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PubMed1995

Head movement trajectory in three-dimensional space during orienting behavior toward visual targets in rhesus monkeys.

Head movement trajectories in three-dimensional space were studied in two monkeys with their heads free during natural and spontaneous orienting behavior toward objects of interest displayed in a horizontal plane. The main interest of this study lies in understanding the process responsible for behavioral variability during the execution of head movements, with special reference to "units of movement." The head movements were recorded by an optoelectronic movement analyzer working with passive markers. Algorithms have been designed to reconstruct the three-dimensional trajectories of the center of gravity of the head. Simultaneously, electromyographic activity in the four pairs of suboccipital muscles was studied. A quantitative evaluation of the involvement of the head in orienting behavior toward visual targets shows that the gaze shift is always produced by eye movements in combination with head movements, even with target eccentricities of less than 10 degrees. On the basis of 80 trials performed by the two monkeys, head trajectories and recruitment patterns of the four pairs of suboccipital muscles have been analyzed. We have been able to identify four elementary kinematic units which can be described as a rightward or leftward turning associated with a contralateral or ipsilateral bending. Each of these four elementary units are underlain by a precise fixed recruitment pattern in the four pairs of suboccipital muscles. These four sets of motor strategies can be combined in order to offer a certain amount of plasticity from which the animal builds its own head trajectory.

Animals↗
PubMed1989

Physiological properties of the motor units of the wrist extensor muscles in man.

The physiological properties of 355 motor units (MUs) recorded in the extensor carpi radialis muscles were studied in 34 healthy human subjects during isometric contractions. MU selective twitches were educed from the whole muscle force using the spike-triggered averaging method. The twitch contraction times and twitch forces were measured. From these data it was attempted to estimate the distribution of fast and slow MUs in the muscles studied. Mu recruitment thresholds were systematically measured during stereotyped slow ramp contractions (force increase = 0.25 N. s-1). Degrees of correlation between contraction times, twitch forces and recruitment thresholds were pair analysed by computing simple regression curves and correlation coefficients. The degrees of correlation were compared between 245 Mus recorded in 34 subjects and 66 MUs recorded in a single subject. Analysis of the instantaneous discharge frequency of 132 MUs showed the existence of a remarkable degree of correlation (correlation coefficient, r = -0.75) between the "frequency rise times" (discharge onset to maximal frequency) and the MU twitch contraction times; i.e., the "frequency rise times" increase when the twitch contraction time decrease. The possibility that muscle contraction may be differentially modulated on the basis of this discharge property of the M Us is discussed. The results are compared to previous data and the limitations of the spike-triggered averaging method applied to long muscles in man are extensively discussed.

Adult↗
PubMed1990

Changes in excitability of motor units during preparation for movement.

Single biceps motor units were recorded in two awake monkeys (Macaca fascicularis) during the preparation for and execution of a forearm flexion movement. The motor sequence was organized as follows: after a control period lasting 500 ms, the animal was informed that a preparatory period (PP) was beginning by a preparatory signal (PS) consisting of diffuse sound and light. One to 1.5 s later, the animal was instructed by a bright light response signal (RS) to perform a rapid forearm flexion movement. Two hundred motor sequences were run during each daily session. The single motor units (MU) were recorded and their discharges analysed. In both monkeys, two extreme groups were found to exist on the basis of their pattern of activity during the preparatory and movement execution phases. 56% of the MUs were silent during the PP and showed a brief burst of discharge after the RS, which was strictly correlated to the movement execution. Their high recruitment threshold and their firing frequency during resting and movement periods suggested that these MUs associated with the movement execution could be called presumed fast or phasic MUs. Among the remaining MUs (44%), 15% were active as early as the beginning of the PP (about 300 ms after the PS) and showed a progressively increasing discharge, which stopped just after the beginning of the movement execution. These MUs associated with the preparatory phase had low recruitment thresholds and firing frequencies, which is compatible with the possibility that they might be slow or tonic MUs. Two functional hypotheses can be proposed on the basis of these results.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Reductions in recruitment force thresholds in human single motor units by successive voluntary contractions.

