lengths

We investigated the recruitment behaviour of low threshold motor units in flexor digitorum superficialis by altering two biomechanical constraints: the load against which the muscle worked and the initial muscle length. The load was increased using isotonic (low load), loaded dynamic (intermediate load) and isometric (high load) contractions in two studies. The initial muscle position reflected resting muscle length in series A, and a longer length with digit III fully extended in series B. Intramuscular EMG was recorded from 48 single motor units in 10 experiments on five healthy subjects, 21 units in series A and 27 in series B, while subjects performed ramp up, hold and ramp down contractions. Increasing the load on the muscle decreased the force, displacement and firing rate of single motor units at recruitment at shorter muscle lengths (P<0.001, dependent t-test). At longer muscle lengths this recruitment pattern was observed between loaded dynamic and isotonic contractions, but not between isometric and loaded dynamic contractions. Thus, the recruitment properties of single motor units in human flexor digitorum superficialis are sensitive to changes in both imposed external loads and the initial length of the muscle.

Effects of spinal cord injury level on the activity of shoulder muscles during wheelchair propulsion: an electromyographic study11No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s) or upon any organization with which the author(s) is/are associated

OBJECTIVE

To determine the influence of spinal cord injury (SCI) level on shoulder muscle function during wheelchair propulsion.

DESIGN

Fine-wire electromyographic activity of 11 muscles was recorded during wheelchair propulsion.

SETTING

Biomechanics research laboratory.

PARTICIPANTS

Convenience sample of 69 men, in 4 groups by SCI level (low paraplegia, n=17; high paraplegia, n=19; C7-8 tetraplegia, n=16; C6 tetraplegia, n=17).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Timing of muscle activity onset, cessation, and duration, and time of peak intensity for each functional group were compared with 1-way analysis of variance. Median electromyographic intensity was also compared.

RESULTS

Two functional synergies were observed: push (anterior deltoid, pectoralis major, supraspinatus, infraspinatus, serratus anterior, biceps) and recovery (middle and posterior deltoid, supraspinatus, subscapularis, middle trapezius, triceps). Push phase activity began in late recovery and ceased in early to late push. Recovery phase muscles functioned from late push to late recovery. Recruitment patterns for the groups with paraplegia were remarkably similar. For subjects with tetraplegia, pectoralis major activity was significantly prolonged compared with subjects with paraplegia (P<.05). Subscapularis activity shifted from a recovery pattern in subjects with paraplegia to a push pattern in persons with tetraplegia.

CONCLUSIONS

Level of SCI significantly affected the shoulder muscle recruitment patterns during wheelchair propulsion. Differences in rotator cuff and pectoralis major function require specific considerations in rehabilitation program design.

Hippocampal activation during transitive inference in humans

Studies in rodents have demonstrated that the integration and flexible expression of memories, necessary for transitive inference, depend on an intact hippocampus. To test this hypothesis in humans, we studied brain activation during the discrimination of a series of overlapping and non-overlapping arbitrary visual stimulus pairs. We report that transitive inference about overlapping pairs is associated with right anterior hippocampal activation, whereas recognition of non-overlapping stimulus pairs is associated with bilateral medial temporal lobe activation centered in the anterior parahippocampal gyrus. We conclude that immediate access to simple stimulus-stimulus relationships is mediated via the parahippocampal gyrus, whereas the flexible representation of memory requires the recruitment of the hippocampus.

Neural correlates of individual ratings of emotional salience: a trial-related fMRI study

Accurate appraisal of meaningful environmental signals involves the interpretation of salient information for their intrinsic emotional value and personal relevance. We examined the neural basis for these components of endogenous salience during such appraisals using trial-related functional magnetic resonance imaging (fMRI). Subjects viewed affective pictures and assessed either the emotional intensity or extent of self-relatedness of the content of those pictures. In a parametric factorial design, individualized subjective ratings of these two dimensions were correlated with brain activity. The nucleus accumbens (NAcc) responded to both increasing emotional intensity and self-relatedness. Activity in the amygdala was specifically related to affective judgments and emotional intensity. The volitional act of appraising the extent of personal association specifically engaged the ventral medial prefrontal cortex (MPFC), and additionally recruited dorsal medial frontal regions and insula as the extent of self-relatedness increased. The findings highlight both overlapping and segregated neural representations of intrinsic value and personal relevance during the appraisal of emotional stimuli.

Independent component analysis of dynamic brain responses during visuomotor adaptation

To investigate the spatial and temporal changes in electro-cortical brain activity and hand kinematics during the acquisition of an internal model of a novel screen-cursor transformation, we employed single-trial infomax independent component analysis (ICA), spectral estimation, and kinematics methods. Participants performed center-out drawing movements under normal and rotated visual feedback of pen movements displayed on a computer screen. Clustering of task-related and adaptation-related independent components identified a selective recruitment of brain activation/deactivation foci associated with the exposure to the distorted visual feedback, including networks associated with frontal-, central-, and lateral-posterior alpha rhythms, and frontal-central error-related negativity potential associated with transient theta and low beta rhythms locked to movement onset. Moreover, adaptation to the rotated reference frame was associated with a reduction in the imposed directional bias and decreases in movement path length and movement time by late-exposure trials, as well as after-effects after removal of the visual distortion. The underlying spatiotemporal pattern of activations is consistent with recruitment of frontal-parietal, sensory-motor, and anterior cingulate cortical areas during visuomotor adaptation.

