Effect of neck posture on patterns of activation of feline neck muscles during horizontal rotation

The electromyographic (EMG) patterns of neck muscles were recorded during whole-body horizontal rotation in head-free, alert cats and head-restrained, decerebrate cats. In some trials the cervical column of the animal was oriented vertically, whereas in others it was oriented more horizontally. In alert cats making head movements that compensated for the motion of the platform, neck muscles with modulated patterns of activity could be divided into a subset whose individual EMG patterns changed significantly when the neck posture was altered (including longissimus capitis, obliquus capitis superior and scalenus anterior) and a subset whose individual EMG patterns were invariant regardless of neck posture (including obliquus capitis inferior, levator scapulae and complexus). In head-restrained, decerebrate cats, electromyograms from all implanted muscles were modulated similarly in phase with the platform position. Changing the orientation of the neck had little effect upon these EMG patterns evoked by the horizontal vestibulocollic reflex. One decerebrate cat with strong extensor tone was tested further under head-free conditions. There was very little compensatory head movement, but individual neck muscles displayed patterns of activity that were more similar to those observed in alert, head-free animals.     

Characteristics of the control of standing posture during pregnancy

During pregnancy, the physical and mental states greatly change. We investigated the influences of pregnancy and anxiety on postural control in pregnant women (P) standing upright in the late trimester. An analysis of posturograms revealed that the area of body sway and length of antero-posterior body sway were greater in P than those in non-pregnant controls (NP). No difference was found in the medio-lateral body sway between P and NP. Fast Fourier transform analysis of body sway showed that the percentile power of the 1.0–10.0 Hz band in the medio-lateral axis was smaller in P than in NP irrespective of whether the eyes were open or closed. P were divided into a high (HA) and low (LA) anxiety group on the basis of state anxiety scored by Spielberger's State- and Trait-Anxiety Inventory. A positive correlation was identified between state anxiety and the area of body sway in HA standing with eyes open. This correlation was diminished when the eyes were closed. Body sway of over 1 Hz is generally stabilized by somatosensory input, therefore, the results show that body sway in the medio-lateral axis is stabilized in P by increasing the sensitivity to somatosensory cues. High anxiety during pregnancy destabilizes the standing posture when the eyes are open. The correlation between anxiety and body sway revealed by our previous studies in college students was also confirmed in P, suggesting that humans with high anxiety abstract visual cues differently from those with low anxiety.     

Contribution of peripheral afferents to the activation of the soleus muscle during walking in humans

Summary Small, rapid stretches were applied to the soleus muscle during the stance phase of walking by lifting the forefoot with a pneumatic device. Stretch responses were induced in the soleus muscle by the disturbance. The amplitude and time course of the responses from the soleus muscle were a function of both the kinematics of the disturbance and the time in the step cycle when the disturbance was applied. The step cycle was divided into 16 equal time parts, and data obtained within each of these parts were averaged together. The electromyographic (EMG) response of the soleus muscle showed a time course that was similar to the time course of the angular velocity induced by the disturbance at the ankle. Three linear equations were used to predict the EMG response from the soleus muscle as a function of the angular kinematics of the disturbance: 1) velocity, 2) velocity and displacement, 3) velocity, displacement and acceleration. Introduction of a pure delay between the EMG and the kinematics substantially improved the predictions. Most of the variance (70%) in the EMG response could be accounted for by the velocity of the disturbance alone with an optimal delay (average 38 ms). Inclusion of a displacement term significantly increased the variance accounted for (85%), but further addition of an acceleration term did not. Since the velocity of the disturbance accounted for most of the variance, the reflex gain was estimated from the velocity coefficient. This coefficient increased in a ramp-like fashion through the early part of the stance phase, qualitatively similar to the increase in the H-reflex. Based on these identified gains, this reflex pathway was estimated to contribute substantially (30% to 60%) to the activation of the soleus muscle particularly during the early part of the stance phase.     

