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At least 577 records · Page 32Linked to original sources
PubMed1994

The biphasic morphology of voluntary and spontaneous single muscle fiber action potentials.

The extracellular morphology of single muscle fiber action potentials (SMFAPs) is anticipated by volume conductor theory to be triphasic. Single muscle fiber action potentials recorded during single muscle fiber studies (500 Hz to 20 kHz) usually appear triphasic; however, when recorded with an open bandwidth (1 Hz to 20 kHz) they are found to be biphasic. Fibrillation potentials recorded with a single fiber electrode and open bandwidth have identical biphasic morphologies as volitional SMFAPs. Computer simulations suggest that the intracellular action potential models currently used to derive simulated extracellularly recorded SMFAPs must have the repolarization phase considerably prolonged to yield the clinically recorded potentials. This implies that either the models presently used require significant modification, or there is some distortion of the transmembrane source current induced in needle recording studies such that biphasic and not triphasic potentials are detected.

Action Potentials↗
PubMed1996

Effect of fatiguing maximal voluntary contraction on excitatory and inhibitory responses elicited by transcranial magnetic motor cortex stimulation.

Vertex transcranial magnetic stimulation (TMS) elicited tibialis anterior motor evoked potentials (MEPs) and silent periods (SPs) that were recorded during and following isometric maximal volitional contraction (MVC). During MVC in 6 healthy subjects, MEP amplitudes in the exercised muscle showed an increasing trend from an initial value of 4539 +/- 809 muV (mean +/- SE) to 550 +/- 908 muV (P < 0.13) while force and EMG decreased (P < 0.01). Also, SP duration increased from 165 +/- 37 ms to 231 +/- 32 ms (P < 0.01). Thus, during a fatiguing MVC both excitatory and inhibitory TMS-induced responses increased. TMS delivered during repeated brief 10% MVC contractions before and after a fatiguing MVC in 5 subjects, showed no change in MEP amplitude but SP duration was prolonged after MVC. This SP prolongation was focal to the exercised muscle. Silent periods recorded after pyramidal tract stimulation were unchanged following the MVC. These results suggest that MEP and SP might have common sources of facilitation during an MVC and that inhibitory mechanisms remain focally augmented following a fatiguing MVC.

Adult↗
PubMed2007

A stress echocardiography study of cardiac function during progressive exercise in pediatric oncology patients treated with anthracyclines.

BACKGROUND: Anthracycline-treated patients (AP) are at risk for cardiac dysfunction years after treatment. Cardiac function has not been evaluated during exercise in AP. The purpose of this study was to assess exercise tolerance, left ventricular (LV) function, and hemodynamics during progressive exercise. PROCEDURE: We studied 47 AP (cumulative dose: 36-504 mg/m(2)) who were in complete remission and 12 healthy controls (CON). AP were further grouped by cumulative dose (LOW or= 260 mg/m(2)) and resting echocardiographic function. All subjects performed 3-min incremental stages on a semi-recumbent cycle ergometer until volitional fatigue. Using echocardiography and Doppler, LV dimensions, posterior wall thickness (LVPWs), peak aortic velocity (PAoV), shortening fraction (SF), rate-corrected mean velocity of fiber shortening (MVCFc), wall stress at peak systole (sigmaPS), stroke volume index (SVI), and cardiac index (CI) were determined. Measurements were performed at rest, during each stage of exercise, and in recovery. RESULTS: AP did less work than CON (P < 0.050). CON and LOW had similar resting function, while HIGH had a lower SF and MVCFc (P < 0.050) and a higher sigmaPS (P < 0.001). Resting SVI and CI were also lower in HIGH. At peak exercise, MVCFc and sigmaPS remained different in HIGH, and both AP groups had a lower SF, SVI, and CI (P < 0.001). AP had an abnormal SVI response to exercise. AP showed a smaller initial increase in SVI that was not maintained throughout exercise. CONCLUSIONS: AP have reduced exercise tolerance. There also appears to be a dose-related effect on myocardial contractility and SVI. These findings may help to guide treatment.

Adolescent↗
PubMed1995

On the neurophysiology of consciousness: I. An overview.

