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PubMed2006

Total sleep deprivation can increase vestibulo-ocular responses.

The effect of sleep deprivation on the vestibular function is largely unknown. Some studies have found that postural balance or vestibular reflexes are decreased in sleep-deprived subjects while others found no change. The aim of this study was to evaluate the effect of sleep deprivation on the vestibulo-ocular reflex (VOR). Horizontal eye movements were recorded in healthy subjects during earth vertical axis rotation in darkness once after an ordinary night sleep and once after 26-29 h of sleep deprivation. In the first experiment (n = 8), for which rotation was a 60 degrees s(-1) velocity step, sleep deprivation induced a significant increase in VOR gain. In the second experiment (n = 12), for which rotation was sinusoidal (0.2 Hz +/- 25 degrees s(-1)), sleep deprivation induced no significant modification in VOR gain. The difference between the two studies was the abrupt onset of the step stimulation in comparison with the sinusoidal rotation. Because of its unexpected onset and the potential threat to postural balance, the step stimulation may activate the system specialized in reorienting attention towards salient or behaviourally relevant events. This system includes the right temporoparietal cortex, an area also involved in VOR control. A number of studies have found that sleep deprivation alters the activity of this cortical area during attentional tasks. It is therefore our hypothesis that the difference between the effects of these two vestibular stimulations results from a sleep deprivation-induced modulation of the right temporoparietal cortex.

Adult↗
PubMed1982

The effect of captopril on postural hemodynamics and autonomic responses in chronic heart failure.

Both postural abnormalities and autonomic dysfunction have been identified in patients with chronic congestive heart failure (CHF). However, the effect of long-term vasodilator therapy on these phenomena has not been assessed. In this study the hemodynamic and plasma norepinephrine (PNE) responses to upright posture, as well as the cold pressor test and Valsalva's maneuver in 12 patients with severe chronic CHF during both acute and long-term captopril (CPT) therapy, were evaluated. This revealed an absence of the normal hemodynamic adjustments to upright posture and a blunted response of PNE. The heart rate and blood pressure responses to the cold pressor test and Valsalva's maneuver were similarly blunted. The reflex adjustments of systemic resistance during tilt improved with CPT therapy, but the absence of reflex tachycardia in the upright posture persisted. Additionally, there was improvement of the PNE response, and the responses of heart rate and blood pressure to the cold pressor test were virtually normalized during long-term CPT therapy. The abnormal response to the Valsalva maneuver persisted. In conclusion, hemodynamic and reflex-mediated responses to upright posture and the standard assessment of autonomic control mechanisms revealed abnormal patterns in heart failure. While the hemodynamic adjustment to postural changes and sympathetic responsiveness were improved with CPT, complete correction of these abnormalities did not occur. Whether the improvement was a nonspecific vasodilator effect or the result of specific CPT therapy remains to be determined.

Adult↗
PubMed1994

Arterial baroreflex regulation of blood pressure in patients with congestive heart failure.

OBJECTIVES: This study was designed to determine whether arterial baroreflex control of blood pressure is altered in patients with congestive heart failure. BACKGROUND: Arterial baroreceptor reflexes normally contribute to cardiovascular homeostasis by preserving blood pressure during changes in volume and posture. METHODS: Arterial baroreceptor reflex function was studied in 18 patients with congestive heart failure and 18 age-matched healthy subjects. The arterial baroreceptor-blood pressure reflex was assessed by measuring the blood pressure response to perturbations in carotid sinus pressure. Carotid baroreceptors were stimulated by applying negative pressure to a custom neck chamber (-10, -20 and -30 mm Hg) and were unloaded by applying neck positive pressure (+10, +20 and +30 mm Hg). RESULTS: Peak carotid baroreceptor-blood pressure reflex sensitivity was lower in patients with heart failure than in normal subjects (0.19 +/- 0.02 vs. 0.30 +/- 0.03 mm Hg/mm Hg, p < 0.05). During neck positive pressure, blood pressure increased less in the heart failure group than in the normal group. During neck suction, however, the decrease in blood pressure was similar in the two groups. CONCLUSIONS: Patients with heart failure are less able than normal subjects to increase blood pressure during arterial baroreceptor unloading, but they can reduce blood pressure appropriately during baroreceptor stimulation. These observations suggest that the resting blood pressure position on the arterial baroreceptor stimulus-response curve, the operational point, is closer to the baroreceptor threshold in patients with heart failure than in normal subjects. As a result, reduced inhibitory signals from arterial baroreceptors most likely contribute to a heightened state of sympathetic activity and vasoconstriction in patients with congestive heart failure.

