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PubMedDate not supplied

The contribution of vestibulo-spinal mechanisms to the maintenance of human upright posture.

The maintenance of upright stance in humans requires the integrative evaluation of vestibular, visual, and proprioceptive information. Recent experiments prove the existence of fast vestibulo-spinal reactions in humans. Their physiological significance for the compensation of sudden external displacements of the body is minor. For low frequency stabilization of the body, however, vestibulo-spinal afferents as well as vision and proprioception are important. Unilateral loss of vestibular function initially causes a tendency to fall to the ipsilateral side as well as nystagmus and vertigo. After the first days of compensation, the remaining functional disturbance can only be detected by exposing the patient to conflicting sensory reafferents or by reducing proprioceptive and visual inputs simultaneously. The loss of vestibular function not only alters low frequency stabilization and to some extent also reflex-like fast motor patterns, but also interferes with the selection of preprogrammed motor synergies.

Afferent Pathways↗
PubMed1990

Comparison between spino-bulbo-spinal and propriospinal reflexes in thalamic cats during stepping.

To analyze changes in the excitability of both the spinal cord and brainstem in thalamic cats stepping on a moving treadmill, we examined the cutaneous propriospinal (PSR) and spino-bulbo-spinal (SBS) reflex responses in 20 adult cats. Tracheal cannulation, spinal transection at the T10 segment, and decerebration at the stereotaxic A12 level were performed under ether anesthesia. Immediately after decerebration, the ether was withdrawn. The head was fixed in a stereotaxic device, the T2 spinal process clamped to a metal frame, and the lumbar region suspended by a hammock, with bilateral forelimb contact on the floor of a treadmill. Electrical stimulation was applied to the superficial radial nerve with a cuff electrode, and two reflex responses (PSR and SBS) were recorded from the biceps brachii muscle in the same forelimb. Shortly before the appearance of forelimb stepping, both PSR and SBS reflex responses were elevated in amplitude. During forelimb stepping, the amplitudes of PSR and SBS reflex responses fluctuated depending on the phase of the step cycle. The PSR response was enhanced in the early phase of the swing, whereas the SBS response was elevated during a wider period from the beginning of the stance to the middle of the swing. The SBS response was completely absent in the late phase of the swing. This period corresponded to the transfer from flexion to extension and the appearance of the EMG of the triceps brachii muscle of the same forelimb. The fluctuation of the SBS response during stepping may be produced at the brainstem level, and not the spinal cord level, because the PSR response was enhanced only during narrow periods. The generation of locomotion thus seems to result in an enhancement of excitability of reflex pathways in the spinal cord and particularly in the brainstem.

Action Potentials↗
PubMed1982

The vestibulospinal system and postural regulation in man.

Upright posture in humans represents a giant step phylogenetically speaking. But upright posture demands refined mechanisms for static and dynamic postural equilibrium that are initiated by the concurrence of visual and proprioceptive impulses from the muscles of the legs and the labyrinth. The static labyrinth and Deiter's nucleus via the LVST seem to be the main centers responsible for static equilibrium. Its basic neurophysiological mechanism has been analyzed by the study of stretch reflexes, which permits a semiquantitative evaluation of the function of the static labyrinth. The cerebellum appears as the main control center of the proprioceptive system in relation to dynamic equilibrium.

Cerebellum↗
PubMed2000

Relationships between quantitative measures and neurologist's clinical rating of tremor and standing steadiness in two epidemiological studies.

OBJECTIVE: The purpose of the present investigation was to 1) compare a neurologist's physical examination (NPx) rating of severity of postural tremor to several summary measures derived from quantitative measurements of tremor and 2) compare a NPx rating of standing steadiness and related clinical findings to quantitative sway speed measurements. METHODS: Comparisons were performed on data obtained from two epidemiological field studies: 1) retired heavy industrial workers (n= 198, approximately half previously exposed to elemental mercury), and 2) small town residents (n=234, approximately 40% with environmental exposure to arsenic). A commercially available tremor measurement instrument was used in both studies. To obtain standing steadiness measures, a head position monitoring device was used in the mercury study, and a force platform was used in the arsenic study. The NPx included assessment of postural tremor, as well as evaluation of vibration perception and proprioception of the great toe, Achilles tendon reflex activity, Romberg test status, and tandem gait. One neurologist performed all the NPx in both studies and results were graded as normal, equivocal, or abnormal. The square root of the proportion of variance accounted for by a linear trend term in ANOVA models was used as an estimate of association between quantitative tests and clinical examination grade. An estimate of agreement, kappa, was also calculated after both NPx and quantitative test results were dichotomized. RESULTS: Most tremor summary measures varied monotonically with NPx tremor severity grade. Moderately good associations were observed between the tremor acceleration measure and NPx postural tremor grade (correlations up to approximately 0.5). Sway speed with eyes open and with eyes closed increased monotonically with NPx grade for most of the clinical signs. The NPx signs showing the strongest relationships with sway speed were Romberg test performance, tandem gait, and proprioception and vibration sensation of the great toe (correlations up to approximately 0.5). CONCLUSIONS: Quantitative tremor measurements were related to a neurologist's grading of postural tremor. Sway speed was associated with several NPx findings related to standing stability. Quantitative measurements of tremor and standing stability may provide more precise and objective measures of neurological function than NPx by a neurologist and are likely to be more consistent across times and examiners.