How certain neural mechanisms (Mc) momentarily endow with subjective awareness (C) percepts and affects represented elsewhere is more likely to be clarified when structures essential to Mc are identified. The loss of C with bilateral thalamic lesions involving the intralaminar nuclei (ILN) contrasts with retention of C after large cortical ablations depriving C of specific contents. A role of ILN in the perception of primitive (i.e., not cortically computed) sensations is suggested by their afference of directly ascending pathways. A role for ILN in awareness of cortical activity is suggested by their widespread afference from cortex, a property shared with striatum. A role for ILN in volition is suggested by their heavy projection to striatum. Unlike striatum, ILN also project widely to almost all neocortex, enabling an effect on ideation; this last property is in common with other structures (e.g., locus coeruleus) but none of them has the same direct cortical afference. And passage through the reticular nucleus (nRt) of ILN efferents to cortex could impact the attention-selective action of nRt. It is suggested that the quickest route to a better understanding of C involves more intensive study of ILN. No other structure seems, in the light of our current knowledge, a more likely site for Mc.

Awareness↗
PubMed2002

In support of Pockett's critique of Libet's studies of the time course of consciousness.

Susan Pockett presents sound arguments supporting her reinterpretations of data that Libet and co-workers used to support a number of intriguing and influential conclusions regarding the microgenesis and timing of (a) conscious sensory experience and (b) volitionally controlled motor responses. The following analysis, extending and elaborating some of her main arguments, proposes that Libet's experimental methodologies and rationales, and thus also his interpretation of data, are flawed and that neglect or ignorance of methodological and empirical constraints well known to sensory psychologists risks drawing premature or faulty conclusions about the timing of conscious experience.

Chronobiology Phenomena↗
PubMed2002

Peri-Ictal Behavioral and Cognitive Changes.

Peri-ictal behavioral and cognitive changes contribute substantially to disability and distress among people with epilepsy. Psychosis, depression, and suicide may all occur as complications of seizures. Greater appreciation and understanding of the peri-ictal period is clinically important and might open novel therapeutic windows. At the same time this period provides a model for understanding basic mechanisms underlying mood and thought disorders and the substrates of cognition, volition, emotion, and consciousness. This review will discuss behavioral and cognitive antecedents of seizures, including the preictal milieu, reflex seizures, and self-induced seizures. Behavioral and cognitive treatment approaches that have been undertaken are reviewed. Both acute and delayed postictal emotional, behavioral, and cognitive changes will be discussed. Finally, possible mechanisms by which epileptic brain activity and behavior may modify each other are considered.

Journal Article↗
PubMed1994

Essential tremor entrains rapid voluntary movements.

The effect of essential tremor on the timing of rapid wrist flexion was examined in 10 patients with moderate to severe disability. The mean reaction time and motor time of the patients did not differ from the mean values of 10 healthy age- and sex-matched controls. The latencies of the triphasic agonist and antagonist EMG bursts did not differ between patients and controls. The initiation of movement was time-locked to the tremor cycle in all patients. The initial agonist muscle activation occurred in phase with the rhythmic bursts of EMG, but the onset of rapid wrist flexion occurred when the momentum of essential tremor opposed the volitional movement. Essential tremor has subtle effects on motor control that transcend oscillation per se. These effects probably contribute to the impaired performance of fine motor tasks in patients with advanced disease.

Aged↗
PubMed2002

What does the frontomedian cortex contribute to language processing: coherence or theory of mind?

The frontomedian cortex (FMC) has been shown to be important for coherence processes in language comprehension, i.e., for establishing the pragmatic connection between successively presented sentences. The same brain region has a role during theory-of-mind processes, i.e., during the attribution of other people's actions to their motivations, beliefs, or emotions. In this study, we used event-related functional magnetic resonance imaging at 3 T to disentangle the relative contributions of the FMC to theory-of-mind (ToM) and coherence processes, respectively. The BOLD response of nine participants was recorded while they listened to pragmatically coherent or unrelated sentence pairs. Using a logic instruction for inanimate sentence pairs, ToM processing was discouraged during the first part of the experiment. Using explicit ToM instructions for sentence pairs mentioning human protagonists, ToM processing was induced during the second part. In three of the resulting four conditions a significant increase in the BOLD response was observed in FMC: when ToM instructions were given, both coherent and incoherent trials elicited frontomedian activation, in replication of previous results showing involvement of the FMC during ToM tasks. When logic instructions were given, the coherent trials, but not the incoherent trials, activated the FMC. These results clearly show that the FMC plays a role in coherence processes even in the absence of concomitant ToM processes. The findings support the view of this cortex having a domain-independent functionality related to volitional aspects of the initiation and maintenance of nonautomatic cognitive processes.