Baroreflex↗
PubMed1991

Effects of body position on crossed extension reflex in decerebrate cat: rectus femoris is more sensitive than is vastus medialis.

Physiological stimuli which increase postural extensor tone also increase excitability of the crossed extension reflex (CER). We report here that such stimuli increase excitability of the CER recorded from rectus femoris (RF) more than that of vastus medialis (VM). The difference might reflect an important role of the biarticular actions of RF, which is also a weak hip flexor, in stabilizing the hip as well as extending the knee during maintenance of posture.

Animals↗
PubMed1992

Vestibular-evoked postural reactions in man and modulation of transmission in spinal reflex pathways.

1. The effects of galvanic stimulation of the vestibular apparatus (with electrodes on the mastoid processes) have been studied in standing human subjects. With the head turned to one side, subjects swayed towards the anode. 2. Forwards sway was preceded by electromyographic (EMG) activity in quadriceps and tibialis anterior muscles. Backwards sway was preceded by EMG activity in soleus and hamstring muscles. 3. Using the method of H reflex conditioning, forward sway was found to be preceded by inhibition of soleus motoneurones. 4. Interaction between the vestibular-evoked inhibition of soleus motoneurones preceding forwards sway and peripheral reflex inhibition was examined by a spatial facilitation method. 5. Interaction was found between vestibular-evoked inhibition and Ia reciprocal, group I non-reciprocal and group Ia-Ia presynaptic inhibitory pathways. It is concluded that vestibular signals converge on spinal interneurones subserving these inhibitory actions. 6. A 'decoupling' of soleus motoneurons and soleus-coupled Renshaw cells was found in the period of soleus activation preceding backwards sway.

Adult↗
PubMed2004

Functional and anatomical correlates of impaired velocity storage.

Velocity storage (VS), a brainstem function, extends the low-frequency response of the vestibular system. To better understand VS mechanisms and characteristics in humans, we analyzed retrospectively functional measures of gait, electrophysiological measures of vestibular function, and imaging studies in an attempt to determine clinical, electrophysiological, and anatomical correlates of abnormalities in VS. Two cohorts of patients referred to our Risk of Falls Assessment Clinic participated in this investigation. Group 1 (control) patients demonstrated normal caloric and rotary chair tests. Group 2 patients with impaired velocity storage (experimentals) differed clinically from Group 1 only by demonstrating abnormal multifrequency vestibulocular reflex phase measures on rotational testing. Results showed that Group 2 patients had greater impairments in postural stability and gait than Group 1 patients. Additionally, 80% of patients in Group 2 and none in Group 1 showed pontine hyperintense lesions on MRI.

Aged↗
PubMed1982

Properties of postural adjustments associated with rapid arm movements.

1. We have examined rapid postural adjustments associated with a class of voluntary movements that disturb postural equilibrium. In the text that follows, these motor activities are termed associated postural adjustments and voluntary focal movements, respectively. Standing human subjects performed a variety of movement tasks on a hand-held manipulandum, resulting in disturbances to their postural equilibrium. The experimental use of movements that interact with the subject's environment in a relatively simple was permitted a more precise comparison of the postural adjustments with their associated focal movements. 2. Subjects either pulled or pushed on a stiff interface (the handle) or they responded in a predetermined way to handle perturbations. These activities were carried out with various degrees of steady-state postural stability. Prior to and during these movements, support surface and handle forces, electromyographic (EMG) signals, and body sway were monitored. 3. In addition to previously shown postural adjustments associated with reaction-time armed movements, we have demonstrated these postural activities occur in concept with segmental stretch reflexes and self-initiated (untriggered) movements. Postural adjustments were initiated shortly before all focal movements tested except the short-latency component of the biceps stretch reflex (25- to 30-ms latency). However, this reflex component was rarely elicited by handle perturbations in free-standing subjects; therefore, postural adjustments usually preceded any biceps activity under this condition. 4. By varying the degree of steady-state postural equilibrium, a reciprocal gain/threshold relationship between postural and focal components was documented, i.e., when stability was high, postural activity was reduced or absent and focal activity enhanced. Conversely, the biceps stretch reflex was difficult to elicit under any condition where the subjects was not fully supported in the direction of movement and reaction times of focal movements were prolonged. 5. Postural activities associated with focal movements were found to share a number of organizational properties with automatic postural adjustments to support surface movements. Specifically, the postural muscle synergies were equivalent in muscle composition, relative activation magnitudes, and relative temporal sequencing. Furthermore, both types of postural adjustments were highly specific in locus and magnitude to the quality of steady-state postural equilibrium (e.g., postural "set"). 6. A conceptual model is proposed that suggests one simple way in which the reciprocal influence of postural set on postural and focal movement components and their temporal sequencing might be accomplished. Furthermore, we propose in this model a common central organization of postural adjustments associated with focal movements and those elicited by support-surface movements.