Neural responses to the production and comprehension of syntax in identical utterances

Following up on an earlier positron emission tomography (PET) experiment (Indefrey et al., 2001), we used a scene description paradigm to investigate whether a posterior inferior frontal region subserving syntactic encoding for speaking is also involved in syntactic parsing during listening. In the language production part of the experiment, subjects described visually presented scenes using either sentences, sequences of noun phrases, or sequences of syntactically unrelated words. In the language comprehension part of the experiment, subjects were auditorily presented with the same kinds of utterances and judged whether they matched the visual scenes. We were able to replicate the previous finding of a region in caudal Broca's area that is sensitive to the complexity of syntactic encoding in language production. In language comprehension, no hemodynamic activation differences due to syntactic complexity were found. Given that correct performance in the judgment task did not require syntactic processing of the auditory stimuli, the results suggest that the degree to which listeners recruit syntactic processing resources in language comprehension may be a function of the syntactic demands of the task or the stimulus material.

Soleus H-reflex recruitment is not altered in persons with chronic spinal cord injury 11No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors(s) or upon any organization with which the author(s) is/are associated

OBJECTIVE

To determine whether spasticity in persons with spinal cord injury (SCI) is associated with elevated monosynaptic reflex excitability.

DESIGN

One-way experimental.

SETTING

Research laboratory.

PARTICIPANTS

Convenience sample of 9 subjects (8 men, 1 woman) with chronic and complete SCI and 20 persons (14 men, 6 women) with no neurologic impairment. Subjects with SCI exhibited lower-extremity spasticity as indicated by velocity-dependent increased resistance to passive muscle stretch, abnormally brisk deep tendon reflexes, involuntary lower-extremity flexion and/or extension spasms, and clonus.

INTERVENTION

Soleus H-reflex recruitment curves were elicited in all subjects.

MAIN OUTCOME MEASURES

Soleus H-reflex threshold (HTH), gain (HGN), and amplitude (HPP).

RESULTS

There was no difference between subjects with and without SCI in HTH, HGN, or HPP.

CONCLUSIONS

Spasticity in people with chronic and complete SCI was not associated with increased excitability of the connections between Ia afferent projections and motoneurons. Factors extrinsic to these connections may have a role in spasticity caused by SCI.

Altered Representation of the Spatial Code for Odors after Olfactory Classical Conditioning

In the olfactory bulb of vertebrates or the homologous antennal lobe of insects, odor quality is represented by stereotyped patterns of neuronal activity that are reproducible within and between individuals. Using optical imaging to monitor synaptic activity in the Drosophila antennal lobe, we show here that classical conditioning rapidly alters the neural code representing the learned odor by recruiting new synapses into that code. Pairing of an odor-conditioned stimulus with an electric shock-unconditioned stimulus causes new projection neuron synapses to respond to the odor along with those normally activated prior to conditioning. Different odors recruit different groups of projection neurons into the spatial code. The change in odor representation after conditioning appears to be intrinsic to projection neurons. The rapid recruitment by conditioning of new synapses into the representation of sensory information may be a general mechanism underlying many forms of short-term memory.

Interpreting cardiac muscle force-length dynamics using a novel functional model

To describe the dynamics of constantly activated cardiac muscle, we propose that length affects force via both recruitment and distortion of myosin cross bridges. This hypothesis was quantitatively tested for descriptive and explanative validity. Skinned cardiac muscle fibers from animals expressing primarily alpha-myosin heavy chain (MHC) (mouse, rat) or beta-MHC (rabbit, ferret) were activated with solutions from pCa 6.1 to 4.3. Activated fibers were subjected to small-amplitude length perturbations [deltaL(t)] rich in frequency content between 0.1 and 40 Hz. In descriptive validation tests, the model was fit to the ensuing force response [deltaF(t)] in the time domain. In fits to 118 records, the model successfully accounted for most of the measured variation in deltaF(t) (R(2) range, 0.997-0.736; median, 0.981). When some residual variations in deltaF(t) were not accounted for by the model (as at low activation), there was very little coherence (<0.5) between these residual force variations and the applied deltaL(t) input function, indicating that something other than deltaL(t) was causing the measured variation in deltaF(t). With one exception, model parameters were estimated with standard errors on the order of 1% or less. Thus parameters of the recruitment component of the model could be uniquely separated from parameters of the distortion component of the model and parameters estimated from any given fiber could be considered unique to that fiber. In explanative validation tests, we found that recruitment and distortion parameters were positively correlated with independent assessments of the physiological entity they were assumed to represent. The recruitment distortion model was judged to be valid from both descriptive and explanative perspectives and is, therefore, a useful construct for describing and explaining dynamic force-length relationships in constantly activated cardiac muscle.

