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PubMed1986

Spinal neuronal inhibition and EEG synchrony by electrical stimulation in subcortical forebrain regions of the cat.

In cats anaesthetized with sodium pentobarbital and 70% N2O, single lumbar dorsal horn neurons were excited by controlled noxious radiant heating of glabrous hindpaw skin. The EEG was recorded from the pericruciate cortex and posterior lateral gyrus. Subcortical forebrain sites where electrical stimulation inhibited dorsal horn neuronal heat-evoked responses contralaterally were identified by mapping the caudate nucleus, internal capsule, septum, nucleus accumbens and basal forebrain regions. Inhibitory sites were mainly located in the ventral forebrain (ventral septum, diagonal band, basal forebrain). The caudate nucleus and internal capsule had a low incidence and effectiveness of inhibitory sites. In the basal forebrain, the incidence and effectiveness of inhibitory sites decreased from caudal to rostral regions. There was a rostral limit of inhibitory sites, both medially and laterally. The magnitude of inhibition increased with graded increases in brain stimulation intensity. The mean incremental increase in inhibition was greater for caudal than for rostral basal forebrain sites. Mean stimulus currents for threshold of inhibition and for inhibition to 50% of control heat responses were lower for caudal than for rostral sites. Responses of the dorsal horn neurons to increasing temperatures of noxious skin heating were monotonic linear functions over the temperature range studied (48-53 degrees C). Stimulation in both rostral and caudal basal forebrain decreased the slope of this stimulus-response function, with a greater decrease for caudal sites. Cortical EEG synchronization was evoked by stimulation in the caudate nucleus and rostral basal forebrain. For both regions, most synchronogenic sites did not produce descending inhibition of dorsal horn neurons. The significance of these findings in relation to descending inhibition from other brain regions and stimulation-produced analgesia is discussed.

Action Potentials↗
PubMed1986

Thalamocortical and intrathalamic interactions during slow repetitive stimulation of n. centralis lateralis.

Extracellular single unit activity was recorded simultaneously in cortex (anterior part of the middle suprasylvian gyrus, MSSG) and thalamus (n. ventralis anterior, VA; n. lateralis posterior, LP) during repetitive low frequency stimulation (RLFS) of n. centralis lateralis (CL) in lightly anesthetized cats. Such stimulation induced typical recruiting responses in the cortical EEG consisting of long-latency, surface-negative waves reversing in polarity at 0.1-2mm depth. These cortical EEG responses were associated with long-latency (8-20 ms) action potential (AP) discharges of cortical neurons appearing with the 2nd stimulus of the train. The number of AP discharges and response latency increased as the train of CL stimuli progressed. In 12 of these neurons there was a short-latency (up to 2-5 ms) response which, however, did not show incremental features during RLFS. Thalamic neurons in VA usually responded to the first stimulus within a train of RLFS of CL, while LP neurons responded only to the 2nd or 3rd stimulus. Peristimulus time histograms (PSTHs) of AP discharges in VA showed an increase in both number and latency of APs as the train of stimuli progressed. This was also observed in those thalamic neurons in LP which changed their firing during RLFS of CL. The peak of firing of 20 VA neurons preceded, and that of 7 followed that of the MSSG neurons, while 6 VA and MSSG neuronal pairs reached their peak firing simultaneous; peak firing of 29 LP neurons preceded, and that of 21 followed the firing peak of MSSG neurons. The thresholds of the incremental responses of MSSG, VA and LP neurons to progressively increased intensity of RLFS of CL were different: MSSG and VA neurons changed their firing pattern at an intensity incapable of modifying the activity of LP neurons. When stimulation intensity was increased to a level sufficient to change the responses of both neurons of a given pair (either MSSG-VA or MSSG-LP) the time sequence of involvement in the incremental process was in the following order: VA, MSSG, LP. Within a range of RLFS extending from 2-20 Hz, stimulation at 6.6 Hz (150 ms interstimulus intervals) induced the most prominent incremental responses both in the cortex and thalamus, i.e. response increases were largest when each subsequent stimulus occurred 50-130 ms before the expected rebound excitation following the preceding response.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1984

EMG patterns in antagonist muscles during isometric contraction in man: relations to response dynamics.