Adult↗
PubMed1995

Single-trial readiness potentials and fatigue.

The authors propose that the cognitive processes related to internal motivation and volition (e.g., intention and preparation of a voluntary action), influenced by central fatigue, could be identified and characterized by cerebral readiness potentials (RP) using methods of chaotic dynamics. The boundaries of single-trial RP and its successive phases can be detected by tracking the data dynamics, and are represented by chaotically behaved short EEG transitions.

Animals↗
PubMed1992

Head movement propensity.

In studies of human eye-head coordinated saccadic gaze shifts, different laboratories have found greatly different magnitudes of head movements for a given amplitude of gaze shift (head movement gain). The present study was conducted to examine why, and to quantify volitional head movements. Fixation/target lights were located at 20 degrees and 40 degrees on either side of a central light. There were two tasks or modes. In the non-aligned mode, gaze fixation (first light) was followed by a gaze step to the target (second light) accurately and quickly. In the head-aligned mode, the head was aligned within 3 degrees of the first fixation light (i.e., initial starting position) before the step sequence began. In both non-aligned and head-aligned modes, subject instructions pertaining to the second target light concerned only gaze; there was no requisite head position. The head movement propensity of nine subjects was ranked according to the mean gain (head/target amplitude) of two 40 degree jumps (0-40 degrees and -20 to +20 degrees) in the non-aligned mode. This ranking method clearly identified extreme head-movers and non-movers. The moderate movers were further characterized by three additional criteria, derived by comparing the gains in different jumps, which varied in starting position and amplitude. First, when the two 40 degree jumps were compared, typically the gain of non-movers was less in the symmetric jump (-20 to +20 degrees) with the gain of the head-movers was greater in the symmetric jump. Second, in the head-aligned mode the gain of non-movers progressively increased when the starting position was progressively moved eccentrically, whereas the gain of head-movers increased only slightly, if at all. Third, when the gains of two symmetric (40 degrees and 80 degrees) jumps were compared, the head-movers consistently had opposite trends from non-movers. These three comparative criteria and the initial criterion together define head movement propensity. To explain the above observations, three effects are proposed. First, a "midline-attraction" effect causes resistance to movement away from the midline in non-movers and an increase in movement amplitude if the jump starts eccentrically. Second, a "resetting" effect occurs when the eccentricity of the jump is varied; the stopping position is reset closer to the target. Third, an "awareness/arousal" effect increases the gain in the head-aligned mode due to the intrinsic nature of the alignment procedure.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1991

Influences of object weight and instruction on grip force adjustments.

This study investigated the influence of object weight and instructions on grip force responses in humans. Using a precision grip, subjects lifted a small instrumented test object to a predetermined height. Prior to each set of 40 trials, subjects were verbally instructed to either "hold" or "let go" of the object in response to any change in weight. Unpredictably on some trials (less than 20%), a sudden sustained increase (load) or decrease (unload) in vertical load was applied to the object. Grip responses to these induced weight changes were evaluated by measuring grip force, object position, and associated electromyographic (EMG) activity. Grip force changes for a load were over three times greater than those for an unload. Such asymmetry may reflect everyday grasp and manipulation in a gravity-influenced world. Grip force adjustments to loads following "hold" instructions were on the average somewhat larger than those following "let go" instructions, but there was no influence of instructions on responses to unloads. These findings contrast with more robust influences of verbal instruction on automatic postural and proximal upper limb responses and also may suggest that grip force adjustments are influenced to a greater extent by intrinsic task variables than by extrinsic volitional intent. Such organization appears tailored to functional task requirements in natural environmental contexts.

Adult↗
PubMed1982

Long latency reflexes to imposed displacements of the human wrist: dependence on duration of movement.