Prediction of myelinated nerve fiber stimulation thresholds: limitations of linear models

Computer models of neurons are used to simulate neural behavior, and are important tools for designing neural prostheses. Computation time remains an issue when simulating large numbers of neurons or applying models to real time applications. Warman et al. developed a method to predict excitation thresholds for axons using linear models and a predetermined critical voltage. We calculated threshold prediction error as a function of the location of an extracellular electrode using two different axon models to examine further threshold prediction using linear models. Threshold prediction error was low (<3% error) under the conditions examined by Warman et al., but under more general conditions, threshold prediction error was as high as 23.6%. Linear models were limited as effective tools for single fiber threshold prediction because accuracy was dependent on the nonlinear and linear models used, and any parameter that affected the extracellular potential distribution. Threshold prediction could be improved by appropriately choosing the membrane conductance of the linear model, but determination of an optimal conductance was computationally expensive. Finally, although single fiber threshold prediction error was partially masked when considering the input-output (I/O) properties of populations of axons, relatively large errors still occurred in population I/O curves generated with linear models.

Impaired hippocampal function during the detection of novel words in schizophrenia

BACKGROUND

Patients with schizophrenia have smaller hippocampal volumes and perform abnormally on most declarative memory tasks. Although these findings are likely related, the impact of hippocampal pathology on cognitive performance in schizophrenia remains unclear. This study examined this relationship by measuring the volume of the hippocampus and its activation during memory task performance.

METHODS

Participants included 15 patients with schizophrenia and 16 age-matched control subjects. Hippocampal volume was determined via three-dimensional volumetric analysis of high-resolution magnetic resonance images. Hippocampal activity was assessed by measuring changes in blood oxygen level-dependent signal during a recognition memory task.

RESULTS

Patients with schizophrenia had smaller hippocampal volumes bilaterally and demonstrated poorer performance on the recognition memory task, largely because of a heightened rate of false alarms to novel stimuli. Both groups showed robust hippocampal activity to old and new items when compared with a low-level baseline task; however, direct comparison of hippocampal activity during recognition task performance revealed that healthy control, but not the schizophrenia, subjects showed significant right anterior hippocampal activation during the evaluation of novel items.

CONCLUSIONS

The impaired ability to classify new items as previously not experienced is associated with decreased recruitment and smaller volume of the hippocampus in schizophrenia.

Referent configuration of the body: a global factor in the control of multiple skeletal muscles

In addition to local biomechanical and reflex factors influencing muscle activation, global factors may be used by the nervous system to control all muscles in a coherent and task-specific way. It has been hypothesized that a virtual or referent (R) configuration of the body determined by muscle recruitment thresholds specified by neural control levels is such a factor. Due to the threshold nature of the R configuration, the activity of each muscle depends on the difference between the actual (Q) and the R configuration of the body. The nervous system modifies the R configuration to produce movement. One prediction of this hypothesis is that the Q and R configurations may match each other, most likely in movements with reversals in direction, resulting in a minimum in the electromyographic (EMG) activity level of muscles involved. The depth of the minima is constrained by the degree of coactivation of opposing muscle groups. Another prediction is that EMG minima in the activity of multiple muscles may occur not only when the movement is assisted but also when it is opposed by external forces (e.g., gravity). To verify these predictions, we analyzed EMG patterns of 16-21 functionally diverse muscles of the legs, trunk, and arms during jumping and stepping in place. One EMG minimum in the activity of all muscles regularly occurred near the apex of the jump. A minimum was also observed near the point of transition of the body from flexion to extension leading to a jump. During stepping in place, the activity of muscles of each side of the body was usually minimized near the beginning and near the end of the stance phase as well as during the maximum elevation of the foot. Since EMG minima occurred not only during gravity-assisted but also gravity-opposed movement reversals, it is concluded that neural factors (such as matching between the Q and R) rather than mechanical factors are responsible for minimizing the EMG activity in these movements.

Comparison of linear regression and probit analysis for detecting H-reflex threshold in individuals with and without spinal cord injury

A major challenge to understanding spinal reflex organization in health and disease is identifying sensitive measures of reflex excitability. The purpose of this study was to determine whether linear regression or probit analysis techniques are more sensitive for detecting H-reflex and M-wave threshold and for identifying differences in H-reflex threshold in individuals with and without spinal cord injury (SCI). Soleus H-reflex recruitment curves were generated in 9 individuals with SCI and 20 able-bodied individuals. H-reflex and M-wave threshold was estimated using three different methods, two that used linear regression of H-reflex peak-to-peak amplitude and one that used probit analysis of quantal H-reflexes. Results indicate that in both groups all three techniques were equally sensitive for detecting H-reflex but not M-wave threshold. When H-reflex threshold was normalized to M-wave threshold, different techniques provided different estimates of H-reflex threshold. However, between-group differences (SCI vs. able-body) in H-reflex and M-wave threshold were not affected by the measurement techniques that were compared in this study. We conclude that these techniques provide equally sensitive estimates of H-reflex but not M-wave threshold in persons with and without SCI. Hence, caution should be used when interpreting normalized and non-normalized values of H-reflex threshold.