Adult↗
PubMed2003

Modulation of afferent inflow during the control of balancing tasks using the lower limbs.

This study investigated the control of sensory inputs during the performance of an inverted-pendulum balancing task. Experiments were conducted to examine modulation of proprioceptive inputs during balance tasks of varying difficulty. It was hypothesized that proprioceptive inputs to both spinal and cortical levels would be facilitated during a challenged balance task. In contrast, during challenged balance control, results revealed task-specific facilitation of sensory inputs to the cortex and inhibition of the spinal reflex pathway. Observations of increased transmission of proprioceptive inputs to the cortex and decreased transmission at the spinal level suggest that the cortex plays an important role in challenged balance, whereas the role for the spinal stretch reflex appears to be less important.

Adult↗
PubMed1986

M.I.T./Canadian vestibular experiments on the Spacelab-1 mission: 3. Effects of prolonged weightlessness on a human otolith-spinal reflex.

Reflex responses that depend on human otolith organ sensitivity were measured before, during and after a 10 day space flight. Otolith-spinal reflexes were elicited by means of sudden, unexpected falls. In weightlessness, "falls" were achieved using elastic cords running from a torso harness to the floor. Electromyographic (EMG) activity was recorded from gastrocnemius-soleus. The EMG response occurring in the first 100-120 ms of a fall, considered to be predominantly otolith-spinal in origin, decreased in amplitude immediately upon entering weightlessness, and continued to decline throughout the flight, especially during the first two mission days. The response returned to normal before the first post-flight testing session. The results suggest that information coming from the otolith organs is gradually ignored by the nervous system during prolonged space flight, although the possibility that otolith-spinal reflexes are decreased independent of other otolith output pathways cannot be ruled out.

Adaptation, Physiological↗
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↗
PubMed2000

Relations between the directions of vibration-induced kinesthetic illusions and the pattern of activation of antagonist muscles.

In humans, tendon vibration evokes illusory sensations of movement that are usually associated with an excitatory tonic response in muscles antagonistic to those vibrated (antagonist vibratory response, AVR), i.e., in the muscle groups normally contracted if the illusory movement had been performed. The aim of the present study was to investigate the relation between the parameters of the illusory sensation of movement and those of the AVR and to determine whether vectorial models could account for the integration of proprioceptive inputs from several muscles, as well as for the organization of the elementary motor commands leading to one unified motor response. For that purpose, we analyzed the relations between the anatomical site of the tendon vibration, the direction of the illusory movement, the muscles in which the AVR develops, and the characteristics of the AVR (surface EMG, motor unit types, firing rates, and activation latencies). This study confirmed the close relationship between the parameters of an AVR and those of the kinesthetic illusion. It showed that, during illusions of movements in different directions, motor units are activated according to a specific pattern correlated with their type, with the direction of the illusory movement and with the biomechanical properties of their bearing muscles. Finally, kinesthetic illusions and AVRs can be effectively represented using similar vectorial computations. These strong relations between the perceptual and motor effects of tendon vibration once again suggest that the AVR may result from a perceptual-to-motor transformation of proprioceptive information, rather than from spinal reflex mechanisms.

Action Potentials↗
PubMed2002

Sensorimotor deficits contributing to glenohumeral instability.