We studied the EMG activity of biceps and triceps in human subjects during isometric force adjustments at the elbow. Rapid targeted force pulses exhibited stereotyped trajectories in which peak force was a linear function of the derivatives of force and the time to peak force was largely independent of its amplitude. These responses were associated with an alternating triphasic pattern of EMG bursts in agonist and antagonist muscles similar to that previously described for rapid limb movements. When the instructions demanded rapid force pulses, initial agonist bursts were of constant duration, and their magnitude was strongly related to peak force achieved. The timing of EMG bursts in antagonist pairs was closely coupled to the dynamics of the force trajectory, and the rising phase of the force was determined by both agonist and antagonist bursts. When peak force was kept constant and rise time systematically varied, the presence and magnitude of antagonist and late agonist bursts were dependent on the rate of rise of force, appearing at a threshold value and then increasing in proportion to this parameter. It is proposed that antagonist activity compensates for nonlinearity in muscle properties to enable the linear scaling of targeted forces which characterizes performance in this task.

Adult↗
PubMed1987

Effects of arm acceleration and behavioral conditions on the organization of postural adjustments during arm flexion.

Nine standing subjects performed unilateral arm flexion movements over an eight-fold range of speeds, under two behavioral conditions. In the visually-guided condition, a visual target informed subjects about the correct movement speed. Seven subjects also made movements of different speeds during a self-paced condition, without a visual target. Angular displacement and acceleration of the arm, and EMG activity from the hamstrings (HM), erector spinae (ES) and the anterior deltoid (AD) muscles were measured. The following results were observed. Mean rectified amplitudes of EMG activity in HM and ES were typically correlated with the average arm acceleration and presumably the disturbance to posture and/or balance. HM and ES amplitudes were correlated for only six subjects. Functions relating the ratios of HM/ES EMG amplitudes to acceleration varied between subjects. HM onset latencies were highly variable for slow movements and usually lagged movement. For movements above a threshold-like point in acceleration, HM latencies were correlated with arm acceleration and recruited before movement. ES latencies were constant for fast movements, and negatively correlated with acceleration for slower movements. The recruitment order of HM and AD was influenced by the behavioral condition but not by arm acceleration for fast movements. HM and AD were recruited coincidentally for visually-guided movements, while for self-paced movements, HM was recruited before AD. We conclude that for the arm flexion task: HM and ES are not tightly coupled; both behavioral and mechanical conditions affect the recruitment of postural muscles; and postural and focal components of the movement are probably organized by parallel processes.

Acceleration↗
PubMed1987

On the intracortical activity during recruiting responses: an analysis of laminar profiles before and after topical application of GABA to the cortex.

Intracortical activity during recruiting responses (RRs) has been studied by recording laminar profiles of intracortical field potentials during repetitive stimulation, at 6 Hz, of nucleus centralis lateralis (CL) and nucleus centralis medialis (NCM) in lightly anesthetized cats, before and after topical application to the cortex of 1% GABA solution. The data obtained underwent current source density analysis (CSD) which disclosed that in the pre-GABA condition, there are two almost simultaneous sinks, one in the most superficial layer and the other in mid-cortical layers. After GABA application, a single large sink was present in mid-cortical layers. Extracellular single cortical unit activity was recorded in different animals, through a microelectrode tangentially inserted into the cortex, during repetitive stimulation of CL or NCM, both before and after GABA application. In 75% of these units there was, after GABA, a mean reduction of about 50% of firing probability while in the remaining 25% there was an increased activity. Topical application of 1% Manganese sulfate to the cortical surface appeared to completely inactivate the whole thickness of the cortex where it was applied, making evident the contribution to RRs of the potentials generated in the cortex buried in the adjacent sulci. Finally, a reciprocal facilitating effect of RRs and augmenting responses (ARs), which was studied by combined stimulation of nucleus ventralis posterolateralis (VPL) and NCM, appeared to be dependent upon an intracortical mechanism. All these data suggest that: RRs are the result of a simultaneous activation of superficial and mid-cortical layers; RRs are contaminated by a volume conducted potential arising from the cortex buried in the sulci; a superficial inhibition following the initial excitation seems to be an usual component of the response; ARs and RRs probably share a similar intracortical mechanism for incrementing the response.