Cerebral and cerebellar motor activation abnormalities in a subject with Joubert syndrome: functional magnetic resonance imaging (MRI) study

Joubert syndrome is an autosomal recessive disorder characterized by hypotonia, ataxia, developmental delay, and a distinctive hindbrain malformation involving the cerebellum and brain stem, visualized radiographically on magnetic resonance imaging (MRI) as the "molar tooth sign." In postmortem brains from subjects with Joubert syndrome, there is an apparent absence of decussation of both corticospinal and superior cerebellar tracts, although the functional significance has not been elucidated. We sought to explore the cerebral and cerebellar activation pattern elicited by finger tapping in an adolescent with Joubert syndrome and in a normal control subject using functional MRI. In contrast to the typical highly lateralized activation seen in our control subject, the subject with Joubert syndrome demonstrated striking bilateral activation of the sensorimotor and cerebellar cortex. Although our functional MRI data do not indicate a clear absence of decussation, the abnormal activation pattern observed suggests altered brain functional organization in relation to anatomic differences. Malformation of the hindbrain could result in recruitment of alternative pathways, similar to what has been observed following ischemic injury to the developing or mature central nervous system.

Neuromuscular response to sustained low-level muscle activation: within- and between-synergist substitution in the triceps surae muscles

The intent of this study was to investigate physiological recruitment strategies employed by the triceps surae (TS) muscles during sustained low-level activation. Surface-recorded electromyographic (EMG) signal data were recorded from the medial gastrocnemius (MG), lateral gastrocnemius (LG), and medial soleus (MS) muscles while eight subjects sustained static plantarflexion contractions at 10% of their maximum voluntary contraction (MVC) for 1 h. Fine-wire EMG activity was simultaneously recorded from electrodes located at three sites within the MG muscle. Correlation coefficients were computed among root mean square (RMS) amplitude values recorded from MG, LG, and MS to investigate between-synergist substitution, as well as between the three wire channels to investigate local substitution within the MG motor unit pool. Over the 1-h test, EMG amplitude in LG and MS increased linearly, and there was a moderately strong ( R(2)=0.662, P<0.023) positive correlation in the detrended activity between LG and MS, suggesting that these muscles generally acted together. When the data were divided into 5-min blocks, regression analysis on the partial correlation data revealed that MS and LG were correlated over the duration of the contraction ( P<0.001) suggesting co-activation synergism, whereas MG and LG and MS and MG demonstrated a tendency toward trade-off synergism. In five of eight individuals at least one wire channel pair within the MG was negatively correlated, suggesting that there was some form of substitution between motor units or motor unit pools. The other three individuals maintained correlated activity between all three pairs throughout the 1-h contraction.

Roles of motor-unit recruitment in producing force variability of simulated muscle contractions

The purpose of this study was to examine the effect of motor-unit recruitment on force variability by using computer simulated isometric contractions of a hand muscle (i.e., first dorsal interosseus). The force was simulated at 10 levels of excitation, ranging from 10 to 100% of maximum. Two recruitment conditions were simulated to compare the relative effect of motor-unit recruitment (MUR) on the relationship of force variability and level of force. One condition (40%MUR) recruited all motor units at 40% of the maximum excitation level, and the other (50%MUR) recruited all motor units at 50% of the maximum. The 40%MUR condition had a greater number of motor units than the 50%MUR group before the excitation level reached 50% of the maximum. The results showed that force variability increased at a faster rate before the completion of motor-unit recruitment and, thereafter, increased at a slower rate. In addition, the 40%MUR group showed greater force variability than the 50%MUR group. These data suggest that motor-unit recruitment is an important factor in causing force variability.

Weakness and strength training in persons with poststroke hemiplegia: Rationale, method, and efficacy

Several converging lines of contemporary evidence suggest that weakness presents a more serious compromise to movement function in poststroke hemiplegia than spasticity. This review examines the clinical and functional phenomena of weakness in poststroke hemiplegia, currently available evidence identifying physiologic substrates contributing to weakness, and reports of early investigations involving high-resistance training targeted at improving strength and the transfer of strength to improvements in functional capacity. Based on this information, we describe some unsolved problems and indicate some likely lines of development to increase our knowledge regarding how resistance training can be included in effective stroke rehabilitation.

Trunk recruitment during spoon use in tetraparetic cerebral palsy

In the present study we investigated the extent to which individuals suffering from spastic tetraparesis as a consequence of cerebral palsy tune their trunk involvement to accuracy demands in a spoon-handling task. Twenty-two participants (ten adolescents with spastic tetraparesis and 12 control participants) had to transport a spoon filled with water or sugar to a small or a large bowl that was placed within reach. Even though trunk displacement was larger in the tetraparetic participants than it was in the control participants, the effects of the imposed accuracy constraints were remarkably similar. Participants in both groups increased trunk displacement with increasing precision requirements. Furthermore, in both groups the largest trunk involvement was found in the initial and final part of the substance-transporting phase, when wrist velocity was lowest. We propose several explanations for these findings and conclude that the large trunk involvement in individuals with tetraparetic cerebral palsy should, in any case, not be regarded as a primary symptom of the disorder, but rather as an adaptive reaction to increased task demands.