Recruitment force thresholds of biceps brachii single motor units were studied in 4 male subjects before and after an isometric muscle contraction, passive muscle stretch, or following successive muscle contractions, muscle stretches or during alternations between muscle stretches and muscle contractions. Isometric muscle contractions of 5 s duration decreased subsequent single motor unit force thresholds. These force thresholds could usually be reset at or near precontraction force threshold values by passive muscle stretch induced by elbow extension. Single motor units showing reduced force thresholds following contraction were momentarily derecruited during and/or after muscle stretch. Successive muscle stretches alone did not significantly alter single motor unit force thresholds. In contrast, single motor unit recruitment force thresholds during successive weaker contractions were progressively lowered. Intercontraction muscle stretches maintained the single motor unit force thresholds at or near the initial force threshold level. The mechanism(s) underlying a muscle contraction-induced lowering of single motor unit force thresholds may reside in stretch reflex pathways.

Electrodes↗
PubMed1994

The relation between the direction dependence of electromyographic amplitude and motor unit recruitment thresholds during isometric contractions.

The activation of muscles can be studied by measuring the activity of a representative set of single motor units in a muscle or by measuring the surface electromyographic (EMG) activity of a muscle that results from the contribution of a large number of motor units. In this study we have developed a model showing how the direction dependence of the amplitude of the EMG activity during isometric contractions can be understood from the recruitment thresholds of single motor units when force is applied in various directions within a plane. The model predicts that the direction with the largest EMG activity (called the "preferred direction") corresponds to the direction in which the largest number of motor units is recruited. If one assumes homogeneous activation of a population of motor units, this preferred direction can be shown to be equivalent to the direction in which the recruitment threshold of the motor units is smallest. The experimental data show that, for most muscles in the human arm, the amplitude of surface EMG activity for a constant, isometric force at the wrist was proportional to the cosine of the angle between the muscle's preferred direction and the direction of force. As predicted by the model, the preferred direction coincided with the direction in which the recruitment threshold of motor units was smallest. Musculus deltoideus anterior had a more complicated directional sensitivity for surface EMG activity that could not be well fit by a single cosine function. This effect could be explained by the finding of two subpopulations of motor units within that muscle, each with a different recruitment behavior.

Adult↗
PubMed1976

Effects of stimulation of the midbrain reticular formation upon thalamo-cortical neurones responsible for cortical recruiting responses.

1. In lightly nembutalized cats, effects of high frequency (60-100/sec) repetitive stimulation of the midbrain reticular formation (RF) were tested upon the thalamo-cortical (T-C) neurones which project from the anterior ventral (VA) nucleus of the thalamus and its vicinities to the parietal association cortex and convey impulses responsible for cortical recruiting and spindling-like responses. 2. Tonic maintained or rhythmic grouped firings of the T-C neurones recorded as extracellular units with microelectrode in the thalamus were in the majority suppressed by high frequency RF stimulation during and often for a short time after the stimulation, and they showed later tonic discharges for many seconds or minutes. Intracellular recording from the T-C neurones revealed hyperpolarizing potential changes corresponding to the suppression on high frequency RF stimulation. Field potential analysis in the VA nucleus indicated that the hyperpolarization is ascribed at least in part to IPSPs elicited in the T-C neurones. Responses in some other types of thalamic neurones to the RF stimulation were exemplified to be compared and related with those of the T-C neurones. 3. Desynchronization of the parietal electrocorticogram coincided with the suppression of the T-C neurones and lasted for the time of the later tonic discharges, which contrasted with the rhythmic grouped discharges of the T-C neurones in association with recruiting and spindling-like responses of the cortex. Relations between the thalamo-cortical recruiting system and the ascending reticular activating system were discussed.

Animals↗
PubMed1975

Thalamo-cortical projections for recruiting responses and spindling-like responses in the parietal cortex.

1. The thalamic neurones sending their axons to the parietal association cortex (middle suprasylvian gyrus) and receiving monosynaptic excitation from the cerebellar (interpositus or lateral) nucleus were recorded with microelectrodes extracellularly and intracellularly around the anterior ventral (VA) nucleus of the thalamus in cats. Such thalamic neurones are known to carry exclusively the impulses responsible for superficial thalamo-cortical (T-C) responses in the parietal cortex, being called superficial T-C neurones (see Sasaki et al., 1972a, b). 2. Repetitive (6--9/sec) stimulation of the centrum medianum-parafascicular complex (CM) or the intralaminar nuclei (IL) of the thalamus elicited grouped spike discharges of the neurone in synchronization with the recruiting responses in the parietal cortex. The grouped discharges usually preceded the respective cortical responses by several milliseconds. Numbers of the spikes in the grouped discharges increased and decreased as the recruiting responses waxed and waned on the repetitive stimulation. 3. The superficial T-C neurones also showed similar grouped discharges in synchronization with spindling-like, surface-negative cortical responses which occurred spontaneously or were evoked by single thalamic stimulation. 4. It was concluded that the superficial T-C neurons can convey impulses for recruiting responses and spindling-like responses from the thalamus directly to the cerebral cortex. They are supposed to constitute the final T-C pathway of the neuronal circuits of the recruiting system, i.e., non-specific T-C projection system.