Adapting reflexes controlling the human posture

Summary Doubt about the role of stretch reflexes in movement and posture control has remained in part because the questions of reflex usefulness and the postural set have not been adequately considered in the design of experimental paradigms. The intent of this study was to discover the stabilizing role of stretch reflexes acting upon the ankle musculature while human subjects performed stance tasks requiring several different postural sets. Task specific differences of reflex function were investigated by experiments in which the role of stretch reflexes to stabilize sway during stance could be altered to be useful, of no use, or inappropriate.Because the system has available a number of alternate inputs to posture (e.g., vestibular and visual), stretch reflex responses were in themselves not necessary to prevent a loss of balance. Nevertheless, 5 out of 12 subjects in this study used long-latency (120 msec) stretch reflexes to help reduce postural sway. Following an unexpected change in the usefulness of stretch reflexes, the 5 subjects progressively altered reflex gain during the succeeding 3–5 trials. Adaptive changes in gain were always in the sense to reduce sway, and therefore could be attenuating or facilitating the reflex response. Comparing subjects using the reflex with those not doing so, stretch reflex control resulted in less swaying when the task conditions were unchanging. However, the 5 subjects using reflex controls oftentimes swayed more during the first 3–5 trials after a change, when inappropriate responses were elicited.Four patients with clinically diagnosed cerebellar deficits were studied briefly. Among the stance tasks, their performance was similar to normal in some and significantly poorer in others. Their most significant deficit appeared to be the inability to adapt long-latency reflex gain following changes in the stance task.The study concludes with a discussion of the role of stretch reflexes within a hierarchy of controls ranging from muscle stiffness up to centrally initiated responses.This research was supported in part by grant NS-02289-16 from the National Institute of Neurological Diseases and Stroke and from the Millicent Foundation of Oregon.     

A re-examination of the effects of instruction on the long-latency stretch reflex response of the flexor pollicis longus muscle

We re-examined the issue of how a subject’s intention to react to a joint perturbation may modulate the long-latency M2 stretch reflex response. The experiments were done on the flexor pollicis longus muscle (FPL) of the human thumb, for which there is evidence that its M2 reflex response is mediated, at least in part, by a pathway that traverses the motor cortex. The participation of the cerebral cortex in the genesis of the M2 reflex response may allow for a modulation of its amplitude, based on the intention of the subject. To test whether the M2 response is genuinely modulated by the subject’s intention, we examined the magnitude of this response as a function of the FPL background level of activation, measured by the surface rectified and filtered EMG. The subject was instructed either to oppose the perturbation as quickly as possible, not to react, or to relax as quickly as possible after the onset of the perturbation. The time integral of the long latency FPL EMG response, computed between 40 and 70 ms following the onset of stretch, was plotted against the mean torque produced by the distal inter-phalangeal joint of the thumb, or against the mean background FPL EMG. There were no significant differences in the FPL M2 EMG responses for different instructions. The amplitude of the reflex response was dependent only - in an approximately linear manner - on the background level of muscle activation. The total joint stiffness (intrinsic plus reflex) was also calculated for each combination of instruction and background torque. This variable was calculated over a time interval (from 75 to 105 ms) that included the torque due to the M2 reflex response superimposed on the background torque, but was well before any voluntary reaction. Again, there were no significant differences in joint stiffness as a result of the instruction. We therefore conclude that, despite a cortical contribution to the M2 stretch reflex response, this response is not influenced by the intention of the subject on how to react to a perturbation.     

Characterisation of the quadriceps stretch reflex during the transition from swing to stance phase of human walking

