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Biomedical subjects

D G Rüegg

Publications and source records attributed to D G Rüegg.

17 recordsLinked to original sources
PubMed1990

Superposition of H reflexes on steady contractions in man.

1. The aim of the investigation was to study the influence of steady isometric contractions on H reflexes of human soleus muscle. 2. Stimulating and recording conditions were hardly affected by plantar flexions which subjects maintained in a force matching task. 3. If the interval between a preceding control and the test stimulus was less than 8 s the test H reflex was depressed in the relaxed subject. The depression was diminished or removed if the test reflex was superimposed on a background activity. The interval between control and test H reflex was at least 8 s in the following experiments. 4. H reflexes were nearly independent of steady plantar flexions on which they were superimposed. In some subjects, there was a slight increase with increasing torque. During dorsal flexions, H reflexes in all subjects were inhibited with increasing torque. 5. The relationship between test H reflexes, control H reflexes and background activity was evaluated by varying pseudo-randomly stimulus intensity and steady flexion torque. The surface defined by this three-dimensional relation approximated a plane suggesting linear properties of the H reflex. In some subjects threshold intensity decreased slightly with torque, in others it was constant. 6. In response to a warning signal, human subjects initiated steady plantar or dorsal flexions in both feet and, at the same time, they started to concentrate on a light at the onset of which they performed a unilateral ballistic plantar contraction as fast as possible. The relations between H reflex and maintained flexion force during the warning period of the reaction time task were identical to those during force matching, showing that the behavioural context did not modulate the relations. 7. The relations were also the same if reflexes were evoked bi- or unilaterally, illustrating the absence of a mutual modification of simultaneously evoked H reflexes. 8. The relation was the same with ipsilateral matching and relaxed contralateral muscles as with bilateral matching. If the ipsilateral side stayed flaccid contralateral matching increased H reflexes by about 20% above control values. 9. It was concluded that various factors can combine to produce an increase of H reflexes with torque, the most important of them being the use of short intervals between H reflexes. We have various evidence from the present experiments for believing that, in the relaxed subjects, the subliminal fringe was small and that although stimulus intensities below threshold could evoke an afferent volley, the effect of this on low-threshold motor units was prevented by presynaptic inhibition at the Ia terminals.

Biomechanical Phenomena↗
PubMed1989

Superposition of ballistic on steady contractions in man.

In a visual reaction time task, human subjects superimposed isometric ballistic contractions on a maintained activity in the soleus or anterior tibial muscle. Since there were good reasons to believe that the supraspinal motor commands for the ballistic contractions were independent of those for the background activity, the interaction between the motor commands for the ballistic and for the steady contractions could be studied at the spinal level. If ballistic and steady contractions were in the same direction, the EMG burst and torque changes associated with the ballistic contraction were nearly constant irrespective of the maintained steady flexion force. This was true if a muscle was activated to about 5% of its maximum force as the soleus muscle during plantar flexions and if it was activated to about 40% of its maximum force as the anterior tibial muscle during dorsal flexions. If ballistic and steady contractions were in opposite directions the torque changes related to the ballistic contraction increased linearly with the background activity. This relation was caused by a reduction in antagonist activity starting about 50 ms before the agonist EMG burst and not by an increased agonist burst, the latter remaining independent of background activity. These results imply that the input-output relationship of the motoneuronal pool is nearly linear. The functional basis of this relation is the size principle which is valid during continuous and ballistic contractions. The number of motor units recruited for the ballistic contraction is adjusted according to their force such that the contraction amplitude remains constant.

Adult↗
PubMed1988

Origin of the specific H reflex facilitation preceding a voluntary movement in man.

1. In a reaction time situation, the monosynaptic spinal reflex (H reflex) is facilitated before the onset of an electromyographic (EMG) response. The aim of the present investigation was to test if the facilitation can be attributed either to a subliminal depolarization of motoneurones or to an increase of the excitatory effect of the afferent volley reaching the motoneurones. 2. At the onset of an acoustic warning signal, human subjects were required to concentrate on a reaction time task and, in addition, to initiate a steady isometric plantar flexion of medium intensity in both feet. In response to a following visual stimulus, they carried out a ballistic plantar flexion randomly with the right or left foot. At different times after the visual reaction signal, H reflexes were elicited bilaterally. 3. The facilitation of the H reflex was similar in the presence and absence of a steady activation. In addition, the facilitations were similar in absolute amplitude and duration when the stimuli evoking the H reflexes were at threshold intensities, or at an intensity which produced control H reflexes of 60% maximum amplitude. 4. In a second series of experiments, no H reflexes were elicited but the strength of the steady plantar flexion was varied. Premotor time, i.e. the interval between the onset of the visual stimulus and the EMG response, and reaction time, i.e. the interval between the onset of the visual stimulus and the mechanical response, were computed. Neither parameter depended significantly on the intensity of steady flexion and they were the same with steady flexion as without. 5. The rectified EMG records and the torque records were aligned by the end of premotor time. Three-dimensional displays of average activity as a function of time and steady activation level were computed. No activation before premotor and reaction time was detected which could have been related to the H reflex facilitation. 6. The present results suggest that all motoneurones, in particular those being activated during the voluntary contraction, can contribute to the H reflex facilitation before movement onset and that the basis of this facilitation is an enhanced excitatory effect of the afferent volley elicited by the H reflex stimulus. Mechanisms leading to the facilitation could be removal of presynaptic inhibition at I a terminals or facilitation of interneurones intercalated in polysynaptic components of the reflex pathways.