Animals↗
PubMed1978

Multiple spatial-frequency tuning of electrical responses from human visual cortex.

Human occipital potentials evoked by stimulation with a counterphase flickering grating were recorded by a digital narrowband filter technique. The data showed a surprising degree of narrow tuning to particular spatial frequencies in addition to the expected narrow temporal frequency tuning. At each temporal frequency, there could be two or more peaks of different spatial frequencies, each distinct from the whole field flicker response. Variations in this multiple spatial frequency tuning were investigated as a function of luminance, electrode location, and temporal frequency for several observers. The results are interpreted in terms of many stimulus-specific resonant neural circuits within the brain, and suggest that it is possible to make a highly detailed exploration of the responses of neural circuits to visual stimulation.

Electroencephalography↗
PubMed1987

Sensitization of nociceptive cutaneous nerve fibers from the rat's tail by noxious mechanical stimulation.

This single fiber study on rat tail nerve afferents attempts to establish a peripheral neural correlate for the hyperalgesia to mechanical stimulation which follows injury to the skin. Mechano-heat sensitive C fibers (MH-C or "polymodal" nociceptors) and high-threshold mechanoreceptive A delta fibers (HTM-A delta) were examined with a series of constant noxious pressure stimulations (4-6-8-4 N on 25 mm2, 120 s each, 5 min intervals). These injurious stimuli were either directed to the most sensitive spot of the receptive fields (central stimulation) or closely outside their borders (1-5 mm). With this protocol no clear sensitization was seen in MH-C fibers apart from a stronger dynamic response to central stimulation in some of them. In contrast, most HTM-A delta units, irrespective of the site of noxious stimulation, developed spontaneous activity, lowering of their von Frey thresholds and expansion of their receptive fields. All HTM-A delta units responded to outside stimulation: upon the first stimulus (4 N) there was a delayed discharge of continuously increasing frequency ("recruited response"), but the onset of the last stimulation (4 N repeated) evoked vigorous dynamic responses in many fibers. The recruitment of HTM-A delta nociceptor activity may contribute to post-injury hyperalgesia to mechanical stimulation and it may counteract adaptation of the single afferent fiber during prolonged noxious influence.

Animals↗
PubMed1979

Input-output properties of motor unit responses in muscles stretched by imposed displacements of the monkey wrist.

Reflex responses are "servo-like" where the output is graded with the input or are "triggered" where the output is independent of input once an input threshold is exceeded. Imposed displacements of monkey or human upper limb joints result in prolonged EMG output in the muscles stretched by the displacements. The longer-latency portions of the prolonged output have been variously reported to be servo-like or to be triggered in nature. In monkeys and humans, angular wrist displacements imposed by step loads result in three peaks (M1, M2 and M3) in the gross EMG recorded from the stretched muscles. Each gross EMG peak largely results from the firing of a separately-responding subpopulation of single motor units (SMUs). We studied the responses of SMUs to loads that were presented to the monkeys in a random order as to magnitude, duration and onset time. Average response histograms were constructed for the SMU responses for individual step load magnitudes. Averages were also constructed for the simultaneously-recorded gross EMG responses for each step load magnitude. The input parameters used were the initial velocity of displacement or the magnitude of step load, while the output was taken as the probability of firing/millisecond/presentation above baseline for the SMUs or the area under the response peaks above baseline for gross EMG. The results establish: 1) That it is not possible to unambiguously determine the input-output properties of the responses to imposed displacements utilizing the analysis of gress EMG activity due to the response characteristics of the various subpopulations of motor units contributing to the gross response. 2) That the SMU activity during all of the peak intervals is monotonically graded with increases in magnitude of the step load or the initial velocity of displacement. Hence, the long-latency portions of the EMG responses are servo-like in nature and are not preprogrammed or triggered responses. 3) That the gain (output/input) of the gross EMG responses almost entirely reflects the variation in the number of motoneurons recruited by changes in magnitude of the step loads rather than variation in the firing rates of motoneurons during the reflex responses.