Reflex EMG responses to angular displacements of the wrist joint were recorded from 12 normal human volunteers. A mechanical stop was used to suddenly arrest displacements at varying times following the onset of the stimulus. With unrestricted movement of the handle, the EMG response consisted of an early component (M1) with a latency of 30-35 ms and a long-latency component (M2-3) beginning 55-65 ms after the onset of the displacement. When the displacements were arrested prior to a critical time occurring between 40 and 50 ms after the onset (mean of 44 ms), the M2-3 component of the response was not present. Increasing the duration of the displacement beyond this time resulted in a rapid increase in the size of M2-3. Facilitation provided by volitional intent to oppose the perturbation was not sufficient to generate an M2-3 response to either a brief, low velocity displacement produced by the torque motor or to a phasic, high velocity stretch produced by a tendon tap. The timing relationships between the onset latency of M2 and the minimum duration of displacement required to generate an M2-3 response are not easily reconciled with the notion that the segmentation of the EMG responses into components is mediated by repeated activation of the same central reflex pathway by phasic afferent bursts. Two mechanisms that could account for these results are either inhibition in response to the sudden stop of phasically-active "linking" interneurons which are part of the long latency pathway, or the loss of an essential convergent facilitatory input which serves to monitoring the continuation of the movement.

Adult↗
PubMed1985

Frontal lobe lesions in man cause difficulties in suppressing reflexive glances and in generating goal-directed saccades.

The frontal eye field (FEF) and superior colliculus (SC) are thought to form two parallel systems for generating saccadic eye movements. The SC is thought classically to mediate reflex-like orienting movements. Thus it can be hypothesized that the FEF exerts a higher level control on a visual grasp reflex. To test this hypothesis we have studied the saccades of patients who have had discrete unilateral removals of frontal lobe tissue for the relief of intractable epilepsy. The responses of these patients were compared to those of normal subjects and patients with unilateral temporal lobe removals. Two tasks were used. In the first task the subject was instructed to look in the direction of a visual cue that appeared unexpectedly 12 degrees to the left or right of a central fixation point (FP), in order to identify a patterned target that appeared 200 ms or more later. In the second "anti-saccade" task the subject was required to look not at the location of the cue but in the opposite direction, an equal distance from FP where after 200 ms or more the patterned target appeared. Three major observations have emerged from the present study. Most frontal patients, with lesions involving both the dorsolateral and mesial cortex had long term difficulties in suppressing disallowed glances to visual stimuli that suddenly appeared in peripheral vision. In such patients, saccades that were eventually directed away from the cue and towards the target were nearly always triggered by the appearance of the target itself irrespective of whether or not the "anti-saccade" was preceded by a disallowed glance. Those eye movements away from the cue were only rarely generated spontaneously across the blank screen during the cue-target time interval. The latency of these visually-triggered saccades was very short (80-140 ms) compared to that of the correct saccades (170-200 ms) to the cue when the cue and target were on the same side, thereby suggesting that the structures removed in these patients normally trigger saccades after considerable computations have already been performed. The results support the view that the frontal lobes, particularly the dorsolateral region which contains the FEF and possibly the supplementary motor area contribute to the generation of complex saccadic eye-movement behaviour. More specifically, they appear to aid in suppressing unwanted reflex-like oculomotor activity and in triggering the appropriate volitional movements when the goal for the movement is known but not yet visible.

Adult↗
PubMed1993

Influence of posture on arm exercise tolerance and physiologic responses in persons with spinal cord injured paraplegia.

This study compared metabolic and cardiopulmonary responses to incremental supine and upright sitting arm crank ergometry (ACE) in nine men with spinal cord injured paraplegia ranging from T1-T5. Both tests consisted of continuous graded ACE from rest to volitional fatigue on a modified electronically braked cycle ergometer with the work rate increased by 8.2 W.min-1. No significant differences (P > 0.05) existed for peak ACE power output (W), oxygen uptake, pulmonary ventilation, respiratory exchange ratio, and O2 pulse between the two tests. Heart rate and O2 pulse responses at six submaximal work rates representing 0-58% peak W were also not significantly different between postures. These data indicate that ACE tolerance in persons with high-lesion paraplegia was not enhanced when ACE was performed in the supine posture.

Adult↗
PubMed1988

Changes in muscle and cutaneous cerebral potentials during standing.