The main objective of this study was to characterize the stretch reflex response of the human thigh muscles to an unexpected knee flexion at the transition from stance to swing during walking. Eleven healthy subjects walked on a treadmill at their preferred speed. Reliable and constant knee flexions (6–12° amplitude, 230–350°/s velocity, 220 ms duration) were applied during the late swing and early stance phase of human walking by rotating the knee joint with a specifically designed portable stretch apparatus affixed to the left knee. Responses from rectus femoris (RF), vastus lateralis (VL), vastus medialis (VM), biceps femoris (BF), medial hamstrings (MH) and medial gastrocnemius (GM) were recorded via bipolar surface electromyograms (EMG). The onset of the response in the RF, VL and VM, remained stable and independent of the time in the step cycle when the stretch was applied. Across all subjects the response onset (mean ± SD) occurred at 23±1, 24±1 and 23±1 ms for RF, VL and VM, respectively. The duration of the initial response was 90–110 ms, at which time the EMG signal returned towards baseline levels. Three reflex response windows, labelled the short latency reflex (SLR), the medium latency reflex (MLR) and the late latency reflex response (LLR), were analysed. The medium and late reflex responses of all knee extensors increased significantly (p=0.008) as the gait cycle progressed from swing to stance. This was not related to the background EMG activity. In contrast, during standing at extensor EMG levels similar to those attained during walking the reflex responses were dependent on background EMG. During walking, LLR amplitudes expressed as a function of the background activity were on average two to three times greater than SLR and MLR reflex amplitudes. Distinct differences in SLR and LLR amplitude were observed for RF, VL and VM but not in the MLR amplitude. This may be related to the different pathways mediating the SLR, MLR and LLR components of the stretch response. As for the knee extensor antagonists, they exhibited a response to the stretch of the quadriceps at latencies short enough to be monosynaptic. This is in agreement with the suggestion by Eccles and Lundberg (1958) that there may be functional excitatory connections between the knee extensors and flexors in mammals.     

A re-examination of the effects of instruction on the long-latency stretch reflex response of the flexor pollicis longus muscle

We re-examined the issue of how a subject's intention to react to a joint perturbation may modulate the long-latency M2 stretch reflex response. The experiments were done on the flexor pollicis longus muscle (FPL) of the human thumb, for which there is evidence that its M2 reflex response is mediated, at least in part, by a pathway that traverses the motor cortex. The participation of the cerebral cortex in the genesis of the M2 reflex response may allow for a modulation of its amplitude, based on the intention of the subject. To test whether the M2 response is genuinely modulated by the subject's intention, we examined the magnitude of this response as a function of the FPL background level of activation, measured by the surface rectified and filtered EMG. The subject was instructed either to oppose the perturbation as quickly as possible, not to react, or to relax as quickly as possible after the onset of the perturbation. The time integral of the long latency FPL EMG response, computed between 40 and 70 ms following the onset of stretch, was plotted against the mean torque produced by the distal inter-phalangeal joint of the thumb, or against the mean background FPL EMG. There were no significant differences in the FPL M2 EMG responses for different instructions. The amplitude of the reflex response was dependent only — in an approximately linear manner — on the background level of muscle activation. The total joint stiffness (intrinsic plus reflex) was also calculated for each combination of instruction and background torque. This variable was calculated over a time interval (from 75 to 105 ms) that included the torque due to the M2 reflex response superimposed on the background torque, but was well before any voluntary reaction. Again, there were no significant differences in joint stiffness as a result of the instruction. We therefore conclude that, despite a cortical contribution to the M2 stretch reflex response, this response is not influenced by the intention of the subject on how to react to a perturbation.     

An enhanced level of motor cortical excitability during the control of human standing

Aim: The study examined the role of the motor cortex in the control of human standing. Methods: Subjects (n = 15) stood quietly with or without body support. The supported standing condition enabled subjects to stand with a reduced amount of postural sway. Peripheral electrical stimulation, transcranial magnetic stimulation (TMS) or transcranial electrical stimulation (TES) was applied to elicit a soleus (SOL) H‐reflex, or motor‐evoked potentials (MEPs) in the SOL and the tibialis anterior (TA). Trials were grouped based on the standing condition (i.e. supported vs. normal) as well as sway direction (i.e. forward and backward) while subjects were standing normally. Results: During normal when compared to supported standing, the SOL H‐reflex was depressed (?11 ± 4%), while the TMS‐evoked MEPs from the SOL and TA were facilitated (35 ± 11% for the SOL, 51 ± 15% for the TA). TES‐evoked SOL and TA MEPs were, however, not different between the normal and supported standing conditions. The data based on sway direction indicated that the SOL H‐reflex, as well as the SOL TMS‐ and TES‐evoked MEPs were all greater during forward when compared to backward sway. In contrast, the TMS‐ and TES‐evoked MEPs from the TA were smaller when swaying forward as compared to backward. Conclusions: The results indicated the presence of an enhanced cortical excitability because of the need to control for postural sway during normal standing. The increased cortical excitability was, however, unlikely to be involved in an on‐going control of postural sway, suggesting that postural sway is controlled at the spinal and/or subcortical levels.     