Adult↗
PubMed1984

Effects of electrical stimulation of low-threshold muscle afferents on visual reaction time.

Human subjects performed plantar flexions of the foot in visual reaction time (RT) situations. In the simple RT task, the same foot had to be moved during the whole session, in the choice RT task, the right or left foot was moved in a random sequence. Low-threshold muscle afferents which evoked a monosynaptic H reflex were electrically activated on both sides at various intervals after the onset of the light stimulus. In both RT situations. RT was shortened equally by the electrical stimulation. The intersensory facilitation in the choice RT task provided support for the preparation enhancement model, since the subjects could not react to the electrical stimulus alone.

Adult↗
PubMed1984

Relation between the specific H reflex facilitation preceding a voluntary movement and movement parameters in man.

In a reaction-time situation, the monosynaptic spinal reflex (H reflex) is facilitated before the onset of an electromyographic (e.m.g.) response. The aim of the present investigation was to study aspects of this facilitation. Human subjects were required to perform isometric plantarflexions of the foot in response to a visual stimulus. The movement was always on the same side in the simple reaction-time situation, and randomly with the right or left foot in the choice reaction-time situation. Stimuli to evoke H reflexes were applied bilaterally 40-400 ms after the onset of the visual stimulus. Pre-motor time, i.e. the interval between the onset of the visual stimulus and the e.m.g. response, and reaction time, i.e. the interval between the onset of the visual stimulus and the response on the torque recording, were computed. In both reaction-time situations, there was a significant facilitation of the ipsilateral H reflex 100-160 ms before e.m.g. onset and, in some subjects, a small facilitation of the contralateral H reflex. The specific facilitation, i.e. the difference between the facilitation on the ipsi- and contralateral side relative to the movement, was not significantly different on the right and left side. Pre-motor time was divided into the interval from the light onset until the onset of the specific facilitation, and the interval from the onset of the facilitation until the onset of the voluntary response. Both intervals increased, and the slope and the amplitude of the facilitation decreased with increasing pre-motor time and reaction time. The specificity of the H reflex facilitation in a choice reaction-time situation implies that the interval from light onset until the onset of the facilitation includes stimulus identification and response selection, and the interval from the onset of the facilitation until the e.m.g. response preparation of the motor system for the required movement. The present results suggest that the specific facilitation of the H reflex before a movement is caused by removal of presynaptic inhibition at I a terminals or by activation of interneurones intercalated in polysynaptic components of the H reflex rather than by a subthreshold activation of motoneurones.

Action Potentials↗
PubMed1983

Peripheral and transcortical loops activated by electrical stimulation of the tibial nerve in the monkey.

Peripheral and supraspinal loops activated by electrical stimulation of the tibial nerve were studied in alert monkeys. Weak conditioning stimuli below the threshold for muscle contraction and strong conditioning stimuli which elicited a direct motor response and an H-reflex were applied to the tibial nerve, while soleus EMG was recorded. The motoneuronal excitability was measured with a test H-reflex for different intervals between conditioning and test stimuli. A facilitation of the motoneuronal pool occurred with a latency of about 50 ms for weak, as well as strong, conditioning stimuli. The facilitation was superimposed on a long-lasting inhibition which was more pronounced when strong stimuli were used. The shape of the excitability curve of the motoneurons after strong conditioning stimuli was studied before and after various lesions. The shape of the curve did not change after an ipsilateral cerebellectomy except that the facilitation was more pronounced. A few days after pyramidotomy, the facilitation diminished in size but it recovered to its initial size after 3 months. Spinal hemisection did not abolish the facilitation. We concluded from these results that a peripheral loop was activated when conditioning stimuli above the threshold for a direct motor and reflex response were applied. The facilitation might be mediated by the muscle contraction and following activation of muscle afferents. Superimposed on thus loop is a previously demonstrated transcortical loop of similar latency.

Animals↗
PubMed1983

Detection of reaction time by an adaptive filter based on the least squares fit.