Afferent Pathways↗
PubMed1999

Development of hindlimb postural control during the first postnatal week in the rat.

The development of the postural control of hindlimbs was investigated during the first postnatal week in the rat. The whole body was tilted in a vertical plane with the nose up. The proportion of animals producing a complete extension of both hindlimbs increased with age until the end of the first postnatal week. Motor responses were evoked by the pitch tilt already at birth with a slight extension of the hips, the knees and the ankles remaining bent in most cases. The extension produced at the ankle level increased gradually during the first postnatal week. This was correlated with a change in the EMG activity recorded from the triceps surae muscles (ankle extensors) during this postural reaction. There was a gradual acquisition of a tonic pattern. Characteristics of EMG responses changed significantly with age demonstrating an important increase in the use of triceps surae muscles in this postural task. These data demonstrate that the first postnatal week is a critical period for the development of postural reactions in the hindlimbs. They also suggest the existence of a proximo-distal gradient in the maturation of postural control. The mechanisms responsible for this reflex and for the maturation of posture are discussed.

Action Potentials↗
PubMed1994

Properties of sympathetic reflexes elicited by natural vestibular stimulation: implications for cardiovascular control.

1. To study the properties of vestibulosympathetic reflexes we recorded outflow from the splanchnic nerve during natural vestibular stimulation in multiple vertical planes in decerebrate cats. Most of the animals were cerebellectomized, although some responses were recorded in cerebellum-intact preparations. Vestibular stimulation was produced by rotating the head in animals whose upper cervical dorsal roots were transected to remove inputs from neck receptors; a baroreceptor denervation and vagotomy were also performed to remove visceral inputs. 2. The plane of head rotation that produced maximal modulation of splanchnic nerve activity (response vector orientation) was measured at 0.2-0.5 Hz. The dynamics of the response were then studied with sinusoidal (0.05- to 1-Hz) stimuli aligned with this orientation. 3. Typically, maximal modulation of splanchnic nerve outflow was elicited by head rotations in a plane near pitch; nose-up rotations produced increased outflow and nose-down rotations reduced nerve discharges. The gains of the responses remained relatively constant across stimulus frequencies and the phases were consistently near stimulus position, like regularly firing otolith afferents. Similar response dynamics were recorded in cerebellectomized and cerebellum-intact animals. 4. The splanchnic nerve responses to head rotation could be abolished by microinjections of the excitotoxin kainic acid into the medial and inferior vestibular nuclei, which is concordant with the responses resulting from activation of vestibular receptors. 5. The properties fo vestibulosympathetic reflexes recorded from the splanchnic nerve support the hypothesis that the vestibular system participates in compensating for posturally related changes in blood pressure.

Animals↗
PubMed1997

Conditioned and unconditioned forelimb reflex systems in the cat: involvement of the intermediate cerebellum.

Temporary inactivation of the cerebellar interposed nuclei was used to assess the role of the intermediate cerebellum in the performance of forelimb cutaneo-muscular reflexes in the cat. The following types of reflexive responses were evaluated: the classically conditioned and unconditioned forelimb withdrawal responses and the forelimb tactile placing, hopping and magnet responses. The experiments tested the hypothesis that the intermediate cerebellum is involved in the performance of all the above forelimb reflexes. The forelimb withdrawal reflex was classically conditioned in a newly developed paradigm in which animals were first operantly conditioned to stand on four elevated platforms. Trained animals were microinjected with a gamma-aminobutyric acid (GABA) agonist, muscimol, in the interposed nuclei, and the effects of inactivation of the intermediate cerebellar output on the forelimb reflexes were examined. The main findings of these experiments are that unilateral muscimol inactivation of the interposed nuclei in the cat abolished the expression of the classically conditioned limb flexion reflex, suppressed the performance of the unconditioned withdrawal reflex and, in parallel, down-regulated the tactile placing, hopping and magnet postural responses in the ipsilateral forelimb. These observations are inconsistent with concepts indicating exclusive involvement of the intermediate cerebellum in the classically conditioned reflexes elicited by aversive stimuli. On the contrary, they support the hypothesis of a more global involvement of this structure in learned and unlearned defensive flexion reflexes and in automatic postural response systems.