The conventional perspective has been that capsuloligamentous structures act as a mechanical restraint to humeral translation at the shoulder. Although this is true, the capsuloligamentous structures also have a sensorimotor influence on the musculoskeletal system, providing stability at the shoulder. The purpose of the current study was to discuss the sensorimotor role that the capsuloligamentous structures play in providing stability, how these mechanisms are disrupted with glenohumeral instability, and how surgical intervention restores such mechanisms. Proprioceptive information transmitted from the mechanoreceptors embedded within the capsuloligamentous structures influence the coordinated motor patterns, reflex activity, and joint stiffness to provide enhanced joint stability. The capsuloligamentous injury that occurs with shoulder instability not only affects mechanical restraint, but also alters this proprioceptive input to the central nervous system. As a result of these deficits in proprioception, alterations in reflex activity and motor programs as evident by muscle firing pattern alterations manifest. Although the main goal of surgical intervention is to restore the mechanical restraint that is lost with joint dislocation or subluxation, surgical intervention whether through open, arthroscopic, or thermal techniques seem to restore the proprioceptive deficits that exist after joint injury.

Humans↗
PubMed1986

[Function-oriented neurophysiologic diagnosis: long loop reflexes in spinal and cerebral lesions].

Neurophysiological methods which allow to identify objectively lesions of the efferent cortico-spinal pathways in humans are with the exception of transcutaneous electrical stimulation of the motor cortex nor available. Long latency EMG responses from leg muscles are mediated by a "transcortical loop" and offer the possibility to detect in combination with sensory evoked potentials lesions both of the afferent proprioceptive and the efferent motor pathways. Subjects stood on a platform which was rotated toe-up (50 degrees/s, 4 degrees) around the axis of the ankle joint. This lead to two EMG responses of short- and medium latency in the stretched triceps surae muscle, and an EMG response of long latency (LL) from the antagonistic anterior tibial muscle. We investigated 135 patients with a spinal or cerebral lesion, detected by neuroradiological methods. We found a significant delay of LL in patients with spinal lesions and in patients with cerebral lesions on the affected side. A significant delay of LL could also be observed in patients with pure motor symptoms and normal latency of the cerebral potential (P 40) alone in the group of patients with spinal lesions revealed in about 45% a pathological result. The rate of pathological findings was about 65% when both methods (SEP and LL) were combined. The results indicate that the recording of LL is helpful to detect lesions of the efferent central pathways. In a follow-up study we investigated patients who underwent successful surgical treatment. We found a close correlation between the decrease in latencies of LL and the improvement of clinical signs.

Adult↗
PubMedDate not supplied

Direct evidence of the anterior cruciate ligament-hamstring reflex arc in humans.

It has been emphasized that the anterior cruciate ligament plays an important role in the proprioceptive feedback system. The anterior cruciate ligament-hamstring reflex has been revealed in animal experiments, but it has not been established in humans. The purpose of this study was to demonstrate direct evidence of the anterior cruciate ligament-hamstring reflex arc. Nine knees in nine healthy subjects were investigated. The anterior cruciate ligament was stimulated by the use of wire electrodes inserted using an arthroscopic technique. Electromyographic signals from the biceps femoris and the semitendinosus muscles were recorded with surface electrodes. The change in electromyographic activity was analyzed after electrical stimulation in the normal knee condition, and again after intraarticular sensation had been interrupted with a local anesthetic. After electrical stimulation, subjects demonstrated increased electromyographic activity of the hamstring muscles in the normal knee condition. This response indicates the existence of an anterior cruciate ligament-hamstring reflex arc. Conversely, there was no change in activity for the hamstring muscle in the anesthetized knee because the afferent impulse from the neural elements of the anterior cruciate ligament had been removed.

Adult↗
PubMed1999

Evidence suggesting that a transcortical reflex pathway contributes to cutaneous reflexes in the tibialis anterior muscle during walking in man.

Stimulation of cutaneous foot afferents has been shown to evoke a facilitation of the tibialis anterior (TA) EMG-activity at a latency of 70-95 ms in the early and middle swing phase of human walking. The present study investigated the underlying mechanism for this facilitation. In those subjects in whom it was possible to elicit a reflex during tonic dorsiflexion while seated (6 out of 17 tested), the facilitation in the TA EMG evoked by stimulation of the sural nerve (3 shocks, 3-ms interval, 2.0-2.5x perception threshold) was found to have the same latency in the swing phase of walking. The facilitation observed during tonic dorsiflexion has been suggested to be -- at least partly -- mediated by a transcortical pathway. To investigate whether a similar mechanism contributes to the facilitation observed during walking, magnetic stimulation of the motor cortex (1.2x motor threshold) was applied in the early swing phase at different intervals in relation to the cutaneous stimulation in 17 subjects. In 13 of the subjects, the motor potentials evoked by the magnetic stimulation (MEPs) were more facilitated by prior sural-nerve stimulation (conditioning-test intervals of 50-80 ms) than the algebraic sum of the control MEP and the cutaneous facilitation in the EMG when evoked separately. In four of these subjects, a tibialis anterior H-reflex could also be evoked during walking. In none of the subjects was an increase of the H-reflex similar to that for the MEP observed. In five experiments on four subjects, MEPs evoked by magnetic and electrical cortical stimulation were compared. In four of these experiments, only the magnetically induced MEPs were facilitated by prior stimulation of the sural nerve. We suggest that a transcortical pathway may also contribute to late cutaneous reflexes during walking.