An adaptive filter based on the least squares fit between a template and individual records was developed. It has been applied to compute reaction times of a voluntary movement. Its performance was tested by comparing these reaction times with the latency of the EMG responses. Different properties of the least squares fit technique and Woody's filter technique (Woody 1967) are discussed.

Electromyography↗
PubMed1978

Spinal projection to the dorsolateral nucleus of the caudal basilar pons in the cat.

In the cat, a spinal projection to a restricted area of the basilar pontine grey has been revealed with use of anterograde degeneration technique (Fink-Heimer). The area was ipsilateral to the spinal lesion, restricted to the far caudal limit of the pons, and included the dorsal and the dorsolateral subdivisions of the pontine nuclei (PN). Comparisons following high cervical, midthoracic and upper lumbar spinal lesions did not reveal any somatotopic organization. Only a few spinopontine fibers had origins below segmental level L4. Lesions of various quadrants of the cord indicated that the spinopontine fibers ascended through the dorsolateral funiculus, and not through either the dorsal or the ventral funiculi. Comparison with the degeneration effects of cerebral cortical lesions showed that the spinal projection to the PN overlapped to some extent with the projection from the first sensorimotor and second somatosensory cortices. In the rat no comparable spinopontine projection was found. It is suggested that the spinopontine pathway might forward information to the cerebellum from visceral sensory receptors or perhaps from pools of spinal interneurons.

Animals↗
PubMed1975

Why transcortical reflexes?

Experiments in humans and in monkeys have indicated that load perturbations, occurring during voluntary movements and postural activity, may be automatically compensated for. Overall muscle stiffness opposing load changes is determined by the visco-elastic properties of the muscle, by segmental reflex actions and finally by long-loop reflexes. Under certain circumstances, for instance when the subject or the experimental monkey is "prepared" to counteract perturbations which are unpredictable in time, the long-loop "reflexes" appear to be responsible for most of the corrective muscle tension. Experiments in anaesthetized monkeys revealed that signals from stretch afferents reach neurons of the motor cortex, possibly via a relay in the cortical area 3a. The latencies of these responses to well controlled muscle stretches were in the same range as motor cortical cell discharges recorded in alert monkeys subjected to load perturbations. Furthermore, these responses of cells in the motor cortex also had the appropriate timing to indicate a causal relationship with the long-latency electromyographic responses to load changes referred to above. These experimental results therefore strongly support the hypothesis, first proposed by Phillips (1969), of a transcortical servo-loop adjusting motor cortical output according to the load conditions in which movements are performed. The major advantage of transcortical regulations as opposed to segmental regulations, seems to be a powerful gain control acting at the cortical level; it was repeatedly shown that the long-loop reflexes are strongly modifiable and under voluntary control. It is suggested that an adaptive gain control at the cortical level is a prerequisite to preserve the complex capabilities of the motor cortex as the chief "executive" for skilled, preprogrammed movements. A loss of this adaptive gain control may be, at least partly, the cause of motor disorders such as rigidity in Parkinsonian patients, as reported by Tatton and Lee (1975). It is suggested that further investigations of the control of transcortical reflexes may aid in the understanding of the pathophysiology of motor disabilities.

Animals↗
PubMedDate not supplied

Electromyographic assessment of central motor disorders.

Experimental procedures are described that were designed to assess central motor disorders quantitatively. Initially delineated is a study of triggered ballistic movement performed in a reaction-time situation. The reciprocal triphasic EMG pattern recorded in an antagonistic muscle pair was clearly abnormal in Parkinsonian patients; increased duration, diminished synchronization of motor units, and a tendency for coactivation of agonist and antagonist muscles and for action tremor were observed. Transport time of elbow movement was prolonged, particularly in a choice reaction-time situation. Rapid, passive displacements of the forearm combined with excitability measurements of hindlimb motoneurons in monkeys revealed the existence of a transcortical loop that may contribute to increased muscle tone in Parkinsonian patients.

Animals↗
PubMedDate not supplied

Responses of neurones of the pontine nuclei to stimulation of the sensorimotor, visual and auditory cortex of rats.

The aim of this study was to investigate the degree of convergence of corticofugal neurones from various cortical areas onto single neurones of the pontine nuclei (PN). Eighty-five% of the PN neurones responded to electrical stimulation of at least one of the following cortical areas: sensorimotor, visual, auditory. Slightly less than half of these neurones displayed a convergent input from two or three of functionally different cortical areas. The sensorimotor cortex, particularly the face areas, provided the most important input to the PN; stimulation of the visual cortex was less effective and stimulation of the auditory cortex rarely excited PN neurones. The electrophysiological results suggest that considerable cross-link exists between the anatomically defined cortico-pontine projection columns.

Animals↗