Animals↗
PubMed1998

Soleus H-reflex gain in elderly and young adults: modulation due to body position.

BACKGROUND: The control of posture and balance in the elderly is a primary health concern. Postural instability directly leads to a greater incidence of falling in the elderly population. One important neuromuscular mechanism instrumental in the control of posture and balance is the reflex system. The purpose of this study was to examine the gain of the soleus H-reflex in young and elderly adults in two different body positions: standing and prone. METHODS: Eighteen neurologically healthy volunteers were categorized by age in two groups: young (n = 9, mean age = 23.3 yr) and elderly (n = 9, mean age = 71.7 yr). In each position, the resting H-max/M-max ratio was determined. The gain of the reflex was also assessed by instructing the subject to perform voluntary contractions of 10, 20 and 30% of their maximum voluntary contraction, using real-time EMG biofeedback. Data were sampled on-line using custom designed software (sample rate = 2 kHz). Dependent variables included the average background EMG of the soleus muscle (40 ms window prior to stimulation) and the peak-to-peak amplitude of the elicited soleus H-reflex. To examine the gain of the reflex, the peak-to-peak amplitude of the H-reflex was plotted against the background EMG activity for each contraction intensity. RESULTS: Results indicated the following: young subjects significantly depressed the H-max/M-max ratio when standing (69.3% prone, 55.1% standing), whereas elderly subjects increased the ratio (36.1% prone, 54.5% standing). Also, the young subjects modulated the gain of the reflex from prone to standing (3.30 prone, 3.68 standing), and the elderly subjects demonstrated no gain modulation in the different body positions (2.23 prone, 1.91 standing). In both body positions the young subjects demonstrated significantly higher gain that the elderly subjects. CONCLUSIONS: The results demonstrate different control strategies for young and elderly subjects between prone and standing body positions.

Adult↗
PubMed1984

Corrective reactions to stumbling in man: functional significance of spinal and transcortical reflexes.

Stumbling reactions were studied in terms of bilateral leg muscle electromyographic (EMG) responses during locomotion on a treadmill. At random times, but fixed points in the stepping cycle, short impulses were applied to the treadmill, either accelerating or decelerating its progress. It was found that acceleration was compensated for by a strong ipsilateral gastrocnemius and contralateral tibialis anterior activation, and deceleration by a bilateral tibialis anterior activation. In both muscles the responses appeared with a latency of about 70 msec and lasted for about 150 msec. It is concluded that sudden displacements induced by acceleration or deceleration during gait are compensated for by a polysynaptic spinal pathway, with an associated depression of monosynaptic responses.

Acceleration↗
PubMed2006

Discrimination between active and passive head movements by macaque ventral and medial intraparietal cortex neurons.

An important prerequisite for effective motor action is the discrimination between active and passive body movements. Passive movements often require immediate reflexes, whereas active movements may demand suppression of the latter. The vestibular system maintains correct body and head posture in space through reflexes. Since vestibular inputs have been reported to be largely suppressed in the vestibular nuclei during active head movements, we investigated whether head movement-related signals in the primate parietal cortex, a brain region involved in self-motion perception, could support both reflex functions and self-movement behaviour. We employed a paradigm that made available direct comparison of neuronal discharge under active and passive movement conditions. In this study, we demonstrate that a population of intraparietal (VIP (ventral) and MIP (medial)) cortex neurons change their preferred directions during horizontal head rotations depending on whether animals have performed active movements, or if they were moved passively. In other neurons no such change occurred. A combination of these signals would provide differential information about the active or passive nature of an ongoing movement. Moreover, some neurons' responses clearly anticipated the upcoming active head movement, providing a possible basis for vestibular-related reflex suppression. Intraparietal vestibular neurons thus distinguish between active and passive head movements, and their responses differ substantially from those reported in brainstem vestibular neurons, regarding strength, timing, and direction selectivity. We suggest that the contextual firing characteristics of these neurons have far-reaching implications for the suppression of reflex movements during active movement, and for the representation of space during self-movement.