Action Potentials↗
PubMed1989

Interactions of cervico-ocular and vestibulo-ocular fast-phase signals in the control of eye position in rabbits.

1. Eye movements in unanaesthetized rabbits were studied during horizontal neck-proprioceptive stimulation (movement of the body with respect to the fixed head), when this stimulation was given alone and when it was given simultaneously with vestibular stimulation (rotation of the head-body). The effect of neck-proprioceptive stimulation on modifying the anticompensatory fast-phase eye movements (AFPs) evoked by vestibular stimulation was studied with a 'conditioning-test' protocol; the 'conditioning' stimulus was a neck-proprioceptive signal evoked by a step-like change in body position with respect to the head and the 'test' stimulus was a vestibular signal evoked by a step rotation of the head-body. 2. The influence of eye position and direction of slow eye movements on the occurrence of compensatory fast-phase eye movements (CFPs) evoked by neck-proprioceptive stimulation was also examined. 3. The anticompensatory fast phase (AFP) evoked by vestibular stimulation was attenuated by a preceding neck-proprioceptive stimulus which when delivered alone evoked compensatory slow-phase eye movements (CSP) in the same direction as the CSP evoked by vestibular stimulation. Conversely, the vestibularly evoked AFP was potentiated by a neck-proprioceptive stimulus which evoked CSPs opposite to that of vestibularly evoked CSPs. 4. Eccentric initial eye positions increased the probability of occurrence of midline-directed compensatory fast-phase eye movements (CFPs) evoked by appropriate neck-proprioceptive stimulation. 5. The gain of the horizontal cervico-ocular reflex (GHCOR) was measured from the combined changes in eye position resulting from AFPs and CSPs. GHCOR was potentiated during simultaneous vestibular stimulation. This enhancement of GHCOR occurred at neck-proprioceptive stimulus frequencies which, in the absence of conjoint vestibular stimulation, do not evoke CSPs.

Animals↗
PubMed2001

Variability in the control of head movements in seated humans: a link with whiplash injuries?

The aim of this study was to determine how context and on-line sensory information are combined to control posture in seated subjects submitted to high-jerk, passive linear accelerations. Subjects were seated with eyes closed on a servo-controlled linear sled. They were asked to relax and received brief accelerations either sideways or in the fore-aft direction. The stimuli had an abrupt onset, comparable to the jerk experienced during a minor car collision. Rotation and translation of the head and body were measured using an Optotrak system. In some of the subjects, surface electromyographic (EMG) responses of selected neck and/or back muscles were recorded simultaneously. For each subject, responses were highly stereotyped from the first trial, and showed little sign of habituation or sensitisation. Comparable results were obtained with sideways and fore-aft accelerations. During each impulse, the head lagged behind the trunk for several tens of milliseconds. The subjects' head movement responses were distributed as a continuum in between two extreme categories. The 'stiff' subjects showed little rotation or translation of the head relative to the trunk for the whole duration of the impulse. In contrast, the 'floppy' subjects showed a large roll or pitch of the head relative to the trunk in the direction opposite to the sled movement. This response appeared as an exaggerated 'inertial' response to the impulse. Surface EMG recordings showed that most of the stiff subjects were not contracting their superficial neck or back muscles. We think they relied on bilateral contractions of their deep, axial musculature to keep the head-neck ensemble in line with the trunk during the movement. About half of the floppy subjects displayed reflex activation of the neck muscles on the side opposite to the direction of acceleration, which occurred before or during the head movement and tended to exaggerate it. The other floppy subjects seemed to rely on only the passive biomechanical properties of their head-neck ensemble to compensate for the perturbation. In our study, proprioception was the sole source of sensory information as long as the head did not move. We therefore presume that the EMG responses and head movements we observed were mainly triggered by the activation of stretch receptors in the hips, trunk and/or neck. The visualisation of an imaginary reference in space during sideways impulses significantly reduced the head roll exhibited by floppy subjects. This suggests that the adoption by the central nervous system of an extrinsic, 'allocentric' frame of reference instead of an intrinsic, 'egocentric' one may be instrumental for the selection of the stiff strategy. The response of floppy subjects appeared to be maladaptive and likely to increase the risk of whiplash injury during motor vehicle accidents. Evolution of postural control may not have taken into account the implications of passive, high-acceleration perturbations affecting seated subjects.