Habituation and conditioning of the human long latency stretch reflex

Summary The effects of stretch repetition rate, prior warning stimuli and self administered stretch were examined on the size of the short and long latency components of the stretch reflex electromyographic EMG response in flexor pollicis longus and the flexor muscles of the wrist and fingers. Stretches of constant velocity and extent were given every 10 s, 5 s, 2 s, or 1 s to either the wrist or thumb during a small background contraction of the flexor muscles. The size of the long latency component of the stretch reflex (measured as the area under the averaged rectified EMG responses) declined dramatically at faster repetition rates, especially in the wrist and finger flexors. The size of the short latency component was relatively unaffected. The size of the electrically elicited H-reflex in forearm muscles also failed to habituate under the same conditions. If each individual trial of a series was examined, the long latency component of the stretch reflex EMG could be seen to decrease in size over the first three to six stretches if stretches were given every 1 s, but not if stretches were given every 10 s. When stretches were given every 5 s to either wrist or thumb, an electrical stimulus applied to the digital nerves of the opposite hand 1 s before stretch reduced the size of the long latency component of the reflex EMG response. The short latency component was unaffected. Self triggering of wrist or thumb stretch by the subject pressing the stimulator button himself with his opposite hand, also decreased the size of the long latency component of the reflex EMG response without affecting the short latency component. It is concluded that factors other than stretch size or velocity can have marked effects on the size of the long latency component of the stretch reflex. These factors must be taken into account when comparing values of reflex size obtained with different stretching techniques and in different disease states in man.     

Control of upright stance over inclined surfaces

The present work investigated the control of upright posture on inclined surfaces (14°). Such conditions could, for example, change the contributions of muscle spindles resulting in alterations in postural sway. Subjects stood in quiet stance over a force platform positioned in one of three different fixed positions: horizontal (H), toes-up (ankle dorsi-flexion, D) and toes-down (ankle plantar-flexion, P). The experiments were done in the presence and also in the absence of vision. The analysis of the resulting sway was based on the power spectrum of the center of pressure displacement in the anterior–posterior direction (CP_ap). Analysis of the electromyogram (EMG) of the leg muscles and evaluation of the level of presynaptic inhibition (PSI) of the soleus (SO) Ia afferents complemented the study. The results showed that the spectrum of the CP_ap changed with the inclination of the surface of support. In condition D a higher instability was found as reflected by the higher spectral amplitudes at lower frequencies (below 0.3 Hz). On the other hand, the CP_ap of subjects in condition P contained increased amplitudes at high frequencies (above 0.3 Hz) and smaller amplitudes at low frequencies. The modifications found in the CP_ap power spectra when standing over an inclined surface may indicate changes in both short-term and long-term systems of postural control. These results do not seem to be associated with changes in group Ia feedback gain since no changes in the level of PSI were found among the three standing conditions. The SO EMG increased in condition P but did not change in condition D. On the other hand, the tibialis anterior had a tendency towards increased bursting activity in condition D. Eye closure caused an increase in the power of the oscillations at all spectral frequencies in the three standing conditions (H, P or D) and also a change in the profile of the CP_ap power spectrum. This may suggest a nonlinearity in the postural control system. The control of the slow component of the postural sway was more dependent on vision when the subject was in condition D, probably in association with the biomechanical constraints of standing on a toes-up ramp. A conclusion of this work was that, depending on the postural demand (direction of the ramp of support), the ensuing proprioceptive and biomechanical changes affect differentially the fast and slow mechanisms of balance control.     