Animals↗
PubMed1977

Properties of the synaptic transmission of the newly formed cortico-rubral synapses after lesion of the nucleus interpositus of the cerebellum.

1. Properties of synaptic transmission during and after repetitive activation of the newly formed cortico-rubral were examined in the red nucleus neurons (RN) of cats after lesions of the nucleus interpositus of the cerebellum (chronic cats) as well as in normal ones. 2. A prominent facilitation of the amplitude of cortico-rubral unitary EPSPs was observed in both normal and chronic cats when a stimulus to the cerebral peduncle (CP) was preceded by another stimulus by 2-50 msec. 3. Time course of the facilitation shows that it attains maximum at the interval of about 3 msec and decays approximately exponentially lasting for 50 msec or more. 4. When three successive stimuli of identical intensity were applied to CP, the degree of facilitation was more prominent than that for double shock. 5. There was a positive correlation between the time to peak of the cortico-rubral EPSPs and their maximum value of facilitation. 6. The posttetanic potentiation of the cortico-rubral EPSPs was observed after tetanic stimulation to CP in chronic and normal cats. It lasts for a few minutes in both cases.

Animals↗
PubMed1980

Time course and effective sites for inhibition from midbrain periaqueductal gray of spinal dorsal horn neuronal responses to cutaneous stimuli in the cat.

Inhibition of spinal dorsal horn neuronal responses to noxious (50 degrees C) skin heating by stimulation of the midbrain periaqueductal gray (PAG) was quantitatively investigated in cats anesthetized with sodium pentobarbital and nitrous oxide. Systematic variation of the interval between onset of PAG stimulation (PAGS) and onset of noxious skin heating revealed that a marked reduction of spinal unit heat-evoked discharges occured immediately upon onset of PAGS, and ceased immediately at offset of PAGS with a post-stimulation excitatory rebound. Stimulation at sites in both ventral and dorsal PAG produced inhibition, the strength of which increased sometimes in a linear manner with increasing strength of PAGS. Thresholds for the generation of descending inhibition were higher in dorsal than ventral PAG. PAGS also inhibited spinal unit responses to non-noxious skin stimulation (brushing of hairs). Descending inhibition from PAG is considered as a possible mechanism for analgesia produced by stimulation of PAG and other brainstem structures.

Animals↗
PubMed1986

Vestibulo-spinal response modification as determined with the H-reflex during the Spacelab-1 flight.

Our laboratory at the Johnson Space Center has employed the H-reflex recorded from the soleus muscle as a method of monosynaptic reflex testing in conjunction with vertical linear acceleration to assess modification of utriculo-saccular function induced through prolonged exposure to microgravity. It was hypothesized that exposure to free fall would reduce the necessity for postural reflexes in the major leg muscles, and that postural modification would reflect a change, not in the peripheral vestibular organs, but more centrally. This postural adjustment would reflect a sensory motor rearrangement where otolith receptor input was reinterpreted to provide an environmentally appropriate response. In addition to the H-reflex (which was the only inflight measurement), vestibulo-spinal EMG from the gastrocnemius, and self-motion reports were obtained in response to a sudden earth vertical fall. Preflight, inflight and postflight motion sickness reports were also recorded, and related to the H-reflex data. The results indicated that early inflight H-reflex amplitude was similar to that recorded preflight, but that measurements obtained later in the flight (day seven) did not show a change in potentiation as a function of the different drop to shock intervals. Immediate postflight H-reflex response in three of the four astronauts tested showed a rebound effect. Postflight gastrocnemius EMG in response to the sudden fall did not show a significant change from preflight values. However, one crewman who was tested early postflight did show an increase in EMG activity in response to the sudden fall. This immediate postflight effect returned to baseline rapidly. Self-motion perception obtained inflight suggested that the early inflight drops were perceived like those preflight. Drops later inflight were described as sudden, fast, hard and translational in nature. Immediately postflight the drops were perceived like those late inflight, and the astronauts said that they did not feel as though they were falling, rather the floor came up to meet them. Post hoc peak H-reflex amplitude, both preflight and postflight was related to inflight space motion sickness.

Humans↗
PubMed1984

Inspiratory on-switch evoked by mesencephalic stimulation: activity of medullary respiratory neurones.