The cerebral potentials produced by electrical stimulation of mechanoreceptive afferents from the foot were recorded in the sitting and standing postures to determine whether transmission to cortex was altered by the postural change. The latencies of the early components of the cerebral potentials produced by muscle afferents (posterior tibial nerve) and cutaneous afferents (sural nerve) did not change with posture. Standing was associated with an approximately 25-35% decline in amplitude of the earliest components of the posterior tibial cerebral potential (N38-P40, P40-N50) for a stimulus intensity associated with a submaximal afferent volley. The amplitude of the equivalent N38-P40 and P40-N50 components produced by sural afferents also declined during quiet stance. In most experiments the subcortical component (P32-N38) was not reduced by stance so that the amplitude attenuation probably occurs in part at cortical level. Qualitatively similar changes in the cerebral potentials were documented for a range of stimulus intensities, including those which evoked a maximal initial component in the nerve volley. For a similar reduction in the initial (N38-P40) component of the cerebral potential, voluntary plantar flexion in the sitting position produced less attenuation in subsequent components than did standing. Thus, attenuation of the cerebral potential during standing may involve specific posture-related factors in addition to those related to volition.

Afferent Pathways↗
PubMed1996

Prepharyngeal dysphagia in Parkinson's disease.

Dysphagia in patients with Parkinson's disease (PD) is most often attributed to pharyngeoesophageal motor abnormalities. In our study of patients with idiopathic PD, attention was focused on prepharyngeal symptoms and motor functions. Using the Hoehn and Yahr disease severity scale, patients were grouped into those with mild/moderate disease [subgroup I (n = 38)] and those with advanced disease [subgroup II (n = 34)]. Dysphagia symptoms were present in 82% of all patients, but subgroup I patients voiced significantly more complaints. Conversely, many prepharyngeal abnormalities of ingestion, including jaw rigidity, impaired head and neck posture during meals, upper extremity dysmotility, impulsive feeding behavior, impaired amount regulation, and lingual transfer movements were statistically more frequent in subgroup II patients. Impaired mastication and oral preparatory lingual movements were the most common aberrations observed during dynamic videofluoroscopy (48/71), with most patients being concordant for both. The motor disturbances of ingestion reported herein reflect the disintegration of volitional and automatic movements caused by PD-related akinesia, bradykinesia, and rigidity.

Aged↗
PubMed1988

Incidence of exercise induced hypoxemia in elite endurance athletes at sea level.

Recent evidence suggests that exercise-induced hypoxemia (EIH) may occur in healthy trained endurance athletes. However, at present, no data exist to describe the regularity of EIH in athletes or non-athletes. Therefore, the purpose of the present investigation was to determine the incidence of EIH during exercise in healthy subjects varying in physical fitness. Subjects (N = 68) performed an incremental cycle ergometer test to volitional fatigue with percent arterial oxyhemoglobin saturation (%SaO2) measured min-by-min. For the purpose of data analysis subjects were divided into three groups according to their level of physical training: 1) untrained (N = 16), 2) moderately trained (N = 27), and 3) elite highly trained endurance athletes (N = 25). EIH was defined as a %SaO2 of less than or equal to 91% during exercise. EIH did not occur in any of the untrained subjects or the moderately trained subjects. However, EIH occurred in 52% of the highly trained endurance athletes tested and was highly reproducible (r = 0.95; P less than 0.05). These findings further confirm the existence of EIH in healthy highly trained endurance athletes and suggests a rather high incidence of EIH in this healthy population. Hence, it is important that the clinician or physiologist performing exercise testing in elite endurance athletes recognize that EIH can and does occur in the elite endurance athlete in the absence of lung disease.

Adult↗
PubMed1987

Influence of a 3.5 day fast on physical performance.

Eight young men were tested for strength, anaerobic capacity and aerobic endurance in a post absorptive state and after a 3.5 day fast. Strength was tested both isokinetically (elbow flexors, 0.52 rad x s-1 and 3.14 rad x s-1) and isometrically. Anaerobic capacity was evaluated by having subjects perform 50 rapidly repeated isokinetic contractions of the elbow flexors at 3.14 rad x s-1. Aerobic endurance was measured as time to volitional fatigue during a cycle ergometer exercise at 45% VO2max. Measures of VO2, VE, heart rate, and ratings of perceived exertion were obtained prior to and during the cycle exercise. The 3.5 day fast did not influence isometric strength, anaerobic capacity or aerobic endurance. Isokinetic strength was significantly reduced (approximately 10%) at both velocities. VO2, VE and perceived exertion were not affected by fasting. Fasting significantly increased heart rate during exercise but not at rest. It was concluded that there are minimal impairments in physical performance parameters measured here as a result of a 3.5 day fast.

Adult↗