Co-ordination within and between verbal and visuospatial working memory: network modulation and anterior frontal recruitment

Attention switching between items being stored and manipulated in working memory (WM) is proposed to be an elementary executive function. Experiment 1 reveals a similar attentional limitation within and between verbal and visuospatial WM and identifies a supramodal switching process required for switching between WM items. By using functional magnetic resonance imaging, Experiment 2 investigated brain activation correlates of parametrically varied attention switching within and between these two WM modalities. Attention switching activation was broadly distributed, was quite similar across the three conditions, and, in almost all areas, increased with increasing switching demand, indicating that attention switching recruits and modulates the entire WM network. Dorsolateral prefrontal cortex was implicated in both within- and between-modality attention switching, but no significant activation was found in ventrolateral areas, supporting dorsal-ventral process models of prefrontal organization. A functional dissociation between anterior frontal and dorsolateral prefrontal cortex was found with the former being more activated when switching attention between modalities was required. The data challenge the notion of an anatomically separate attention switching executive function, but suggest that anterior frontal areas are recruited for the additional demand of coordinating the verbal and visuospatial WM slave systems.

Frontal recruitment during response inhibition in older adults replicated with fMRI

Recent research has explored age-related differences in multiple areas of cognitive functioning using fMRI, PET, and SPECT. However, because these studies used different tasks, subjects, and methods, little is known about whether the results of these studies are generalizable or repeatable. The present study replicated a previous study [Psychol. Aging 17 (2002) 56] using the same Go/No-go task with a subset of 11 of the original older adult subjects, and using the same fMRI scanner and imaging methods. A direct comparison was made between these participants at Time 1 and Time 2 for both behavioral and functional data. These participants were also compared to a new young adult group of 11 participants. Although the current young adult group did not perform as well as the original young adult group, the original finding of enhanced left prefrontal activation in older adults relative to younger adults was replicated. Furthermore, when comparing Time 1 to Time 2, older adults exhibited comparable areas of activation, but significantly greater magnitude of activation at Time 1 in a few clusters. The findings indicate that older adults exhibit more bilateral brain activity during this task than young adults, which appears compensatory and is repeatable over time. The magnitude of regional activation, however, may vary with extraneuronal factors such as signal-to-noise ratio or task experience. This study adds to existing research suggesting that bilateral frontal activation is a predominant finding in the aging literature, and not specific to certain tasks in age group comparisons.

How the mesencephalic locomotor region recruits hindbrain neurons

This chapter summarizes experiments which were designed to reveal how repetitive electrical stimulation of the mesencephalic locomotor region (MLR) recruits nearby hindbrain neurons into activity, such that locomotion can ensue in the tiger salamander, A. tigrinum. The MLR stimulus strength was subthreshold or near-threshold for locomotor movements to ensue. Such relatively weak stimulation of the MLR produced locomotor movements after a relatively long delay, which featured neuronal interactions in the hindbrain. MLR-evoked spike responses of single hindbrain neurons were recorded before locomotor movements began. This allowed consideration of the build-up of the hindbrain neuronal activity, which was subsequently impressed upon the spinal cord such as to evoke locomotor movements. Each train of MLR stimulus pulses evoked monosynaptic responses in but a small proportion of the hindbrain's neurons. Rather, oligosynaptic responses were routinely evoked, even in the "input" neurons that were activated monosynaptically. Consecutive stimulus volleys recruited a given neuron after a variable number of synaptic translations. It is argued that the hindbrain's input neurons excited a much larger number of other hindbrain neurons. By this means, an MLR-evoked, short-lived propagating wave of excitation (i.e., approximately 2-4 successive synaptic activations) can be spread throughout the hindbrain.

Arthrogenic Muscle Inhibition Is Not Present in the Limb Contralateral to a Simulated Knee Joint Effusion

OBJECTIVE

The purpose of this investigation was to estimate bilateral neuromuscular activity in the vastus medialis on induction of a unilateral knee joint effusion.

DESIGN

Eight subjects each were assigned to effusion or control groups. The effusion group had 60 ml of sterile saline injected into their superolateral knee joint capsules. The control group rested for 8 mins. Bilateral recruitment curves for the Hoffmann reflex (H-reflex) and muscle response (M-wave) were obtained before and 10, 20, and 30 mins after the effusion or rest. The maximum H-reflex (Hmax), maximum M-wave (Mmax), and the Hmax/Mmax ratio were utilized for data analysis.

RESULTS

Both Hmax and Hmax/Mmax ratios decreased from the baseline measure in the effused vastus medialis, whereas no changes were detected on the contralateral side. Effused subjects demonstrated decreased motoneuron pool excitability in the effused limb, whereas control subjects did not differ from baseline.

CONCLUSIONS

Knee joint effusion results in ipsilateral but not contralateral impairment of quadriceps function. Rehabilitation protocols after knee joint injury should focus on ipsilateral neuromuscular and mechanical alterations that occur as the result of joint damage.