Animals↗
PubMed1989

Differences in the activation of m. biceps brachii in the control of slow isotonic movements and isometric contractions.

We have compared muscle activation in the control of slow isotonic movements and isometric contractions. Specific attention has been given to the contribution of the two force-grading mechanisms, the recruitment of motor units and the modulation of firing frequency in motor units that have already been recruited. The recruitment order of the m. biceps motor units under study was the same during isometric contractions and slow isotonic movements. However, the recruitment thresholds of the m. biceps units were considerably lower for both isotonic flexion and extension movements, even at velocities as low as 2 deg/s, than for isometric contractions. Furthermore, firing frequency at recruitment was found to depend on the motor task: at recruitment the motoneurone starts firing with a higher firing frequency during isotonic flexion movements and a lower firing frequency during isotonic extension movements than during isometric contractions. Two main conclusions can be drawn from these results. First of all, the concept of one single activation parameter (total synaptic drive?) cannot account for the motor-unit behaviour observed during our experiments: the relative contribution of the two force-grading mechanisms is different for different tasks. Secondly, the distribution of activity among flexor motoneurone pools is different for isometric contractions and isotonic movements.

Humans↗
PubMed1984

Delays in recruitment at different trophic levels: effects on stability.

Predator prey models in which there is a delay in recruitment in both species and only adults interact are formulated and studied. Stability results show that the length of delays in recruitment in the prey are more critical than the length of recruitment delays in the predator. Thus the destabilizing effect of recruitment delays may be less important in higher trophic levels than some single species models indicate. Variance in the recruitment delay is shown to be an important stabilizing influence.

Age Factors↗
PubMed1976

Effects of threshold, low frequency, long lasting stimulation on single motor unit electrical responses from the extensor digitorum brevis muscle in normal and myasthenic subjects.

The authors studied the recruitment of motor units of EDB muscle by long lasting threshold intensity stimulation at 1 c/sec of external popliteal nerve at the ankle in healthy subjects and in myasthenic patients. In the healthy volunteers, at steady intensity of threshold stimuli, maximum number of motor units never exceeded 5--6 electrical increments with a final incremental response of constant amplitude during 60 min of stimulation. In myasthenic patients, both an early exhaustion of threshold motor units and a marked facilitation phenomenon occurred with massive recruitment of incremental responses, while in a small number of cases no changes were observed. These data are discussed in the light of various morphofunctional hypotheses regarding the localization of the myasthenic impairment.

Edrophonium↗
PubMed1977

[Comparative investigation of somatosensory spinal and cortical evoked potentials in children (author's transl)].

Somatosensory spinal (spinEP) and primary cortical evoked responses (ssEP) to median and tibial nerve stimulation (at forefinger, wrist, and ankle respectively) were investigated by means of summation techniques in 23 normal children aged 6 to 14 years. Amplitude recovery functions of cervical spinEP were tested by paired stimuli and short tetanic stimulation at the wrist; spinEP amplitudes were unchanged for stimulus intervals down to 5 ms. The amplitudes of the cervical spinEP after strong stimuli to the finger were only a quarter as great as those obtained by stimulation of the wrist at motor threshold strength. In one patient with the Brown-Séquard syndrome cervical spinEP were absent for stimuli on the side of position sense impairment, but were unaffected for stimuli on the side of dissociated sensory loss. The normal latencies of spinEP (to the onset of the negative potential) and ssEP (to first negative peak) are presented as functions of body height. The difference between these two latencies yielded a central latency from the lower cervical spinal cord of about 9--10 ms. The spinal afferent conduction velocity, calculated from the difference between the lumbar and cervical latencies after tibial nerve stimulation at the ankle, was found to be 74m/s.

Adolescent↗
PubMed1993

Motor unit recruitment during prolonged isometric contractions.

