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This article examines the relation between the perception of one's own body position and the distribution of fluid along the subject's spinal (z-) axis. Two experiments are reported. The first one is a replication of the Vaitl et al. study [J. Psychophysiol. 27 (1997) 99] which has shown that changes in shifts of blood volume into or out of the thoracic cavity induced by lower body positive pressure (LBPP: +30 mmHg) or lower body negative pressure (LBNP: -30 mmHg) exerted on the lower body led subjects to feel tilted head-up or head-down, respectively. The second experiment was designed to differentiate between the influence of the otoliths and of the changes in fluid distribution on the perception of body position by means of a sled centrifuge in combination with LBPP and LBNP. In both experiments, changes in blood distribution within the thoracic cavity were measured by impedance plethysmography. Forty healthy volunteers (17 females) participated in experiment 1. They were positioned on the side (right-ear-down head position) on a tiltable board which the subject and the experimenter could tilt via remote control around the subjects' z-axis. Subjects were asked to rotate the board until they felt they were in a horizontal posture. The results clearly show that the perception of posture is influenced by the shift in blood distribution. During LBNP subjects perceived being tilted head-up, whereas LBPP led them feel tilted head-down. Thus, the results obtained in the 1997 study were replicated. Fourteen males volunteered in experiment 2. They were positioned on the sled on a centrifuge in the same manner as in experiment 1. The sled could be moved via remote control by both the subject and the experimenter. While the centrifuge rotated (omega=2 pi times 0.6 rotations per second) the subjects were asked to move the sled until they felt they were in a horizontal position. As in experiment 1, shifts in blood volume were induced by LBPP and LBNP. The distance between the binaural axis (position of the otoliths) and the centrifuge axis served as dependent measure indicating the subjective horizontal position. Due to the additional centrifugal forces exerted on the body the shifts in blood volume were more pronounced than in experiment 1 where only gravitational forces were produced. The changes in the perception of posture were influenced by both the otoliths and the fluid distribution in such a way that both interact in a compensatory manner. These results again corroborate the evidence that afferent inputs from the cardiovascular system play a major role in the perception of the body posture. This phenomenon of graviception needs to be further elucidated with respect to the origins of the afferent inputs and the site and type of graviceptors (mechanoreceptors) involved.
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Recent experiments have shown that somatic graviceptors exist in humans. Traditionally, extravestibular gravity information has been thought to originate from mechanoreceptors in the joints, muscles and skin. Experiments with normal, paraplegic and nephrectomized subjects revealed that the kidneys and the cardiovascular system are involved in providing truncal gravity information. The present study intends to determine the influence of shifts in body fluid, especially of the distribution of blood along the subjects' spinal (Z-) axis, on the perception of posture. To this end, the distribution of body fluids was altered by means of the technique of lower body negative and positive pressure (LBNP and LBPP). LBNP leads to venous pooling of blood in the legs, whereas LBPP prevents venous blood from pooling, increasing central volume. Changes in blood distribution were measured by segmental impedance cardiography for four body segments: the upper torso (thoracic cavity), lower torso (abdominal and pelvic region), thigh and calf. Seventeen healthy subjects (mean age: 27.3 years) participated in the experiment. They were positioned on the side (right-ear-down head position) on a tilt table which the subjects and the experimenter could tilt via remote control around an axis parallel to the subjects' visual (X-) axis. The experimenter set the initial tilt in total darkness to arbitrary angles while strictly alternating between head-up and head-down tilts. Subjects were then asked to rotate the board until they felt they were in a horizontal posture. Means and variances of eight pairs of settings were taken as a measure of the subjective horizontal posture (SHP). During LBNP (-30 mmHg), subjects perceived being tilted head-up, whereas LBPP (+30 mmHg) led them to feel tilted head-down. The results corroborate the hypothesis of an effect of the blood's mass on graviception and also indicate supplementary contributions of other visceral afferences.
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The investigation of afferent cardiovascular influences upon central nervous processes needs a methodology which allows for independent and systematic manipulation of circulatory processes. By mechanical manipulation of posture (tilt table, orthostasis) and compression of lower body parts (by anti-G-suit) reliable changes in heart rate and blood pressure can be induced. In fourty subjects (study 1) it could be shown that sustained (30 min.) changes in heart rate (e.g. orthostatic tachycardia, decrease of heart rate during compression) and mean arterial blood pressure (increase during orthostasis with and without compression) occur. Although changes in heart rate could be achieved irregardless of whether the venous "pooling" was suppressed by an air- or water-filled pressure suit, the pressor effect did differ quite considerably. In fourty-four subjects (study 2) it could be demonstrated that only by means of an water-filled suit further increases in mean arterial blood pressure could be evoked during orthostasis. Changes in hemodynamic also lead to changes in sympatho-vagal control of cardiac activity. In study 3 (ten subjects) it could be shown, that orthostasis mainly evokes alterations in sympathetic activity whereas lower body compression leads to additional increases of vagal activity and respiratory sinus arrhythmia during orthostasis.
Erkunden
Team
- Vaitl (3)
Eintragsart
Thema
- Lower Body Negative Pressure
- Adolescent (1)
- Adult (3)
- Arousal (1)
- Blood Pressure (2)
- Blood Volume (1)
- Blood Volume/physiology (1)
- Centrifugation (1)
- Decompression (1)
- Female (3)
- Fluid Shifts (1)
- Fluid Shifts/*physiology (1)
- *Gravitation (1)
- Gravity Sensing (1)
- Heart Rate (1)
- Humans (3)
- Male (3)
- Mechanoreceptors/physiology (1)
- Middle Aged (2)
- Otolithic Membrane/physiology (1)
- Perception/*physiology (1)
- Posture (2)
- Posture/*physiology (1)
- Regional Blood Flow/physiology (1)
- Thorax/blood supply (1)
- Tilt-Table Test (1)