Comparison of joint torque evoked with monopolar and tripolar-cuff electrodes

Using a self-sizing spiral-cuff electrode placed on the sciatic nerve of the cat, the joint torque evoked with stimulation applied to contacts in a monopolar configuration was judged to be the same as the torque evoked by stimulation applied to contacts in a tripolar configuration. Experiments were carried out in six acute cat preparations. In each experiment, a 12-contact electrode was placed on the sciatic nerve and used to effect both the monopolar and tripolar electrode configurations. The ankle torque produced by electrically evoked isometric muscle contraction was measured in three dimensions: plantar flexion, internal rotation, and inversion. Based on the recorded ankle torque, qualitative and quantitative comparisons were performed to determine if any significant difference existed in the pattern or order in which motor nerve fibers were recruited. No significant difference was found at a 98% confidence interval in either the recruitment properties or the repeatability of the monopolar and tripolar configurations. Further, isolated activation of single fascicles within the sciatic nerve was observed. Once nerve fibers in a fascicle were activated, recruitment of that fascicle was modulated over the full range before "spill-over" excitation occurred in neighboring fascicles. These results indicate that a four contact, monopolar nerve-cuff electrode is a viable substitute for a 12 contact, tripolar nerve-cuff electrode. The results of this study are also consistent with the hypothesis that multicontact self-sizing spiral-cuff electrodes can be used in motor prostheses to provide selective control of many muscles. These findings should also apply to other neuroprostheses employing-cuff electrodes on nerve trunks.

MRI determination of muscle recruitment variations in dynamic ankle plantar flexion exercise

OBJECTIVE

The purpose of this study was to investigate the muscle recruitment variations in the dynamic ankle plantar flexion.

DESIGN

A total of 17 subjects participated in this study and performed the ankle plantar flexion exercise. Magnetic resonance T2-weighted images were obtained from the calf before and immediately after exercise to calculate each T2 relaxation time in the medial and lateral gastrocnemius, soleus, tibialis posterior, flexor digitorum/hallucis longus, peroneus longus, and dorsiflexors.

RESULTS

All the muscles except the dorsiflexors showed significantly increased T2 relaxation time and signal intensity on T2-weighted images after exercise. Above all, both gastrocnemius muscles showed significantly greater postexercise T2 relaxation time than the soleus, tibialis posterior, flexor digitorum/hallucis longus, and dorsiflexors. In addition, the peroneus longus had a tendency to show the greatest T2 relaxation time next to the gastrocnemius, but there was no significant difference between them.

CONCLUSIONS

The present study may suggest that the gastrocnemius muscle, especially the medial side, was best recruited in the dynamic ankle plantar flexion exercise. In addition, it is possible that the peroneus longus was most recruited next to the gastrocnemius in this exercise mode.

Use of surface EMG for studying motor unit recruitment during isometric linear force ramp

The aim of this work was to demonstrate the rank order of motor unit (MU) recruitment by surface EMG based on a Laplacian detection technique and to document the MU features at their recruitment threshold. Surface EMG signals were recorded on the biceps brachii of 10 healthy subjects during linear force ramps. When achievable, the signals were decomposed into MU action potential (MUAP) trains. MU inter-pulse interval (IPI), conduction velocity (MUCV) and amplitude were estimated on the first 12 MUAPs of each detectable train in order to characterize the MU features at their firing onset. A strong correlation was found between MU recruitment threshold and IPI, MUCV, and amplitude, showing that the size principle can be demonstrated by a fully non-invasive EMG technique. However, signal decomposition was not possible on seven subjects due to the effects of the volume conductor when the skinfold thickness was too large. When requirements for an optimal detection of MUAP trains are met, surface EMG may be used to improve our understanding of MU activity.

Changes in muscle fiber conduction velocity indicate recruitment of distinct motor unit populations

To obtain more insight into the changes in mean muscle fiber conduction velocity (MFCV) during sustained isometric exercise at relatively low contraction levels, we performed an in-depth study of the human tibialis anterior muscle by using multichannel surface electromyogram. The results show an increase in MFCV after an initial decrease of MFCV at 30 or 40% maximum voluntary contraction in all of the five subjects studied. With a peak velocity analysis, we calculated the distribution of conduction velocities of action potentials in the bipolar electromyogram signal. It shows two populations of peak velocities occurring simultaneously halfway through the exercise. The MFCV pattern implies the recruitment of two different populations of motor units. Because of the lowering of MFCV of the first activated population of motor units, the newly recruited second population of motor units becomes visible. It is most likely that the MFCV pattern can be ascribed to the fatiguing of already recruited predominantly type I motor units, followed by the recruitment of fresh, predominantly type II, motor units.

Effects on the right motor hand-area excitability produced by low-frequency rTMS over human contralateral homologous cortex

Repetitive transcranial magnetic stimulation (rTMS) has long lasting effects on cortical excitability at the site of stimulation, on interconnected sites at a distance and on the connections between them. In the present experiments we have used the technique of transcallosal inhibition between the motor cortices to examine all three effects in the same protocol. Ten healthy subjects received 900 rTMS stimuli at 1 Hz from a figure of eight coil over the left motor hand area. The intensity of rTMS was above the threshold for inducing short latency interhemispherical inhibition with a single stimulus (equivalent to 115-120 % resting motor threshold). Before and after the rTMS we evaluated: (1) in the left hemisphere, the amplitude of motor-evoked potentials (MEPs), and contralateral and ipsilateral cortical silent periods (CSP, ISP); (2) in the right hemisphere, MEP, CSP, ISP and short-interval intracortical inhibition and intracortical facilitation (SICI/ICF), and (3) interhemispherical inhibition (IHI) from the left-to-right hemisphere using a paired-pulse method. There were two main effects after rTMS to the left hemisphere: first, the amplitude of MEPs from the right hemisphere increased; second, there was a reduction in the IHI from the left-to-right hemisphere at interstimulus intervals of 7 and 10 ms but not at longer intervals (15-75 ms). Control experiments showed that these effects were not due to afferent inputs produced by the muscle twitches induced during the rTMS. The data are compatible with the notion that rTMS to the left hemisphere leads to reduced interhemispherical inhibition of the right hemisphere and a consequent increase in corticospinal excitability in that hemisphere.