The activity of medullary inspiratory and expiratory neurones was studied in urethan-chloralose anaesthetized cats during stimulus-evoked inspiratory phase (inspiratory on-switch). All neurones were characterized according to their axonal destination (i.e. bulbospinal neurons or vagal motoneurones) or the absence of such axonal projections (i.e. propriobulbar neurones), and to their location in the dorsal or ventral respiratory nuclei. The inspiratory on-switch effects were elicited during expiration (E phase) by brief tetanic electrical stimulation (50 to 100 ms duration; 0.5 mA; 300 Hz) delivered to the mesencephalic periaqueductal central gray and the adjacent reticular formation. The evoked inspiratory effects observed on the phrenic nerve discharge consisted of: an immediate response (latency 20 +/- 5 ms) of stable duration related to the stimulus (primary response: Prim.R.), a delayed response (patterned response: Patt.R.) appearing after a latent period (silent phase: Sil.P.) of 100 ms maximal duration. The later the stimulus in the E phase, the longer was the duration of the Patt.R. (300 to 1000 ms). The stimulation evoked an earlier activation of the inspiratory bulbospinal neurons (latency 12 +/- 6 ms) than that obtained in the phrenic nerve (Prim.R.). Hence, the Prim.R. originated from the bulbospinal pathway and not from a pathway directly impinging on the motoneurones. Conversely during stimulation very few inspiratory propriobulbar neurones were activated and no expiratory neurone discharged. During the phrenic Sil.P., 46% of the inspiratory bulbospinal neurones continued to discharge with a firing rate lower than that during the stimulus train, while most of the inspiratory propriobulbar and expiratory neurones were not active. During the Patt.R. all inspiratory bulbospinal neurones discharged early and were strongly activated whatever the Patt.R. duration whereas the expiratory neurones were not active. Inspiratory propriobulbar neurones were either not recruited or recruited later, and the number of active neurones increased as the duration of the Patt.R. lengthened. Our results suggest that the eliciting of the stimulus-evoked inspiration (Patt.R.) primarily depends on the activation of the inspiratory bulbospinal neurones. These neurones therefore would not only be the output neurones of the medullary respiratory centres, but they would serve other roles such as building up of the excitation in other respiratory neurones, thus acting as a component of the inspiratory ramp generator.

Animals↗
PubMed1978

Cutaneous fiber groups involved in the inhibition of fictive locomotion in the rabbit.

Decorticate, paralyzed unanaesthetized rabbit preparations can display motoneuron discharges (spontaneous or elicited through various somatic stimulations) which are related to locomotion. These activities are suppressed when manual pressure is exerted on the dorso-lumbar skin. The present study indicates that: (1) locomotor discharges can also be inhibited through repetitive electrical stimulation of one of the skin nerves belonging to dorsal dermatomes (TH5 to L5); (2) other skin nerves, especially those of the limbs, do not present the same inhibitory properties; (3) among the various fiber groups composing these dorsal skin nerves, only A delta are inhibitory; other groups, on the contrary, display moderate (A alpha and A beta) or strong (unmyelinated C fibers) excitatory actions; (4) the inhibitory action of A delta fibers counteracts the excitatory action of C fibers, when both groups are stimulated together. The relationship between this form of motor inhibition and the well known phenomenon of "reflex immobility" is briefly considered.

Animals↗
PubMed1979

Hemisphere contributions to the composition of the pattern-evoked potential waveform.

The transverse distribution of scalp-recorded potentials evoked by pattern reversal stimulation was studied in 50 healthy subjects. In most individuals the full-field responses were symmetrical over the occipital scalp, but important variations in distribution, symmetry and waveform were recorded in some cases. Asymmetrical responses were similar for each eye (i.e., they were "uncrossed" or homonymous asymmetries). Full-field peak latencies and amplitudes in the lateral channels were more variable than those at midline electrodes. Half-field responses were markedly asymmetric with well-lateralised components widespread over occipital-parietal scalp. In contrast to the full-field responses, component values measured near the midline were less consistent than those from lateral channels due to waveform distortions in this area ("transitional zone"). Upper field stimulation is particularly likely to produce such midline waveform distortions. Activity recorded from the scalp contralateral to the half-field stimulated shows more inter-individual and inter-hemispheric variation than that recorded from ipsilateral electrodes. Variants in the full-field waveform can be accounted for by relative differences in amplitude and distribution of the ipsilateral and contralateral components from each half field. The algebraic sum of these half-field components does not differ significantly from the components of the separately recorded full-field response. Furthermore, responses from the surviving half-field in patients after total hemispherectomy contain all the ipsilateral and contralateral half-field components seen in healthy subjects.

Dominance, Cerebral↗