Acceleration↗
PubMed2005

Effects of hip joint angle changes on intersegmental spinal coupling in human spinal cord injury.

Pathological expression of movement and muscle tone in human upper motor neuron disorders has been partly associated with impaired modulation of spinal inhibitory mechanisms, such as reciprocal or presynaptic inhibition. In addition, input from specific afferent systems contributes significantly to spinal reflex circuits coupled with posture or locomotion. Accordingly, the objectives of this study were to identify the involved afferents and their relative contribution to soleus H-reflex modulation induced by changes in hip position, and to relate these effects with activity of spinal interneuronal circuits. Specifically, we investigated the actions of group I synergistic and antagonistic muscle afferents (e.g. common peroneal nerve, CPN; medial gastrocnemius, MG) and tactile plantar cutaneous afferents on the soleus H-reflex during controlled hip angle variations in 11 motor incomplete spinal cord injured (SCI) subjects. It has been postulated in healthy subjects that CPN stimulation evokes an inhibition on the soleus H-reflex at a conditioning test (C-T) interval of 2-4 ms. This short latency reflex depression is caused mainly by activation of the reciprocal Ia inhibitory pathway. At longer C-T intervals (beyond 30 ms) the soleus H-reflex is again depressed, and is generally accepted to be caused by presynaptic inhibition of soleus Ia afferents. Similarly, MG nerve stimulation depresses soleus H-reflex excitability at the C-T interval of 6 ms, involving the pathway of non-reciprocal group I inhibition, while excitation of plantar cutaneous afferents affects the activity of spinal reflex pathways in the extensors. In this study, soleus H-reflexes recorded alone or during CPN stimulation at either short (2, 3, 4 ms) or long (80, 100, 120 ms) C-T intervals, and MG nerve stimulation delivered at 6 ms were elicited via conventional methods and similar to those adopted in studies conducted in healthy subjects. Plantar skin conditioning stimulation was delivered through two surface electrodes placed on the metatarsals at different C-T intervals ranging from 3 to 90 ms. CPN stimulation at either short or long C-T intervals and MG nerve stimulation resulted in a significant facilitation of the soleus H-reflex, regardless of the hip angle tested. Plantar skin stimulation delivered with hip extended at 10 degrees induced a bimodal facilitation reflex pattern, while with hip flexed (10 degrees , 30 degrees ) the reflex facilitation increased with increments in the C-T interval. This study provides evidence that in human chronic SCI, classically key inhibitory reflex actions are switched to facilitatory, and that spinal processing of plantar cutaneous sensory input and actions of synergistic/antagonistic muscle afferents interact with hip proprioceptive input to facilitate soleus H-reflex excitability. These actions might be associated with the pathological expression of neural control of movement in individuals with SCI, and potentially could be considered in rehabilitation programs geared to restore sensorimotor function in these patients.

Adolescent↗
PubMed2004

Effect of head position and visual condition on balance control in inverted stance.

This study analyzed the effect of head position and visual condition on the control of balance in handstand, a gymnastics posture that necessitates adaptation of sensory information processing. Five expert gymnasts participated. Centre of pressure trajectories and kinematics of different body segments were recorded. The gymnasts were instructed to maintain three handstands as long as possible in four head positions, with and without vision. Performances and postural stability was much better in the standard and dorsiflexion positions than in the aligned and ventroflexion positions under the two conditions of vision. Performances were lower without vision in the standard and dorsiflexion position. If vision clearly plays an important role, yet the tonic neck reflexes also seem to contribute greatly to control body sways during inverted posture.

Adult↗
PubMed2004

The coordination of rotations of the eyes, head and trunk in saccadic turns produced in natural situations.

In real life situations large gaze saccades may involve rotations of the trunk, as well as the eyes and head. When this happens the rotation of the head-in-space is similar whether or not the trunk is also rotating. However, the rotation of the head on the trunk (i.e. the neck movement) is very different in the two circumstances. For similar head-in-space rotations to occur, the neck and trunk movements cannot simply add independently: they must be coordinated. It is argued that this is achieved via a feedback loop in which the semi-circular canals monitor the rotation of the head-in-space, and the neck is driven by an error signal representing the difference between the intended head-in-space trajectory and the actual trajectory. This mechanism, which is essentially the same as the vestibulo-collic reflex, nulls out disturbances to the head-in-space trajectory, whether these are caused by active or passive trunk rotation.

Abdomen↗