Action Potentials↗
PubMed2002

Down training of the elderly soleus H reflex with the use of a spinally induced balance perturbation.

The purpose of this study was to determine the ability of the elderly central nervous system to modulate spinal reflex output to functionally decrease a spinally induced balance perturbation. In this case, the soleus H reflex was used as the source of perturbation. Therefore, decreasing (down training) of the soleus H reflex was necessary to counteract this perturbation and to better maintain postural control. In addition to assessing the effect of this perturbation on the H reflex, static postural stability was measured to evaluate possible functional effects. Ten healthy young subjects (age: 27.0 +/- 4.6 yr) and 10 healthy elderly subjects (age: 71.4 +/- 5.1 yr) participated in this study. Subjects underwent balance perturbation on 2 consecutive days. On day 1 of perturbation, significant down training of the soleus H reflex was demonstrated in both young (-20.4%) and elderly (-18.7%) subjects. On day 2 of perturbation, significant down training of the soleus H reflex was again demonstrated in both young (-24.6%) and elderly (-21.0%) subjects. Analysis of static stability after the 2 days of balance perturbation revealed a significant 10.1% decrease in the area of sway in elderly subjects. In conclusion, this study demonstrated that healthy, elderly subjects compared with young subjects were equally capable of down training the soleus H reflex in response to a balance perturbation. Furthermore, the improvement in static stability through balance training may provide further evidence that balance can be retrained and rehabilitated in subjects with decreased reflex function.

Adult↗
PubMed1993

Hormonal responses to head-out water immersion diminish after exposure to head-down tilt.

It has been postulated that gravitational change from 1 g to microgravity may cause cephalad fluid shift, resulting in suppression of antidiuretic hormone (ADH) secretion and diuresis (Gauer-Henry's reflex). However, results obtained in space flights did not confirm this. Since astronauts are confined at supine position for hours before launch, this posture may abolish the reflex in space flight. To investigate this possibility, effects of head-out water immersion (WI) after 2-hour head-down tilt (HDT) on hormonal and metabolic responses were examined and compared with those after 1-hour upright posture (UP). Hematocrit decreased by WI after UP, indicating hemodilution, but it did not change by WI after HDT. Plasma ADH, renin activity and aldosterone fell and atrial natriuretic peptide (ANP) rose by WI after UP, resulting in increased urine flow. On the other hand, appreciable hormonal changes were not elicited by WI after HDT, and urine flow stayed unchanged. These results indicate that fluid shift and hormonal and metabolic responses to WI are strongly attenuated by the prior exposure to HDT. Pre-launch posture of astronauts may at least partly explain why either suppression of ADH or diuresis was not observed on arrival at space.

Adult↗
PubMed2005

Assessment of cardiac baroreflex function during fixed atrioventricular pacing using baroreceptor-stroke volume reflex sensitivity.

UNLABELLED: Baroreflex sensitivity in paced patients. INTRODUCTION: The baroreceptor-heart rate (HR) reflex has prognostic value in cardiovascular medicine. However, it cannot be used in chronotropically incompetent or paced patients. In healthy subjects, the baroreceptor-stroke volume (SV) reflex, with power spectral analysis of SV and blood pressure (BP) variations in the low-frequency band, serves as an alternate measure of the baroreceptor-cardiac reflex. This study examined the baroreceptor-stroke volume (SV) reflex sensitivity in the supine and 60 degrees upright positions in paced patients. METHODS AND RESULTS: We studied 16 recipients of dual-chamber pacemakers paced at a fixed rate. The hemodynamics and baroreceptor-SV reflex sensitivity were measured during atrioventricular (AV) sequential pacing every 5 minute in the supine and 60 degrees upright positions. Mean SV decreased from 42.0+/-20.1 mL in the supine to 36.6+/-16.1 mL in the upright position (P<0.05), whereas BP and total peripheral resistance did not change. A significant fall in baroreceptor-SV reflex sensitivity from 29.2+/-18.0%/mmHg to 19.5+/-15.5%/mmHg was observed during upright tilt (P<0.005). CONCLUSION: Fixed-rate AV sequential pacing did not blunt the decrease in baroreceptor-SV reflex sensitivity consistent with the arterial baroreflex gain response to upright posture. The decreased baroreceptor-SV reflex sensitivity occurring with the upright posture may reflect a baroreflex-induced inotropic effect secondary to vagal withdrawal and sympathetic activation.

Aged↗