Utility of laryngeal electromyography in predicting recovery after vocal fold paralysis

OBJECTIVES

To determine the value of laryngeal electromyography in predicting recovery after vocal fold paralysis and to determine what elements of laryngeal electromyography are most predictive of outcome.

DESIGN

Cohort study with retrospective data review.

SETTING

University voice center.

PARTICIPANTS

Consecutive sample of 31 patients with vocal fold paralysis diagnosed by indirect laryngoscopy, using either nasal insertion with a flexible nasopharyngoscope or peroral with a 70 degrees Hopkins rod. Inclusion criteria were that laryngeal electromyography had to have been performed between 21 days and 6 months from the onset of symptoms and patients must have been followed a minimum of 6 months.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURE

Vocal fold motion at 6 months from onset of symptoms.

RESULTS

Nine subjects (29%) had resolved vocal fold motion, whereas 22 (71%) had persistent vocal fold paralysis. By using our laryngeal electromyography paradigm, 4 of 6 patients with a laryngeal electromyography prognosis of excellent resolved. The predictive value for a negative test was 66.7%. For patients with a fair or poor prognosis, 5 of 25 resolved. The predictive value for a positive test was 80%. Only 8 of 22 patients with persistent vocal fold paralysis had fibrillations, and spontaneous activity was not significantly related to outcome. Patients with absent or greatly decreased motor unit recruitment had a significantly higher proportion of permanent vocal fold paralysis (P<.05). Stepwise forward logistic regression determined that prognosis by laryngeal electromyography and onset time were significant predictors of outcome (P<.01). The model predicted 44.4% of resolved cases, which represents a moderate prediction.

CONCLUSIONS

This study confirms the utility of laryngeal electromyography in assessing prognosis for recovery of vocal fold motion after laryngeal nerve injury. The results support our hypothesis that preset decision rules based on laryngeal electromyography data can be effectively used to determine a prognosis for recovery of vocal fold motion.

A functional magnetic resonance imaging study of patients with secondary progressive multiple sclerosis

Although several functional magnetic resonance imaging (fMRI) studies have shown adaptive cortical changes in patients with early multiple sclerosis (MS), the presence of brain plasticity and its role in limiting the functional consequences of brain tissue damage in patients with secondary progressive (SP) MS have not been fully investigated yet. In this study, we assessed the movement-associated brain pattern of cortical activations in patients with SPMS and investigated whether the extent of cortical brain activations is correlated with the extent of brain structural changes. From 13 right-handed SPMS patients and 15 sex- and age-matched healthy volunteers, we obtained: (a) brain dual-echo scans; (b) brain mean diffusivity and fractional anisotropy maps of the normal-appearing white (NAWM) and gray matter (NAGM); (c) fMRI during the performance of simple motor tasks [flexion-extension of the last four fingers of the right hand (task 1) and flexion-extension of the right foot (task 2)]. Compared to healthy volunteers, during task 1 performance, SPMS patients showed more significant activations of the ipsilateral inferior frontal gyrus, middle frontal gyrus, bilaterally, and contralateral intraparietal sulcus. During task 2 performance, SPMS patients had more significant activations of the contralateral primary sensorimotor cortex and thalamus and of the ipsilateral upper bank of sylvian fessure. For both tasks, strong correlations (r values ranging from -0.83 to 0.88) were found between relative activations of cortical areas of the motor network and the severity of structural changes of the NAWM and NAGM. This study demonstrates that cortical plasticity does occur in patients with SPMS and that it might have a role in limiting the clinical impact of MS-related damage. It also suggests that, in these patients, functional abilities are sustained by increased recruitment of highly specialized cortical areas.

Cognitive reserve and lifestyle

The concept of cognitive reserve (CR) suggests that innate intelligence or aspects of life experience like educational or occupational attainments may supply reserve, in the form of a set of skills or repertoires that allows some people to cope with progressing Alzheimer's disease (AD) pathology better than others. There is epidemiological evidence that lifestyle characterized by engagement in leisure activities of intellectual and social nature is associated with slower cognitive decline in healthy elderly and may reduce the risk of incident dementia. There is also evidence from functional imaging studies that subjects engaging in such leisure activities can clinically tolerate more AD pathology. It is possible that aspects of life experience like engagement in leisure activities may result in functionally more efficient cognitive networks and therefore provide a CR that delays the onset of clinical manifestations of dementia.