Recovery of stretch reflex responses following mechanical stimulation

Summary The recovery behaviour of mechanically evoked stretch responses was investigated. Stimuli which promoted identical dorsiflexing movements around the ankle joint were applied to ten subjects in two positions, seated and upright. The experimental sets comprised single as well as double dorsiflexing displacements. In the latter the stimuli were elicited for durations of either 100, 200 or 400 ms. Stretch responses following the first displacements were related to the stretch velocity but not to the amplitude. The responses of the plantar flexors following the second mechanical dorsiflexion were reduced with respect to the delay time between the first and second displacement. In addition, the magnitudes of these responses depended on the functional task: the stretch responses recovered much faster in the standing position when the triceps surae muscle was only slightly activated, whereas in the relaxed sitting position the reflexes remained suppressed. Both reciprocal inhibition, as well as the time course of the reformation of intrafusal cross-bridge links, may help to explain the depression of the monosynaptic stretch reflex.     

Body oscillations in balancing due to segmental stretch reflex activity

Summary While subjects balanced on a seesaw consisting of a platform with a curved base, the antero-posterior sway of head and body as well as changes in the angle of the ankle joint were recorded and analysed for their frequency power spectrum. The EMG of leg muscles and the position of the resultant force exerted by the seesaw on a force-measuring platform were simultaneously registered and analysed. Balancing oscillations of 4–5 Hz were observed under this condition. They were accompanied by short, reciprocally organized bursts of EMG activity in the leg muscles. When stimulating the tibialis nerves to produce a displacement, the delay until the counterbalancing EMG activity started (about 40 ms) was in the time range of a fast-conducting segmental reflex. After partial ischaemic blocking of group I afferents from the leg muscles or fixation of the ankle joints, the predominant sway frequency was lacking, bursts of EMG activity became longer and stronger, and body balance was more unstable. Altering the height of the seesaw showed that a threshold change in the ankle angle was the determining factor in the production of spinal stretch reflex activity for fast regulation of balance.This work was supported by the Deutsche Forschungsgemeinschaft (SFB 70 — Hirnforschung und Sinnesphysiologie)     

Stretch reflex modulation during imposed static and dynamic hip movements in standing humans

The purpose of this study was to investigate the effects of hip proprioceptors on soleus stretch reflex excitability in standing humans. A custom-made device to stretch the ankle extensors was mounted on the lower leg portion of a gait orthosis and was used to elicit stretch reflex responses while standing. Six subjects with motor complete spinal cord injury (SCI) and six spinal intact subjects were placed in the orthosis, and stretch reflex responses were elicited when static and/or dynamic hip joint angle changes were imposed. We found that static hip extension significantly enhanced the stretch reflex responses as compared to the neutral position and the hip flexion position only in the SCI group. The EMG magnitude induced by hip extension was 142 ± 16.6% greater than that induced by the neutral position. When the leg was dynamically swung, the reflex responses also changed with the phase of the hip angle in the SCI group; in particular, the reflex amplitude was enhanced with hip extension and in the transition phase from flexion to extension. Although the magnitude of the changes was less than that in the SCI group, a similar type of modulation was found in the normal group. Given the fact that the persons with SCI had lost the neural connection between higher nervous center and the paralyzed lower limb muscles, the mechanism underlying the present results can be attributed to the peripheral afferent input due to the hip angle changes. We concluded that hip mediated afferent input has a significant influence on the excitability modulation of the soleus stretch reflex pathway. Such neural modulation may play a role in the mechanism responsible for the phase-dependent modulation of the stretch reflex while walking.     