Motor unit recruitment patterns were studied during prolonged isometric contraction using fine wire electrodes. Single motor unit potentials were recorded from the brachial biceps muscle of eight male subjects, during isometric endurance experiments conducted at relative workloads corresponding to 10% and 40% of maximal voluntary contraction (MVC), respectively. The recordings from the 10% MVC experiment demonstrated a characteristic time-dependent recruitment. As the contraction progressed both the mean number of motor unit spikes counted and the mean amplitude of the spikes increased significantly (P < 0.01). This progressive increase in spike activity was the result of a discontinuous process with periods of increasing and decreasing activity. The phenomenon in which newly recruited motor units replace previously active units is termed "motor unit rotation" and appeared to be an important characteristic of motor control during a prolonged low level contraction. In contrast to the 10% MVC experiment, there was no indication of de novo recruitment in the 40% MVC experiment. Near the point of exhaustion a marked change in action potential shape and duration dominated the recordings. These findings demonstrate a conspicuous difference in the patterns of motor unit recruitment during a 10% and a 40% MVC sustained contraction. It is suggested that there is a close relationship between intrinsic muscle properties and central nervous system recruitment strategies which is entirely different in fatiguing high and low level isometric contractions.

Action Potentials↗
PubMed1996

Motor unit recruitment strategy changes with skill acquisition.

The modifications of motor unit recruitment strategy due to skill acquisition was determined in the elbow flexor-extensor muscles of normal human subjects. The median frequency of the power density spectra of the electromyograms recorded from the biceps and triceps muscles during a 3-s linear increase in flexion force in the range of 0-100% maximal voluntary contraction (MVC) was calculated for each subject, every 2 weeks over a total 6-week period during which subjects practiced linear flexion force increase three times a week. Electromyograms were recorded with two pairs of electrodes of different size and electrode spacing. It was shown that skill acquisition due to the 360 practice trials over the 6-week period caused an increase in the initial motor unit recruitment phase of the agonist's force generation cycle from about 0-65% MVC to about 0-85% MVC. The increase in the recruitment range was gradual and statistically significant for the measurements made every 2 weeks. The recruitment range of the antagonist triceps demonstrated a minor, but statistically insignificant, decrease over the same training period. There was a minor, but statistically insignificant, advantage of using small electrodes and inter-electrode spacing. It was concluded that skill acquisition, due to repeated functional use of a muscle in the same contraction mode, results in a slower, prolonged recruitment of motor units in the initial segment of the force generation cycle, thereby allowing a more precise and accurate control of the increments of force increase. Such conclusions reinforce the concept advocating the plasticity of motor unit control according to the functional demands imposed on the muscle. The results have significant implications in the design of various athletic, occupational and rehabilitation training modalities for optimal performance of various movement functions.

Adult↗
PubMed1980

Fast and slow motor units: their recruitment for tonic and phasic contraction in normal man.

Most skeletal muscles include at least two types of MU's. Then, it has been stated that the properties of each MU may imply its specific recruitment in static or dynamic work. To test this hypothesis, the activities of biceps brachii MU's have been recorded during voluntary static contraction followed without any break by dynamic contraction. The MU's activity was recorded by means of wire-electrodes. From 45 tests carried out on 15 normal subjects in which 78 MU's were studied it has been found that: (1) the same MU may discharge both in static and in dynamic work; (2) in both types of work the MU's firing rate and recruitment only depend on the external force. For example, a single MU which does not discharge for a given level of static force will discharge during the following dynamic work, but can also be recruited for a higher level of static force. Thus, it can be said that the qualification "tonic" and "phasic", characterize the MU's twitches and do not imply any static-dynamic differentiation. A comparison has been made between the patterns of MU activity which occur in movement against load and those which have been found in movement against inertia. It is suggested that while the latter are preprogrammed, the former fall under a feed-back control.

Action Potentials↗
PubMed1980

The blood pressure response during isometric exercise in fast and slow twitch skeletal muscle in the cat.