Exploring the neural basis of cognitive reserve

There is epidemiologic and imaging evidence for the presence of cognitive reserve, but the neurophysiologic substrate of CR has not been established. In order to test the hypothesis that CR is related to aspects of neural processing, we used fMRI to image 19 healthy young adults while they performed a nonverbal recognition test. There were two task conditions. A low demand condition required encoding and recognition of single items and a titrated demand condition required the subject to encode and then recognize a larger list of items, with the study list size for each subject adjusted prior to scanning such that recognition accuracy was 75%. We hypothesized that individual differences in cognitive reserve are related to changes in neural activity as subjects moved from the low to the titrated demand task. To test this, we examined the correlation between subjects' fMRI activation and NART scores. This analysis was implemented voxel-wise in a whole brain fMRI dataset. During both the study and test phases of the recognition memory task we noted areas where, across subjects, there were significant positive and negative correlations between change in activation from low to titrated demand and the NART score. These correlations support our hypothesis that neural processing differs across individuals as a function of CR. This differential processing may help explain individual differences in capacity, and may underlie reserve against age-related or other pathologic changes.

Cognition and aging: a highly selective overview of event-related potential (ERP) data

An overview of highly selected cognitive aging investigations of deviance detection, episodic memory and working memory reveals two primary themes: (1) when variability in elderly samples has been assessed, it has proven useful in understanding age-related changes in cognition; and (2) there is a frontal lobe contribution to at least some age-related changes in cognition. However, there are too few ERP age-related investigations of individual differences to determine whether the changes in patterns of ERP responding can be deemed "compensatory" or "inefficient." It is suggested that, to the extent possible, future electrophysiological investigations of cognitive aging (as well as other physiological measurement techniques) include individual difference measures that will enable the determination of the implication of a given neural pattern in the genesis of a given, age-related behavioral outcome pattern.

Differences in recruitment properties of the corticospinal pathway between lengthening and shortening contractions in human soleus muscle

The purpose of this study was to investigate how the recruitment properties of the corticospinal pathway are modulated in the soleus muscle of the lower limb during lengthening (LEN) and shortening (SHO) contractions by comparing the shape of the input-output (I/O) relation of the corticospinal pathway. To this end, we investigated the relationship between various stimulus intensities applied via transcranial magnetic stimulation and the size of motor-evoked potentials in 14 healthy subjects during voluntary plantarflexion and dorsiflexion (active lengthening) with a similar background activity (BGA) level. The shape of this relationship was sigmoidal and was characterized by a plateau value, maximum slope and threshold. The plateau value was clearly lower during LEN contractions than during SHO contractions. Likewise, the maximum slope was lower during LEN contractions. However, the threshold did not differ significantly between the two tasks. Since the plateau value and the maximum slope clearly differed between LEN and SHO contractions despite the similarity of their BGA levels, the central nervous system appears to have a different activation strategy for each of these tasks. Namely, the relative balance between excitatory and inhibitory components of the corticospinal volleys, as well as the subliminal fringe of the corticospinal pathway, were reduced during LEN contractions compared with SHO contractions. These strategies may help to avoid reflexive contractions brought about by higher discharge of muscle spindles and enable fine motor actions in voluntary lengthening contractions.

Scapular muscle recruitment patterns: trapezius muscle latency with and without impingement symptoms

BACKGROUND

Altered muscle activity in the scapular muscles is commonly believed to be a factor contributing to shoulder impingement syndrome. However, one important measure of the muscular coordination in the scapular muscles, the timing of the temporal recruitment pattern, is undetermined.

PURPOSE

To evaluate the timing of trapezius muscle activity in response to an unexpected arm movement in athletes with impingement and in normal control subjects.

STUDY DESIGN

Prospective cohort study.

METHODS

Muscle latency times were measured in all three parts of the trapezius muscle and in the middle deltoid muscle of 39 "overhand athletes" with shoulder impingement and compared with that of 30 overhand athletes with no impingement during a sudden downward falling movement of the arm.

RESULTS

There were significant differences in the relative muscle latency times between the impingement and the control group subjects. Those with impingement showed a delay in muscle activation of the middle and lower trapezius muscle.

CONCLUSION

The results of this study indicate that overhand athletes with impingement symptoms show abnormal muscle recruitment timing in the trapezius muscle. The findings support the theory that impingement of the shoulder may be related to delayed onset of contraction in the middle and lower parts of the trapezius muscle.

Experimental muscle pain changes feedforward postural responses of the trunk muscles

Many studies have identified changes in trunk muscle recruitment in clinical low back pain (LBP). However, due to the heterogeneity of the LBP population these changes have been variable and it has been impossible to identify a cause-effect relationship. Several studies have identified a consistent change in the feedforward postural response of transversus abdominis (TrA), the deepest abdominal muscle, in association with arm movements in chronic LBP. This study aimed to determine whether the feedforward recruitment of the trunk muscles in a postural task could be altered by acute experimentally induced LBP. Electromyographic (EMG) recordings of the abdominal and paraspinal muscles were made during arm movements in a control trial, following the injection of isotonic (non-painful) and hypertonic (painful) saline into the longissimus muscle at L4, and during a 1-h follow-up. Movements included rapid arm flexion in response to a light and repetitive arm flexion-extension. Temporal and spatial EMG parameters were measured. The onset and amplitude of EMG of most muscles was changed in a variable manner during the period of experimentally induced pain. However, across movement trials and subjects the activation of TrA was consistently reduced in amplitude or delayed. Analyses in the time and frequency domain were used to confirm these findings. The results suggest that acute experimentally induced pain may affect feedforward postural activity of the trunk muscles. Although the response was variable, pain produced differential changes in the motor control of the trunk muscles, with consistent impairment of TrA activity.