The blood pressure response during fatiguing isometric contractions was examined in a slow twitch muscle (the soleus) and a mixed muscle (the medial gastrocnemius) of the cat. The results of these experiments showed that electrical stimulation of the ventral roots of the spinal cord which carried the efferent innervation to the soleus muscle failed to result in a blood pressure response during isometric exercise. Further, although stimulation of the fast twitch motor units in the medial gastrocnemius muscle was associated with a potent pressor response to isometric exercise, stimulation of the slow twitch motor units was associated with a markedly reduced response throughout the duration of the exercise. These findings infer that the pressor response to isometric exercise may be a function of the fast twitch motor units in the muscle.

Animals↗
PubMed1981

The influence of recruitment order and temperature on muscle contraction with special reference to motor unit fatigue.

Although recruitment in man often proceeds from the slowest to fastest motor units, a variety of stimuli can alter this order. To study the effect of recruitment order on isometric performance, the tetanic tension of individual fast and slow twitch motor units was measured during fatiguing isometric contractions of the medial gastrocnemius muscle of the cat with recruitment proceeding from either the slowest to fastest or fastest to slowest motor units throughout the contractions. The muscles were stimulated electrically through the ventral roots of the spinal cord. Fatiguing isometric tensions were examined at 10, 20, 40, 70, or 100% of the initial strength of the muscles at 28 and 38 degrees C. The results of these experiments showed that for contractions at low isometric tensions (20 and 40% of the muscles' strength), especially at the lower temperature, the endurance was longer when recruitment proceeded from the slowest to fastest units. The reason for this appeared to be linked to the more rapid rate of fall of tension over time in fast as opposed to slow twitch motor units.

Animals↗
PubMed1976

Endolymphatic hydrops: mechanical causes of hearing loss.

An explanation for the mechanical origin of the hearing loss in endolymphatic hydrops is presented that is based on studies in mechanical cochlear models. An elastic bias of the basilar membrane and/or a mass loading of the cochlear duct account for the low-frequency hearing loss, diplacusis, and even-harmonic distortion. In addition, the static shearing displacement between the tectorial membrane and the organ of Corti, caused by the displacement of the basilar membrane, may partially decouple the hair cells from the tectorial membrane, an event that would explain the tinnitus, recruitment, and perhaps even the disportional loss of speech intelligibility associated with endolymphatic hydrops.

Basilar Membrane↗
PubMed1977

Comparative study between the normally hearing child and the hard of hearing child, by acoustic impedance measurements of the ear.

In the first part, the author makes a comparative study of the tympanometric results obtained from the normally hearing child and the hard of hearing child. The frequency of serous otitis media is statistically higher in the deaf child. The author compares such results of tympanometry as obtained from the child, considers the contribution of this technique to the field of pediatric audiology and suggests a classification of tympanograms in terms of the probe-tone frequency of the impedancemeter. The second part concerns the acoustico-facial reflex. This study compares the recording of the stapedius reflex obtained by pure tone and Narrow-Band noise acoustic stimulation, thanks to Madsen electro-acoustic bridge, in two groups of children, the one consisting of normal hearing children and the other of children suffering from severe perception hearing loss. The study of the stapedius reflex threshold makes it possible to analyse the dynamics of the field of perception in both groups. After an analysis of the results, the author outlines the importance attributable to the study of the stapedius reflex in the child as a means of objectively examining the auditive function and a means of differential diagnosis in cases of sensorineural impairment of the hearing function.

Acoustic Stimulation↗
PubMed1977

[The comparison of binaural balance-tests (author's transl)].

14 normal-hearing persons were examined in 7 different versions of the balance-test. Thereby a severe dependence to the methods was found out, caused by psychological factors. By randomization of the side and the intensity (computer-controlled test-procedure) no case showed a significant recruitment. In 23 persons with inner-ear disturbance there was not only the randomized test performed, but also the methods of Jerger (keeping the intensity constant on the worse ear) and Hood (constant on the better ear). By using the randomized test all patients showed a significant recruitment, calculated as the deviation of the angle in relation to the diagonal of the diagram. In 7 cases the method of Jerger and in 20 of all the 23 persons the method of Hood brought a compensation of loudness.

Adult↗