Inability to activate muscles maximally during cocontraction and the effect on joint stiffness

In order to determine the maximum joint stiffness that could be produced by cocontraction of wrist flexor and extensor muscles, experiments were conducted in which healthy human subjects stabilized a wrist manipulandum that was made mechanically unstable by using positive position feedback to create a load with the characteristics of a negative spring. To determine a subject's limit of stability, the negative stiffness of the manipulandum was increased by increments until the subject could no longer reliably stabilize the manipulandum in a 1° target window. Static wrist stiffness was measured by applying a 3° rampand-hold displacement of the manipulandum, which stretched the wrist flexor muscles. As the load stiffness was made more and more negative, subjects responded by increasing the level of cocontraction of flexor and extensor muscles to increase the stiffness of the wrist. The stiffness measured at a subject's limit of stability was taken as the maximum stiffness that the subject could achieve by cocontraction of wrist flexor and extensor muscles. In almost all cases, this value was as large or larger than that measured when the subject was asked to cocontract maximally to stiffen the wrist in the absence of any load. Static wrist stiffness was also measured when subjects reciprocally activated flexor or extensor muscles to hold the manipulandum in the target window against a load generated by a stretched spring. We found a strong linear correlation between wrist stiffness and flexor torque over the range of torques used in this study (20–80% maximal voluntary contraction). The maximum stiffness achieved by cocontraction of wrist flexor and extensor muscles was less than 50% of the maximum value predicted from the joint stiffness measured during matched reciprocal activation of flexor and extensor muscles. EMG recorded from either wrist flexor or extensor muscles during maximal cocontraction confirmed that this reduced stiffness was due to lower levels of activation during cocontraction of flexor and extensor muscles than during reciprocal contraction.     

Independent control of reflex and volitional EMG modulation during sinusoidal pursuit tracking in humans

It is well known that during volitional sinusoidal tracking the long-latency reflex modulates in parallel with the volitional EMG activity. In this study, a series of experiments are reported demonstrating several conditions in which an uncoupling of reflex from volitional activity occurs. The paradigm consists of a visually guided task in which the subject tracked a sinusoid with the wrist. The movement was perturbed by constant torque or controlled velocity perturbations at 45° intervals of the tracking phase. Volitional and reflex-evoked EMG and wrist displacement as functions of the tracking phase were recorded. The relationship of both short-latency (30–60 ms) and longer-latency (60–100 ms) reflex components to the volitional EMG was evaluated. In reflex tracking, the peak reflex amplitude occurs at phases of tracking which correspond to a maximum of wrist joint angular velocity in the direction of homonymous muscle shortening and a minimum of wrist compliance. Uncoupling of the reflex and volitional EMG was observed in three situations. First, during passive movement of the wrist through the sinusoidal tracking cycle perturbation-evoked long-latency stretch reflex peak is modulated as for normal, volitional tracking. However, with passive joint movement the volitional EMG modulation is undetectable. Second, a subset of subjects demonstrate a normally modulated and positioned long-latency reflex with a single peak. However, these subjects have distinct bimodal peaks of volitional EMG. Third, the imposition of an anti-elastic load (positive position feedback) shifts the volitional EMG envelope by as much as 180° along the tracking phase when compared with conventional elastic loading. Yet the long-latency reflex peak remains at its usual phase in the tracking cycle, corresponding to the maximal velocity in the direction of muscle shortening. Furthermore, comparison of the results from elastic and anti-elastic loads reveals a dissociation of short- and long-latency reflex activity, with the short-latency reflex shifting with the volitional EMG envelope. Comparable results were also obtained for controlled velocity perturbations used to control for changes in joint compliance. The uncoupling of the reflex and volitional EMG activity in the present series of experiments points to a flexible relationship between reflex and volitional control systems, altered by peripheral input and external load.     

Non-linear eye movements during visual-vestibular interaction under body oscillation with step-mode lateral linear acceleration

We investigated visual-vestibular interactions in normal humans, where a constant speed of optokinetic stimulation was combined with whole body oscillation of lateral linear acceleration (10 m stroke). The acceleration mode was not sinusoidal, but rectangular (step). The pure optokinetic reflex (reference OKR) and the OKR under combined stimulation (combined OKR) were induced by a random-dot pattern projected onto a hemispherical dome-screen affixed to a chair on a linear accelerator. The translational vestibulo-ocular reflex (tVOR) was determined separately in the dark during acceleration-step oscillation. Since the tVOR was masked by the OKR under combined stimulation, the interaction was assessed as changes in combined-OKR velocity at two segments of opposing acceleration; in other words, tVOR directions identical to (agonistic) and opposite to (antagonistic) the OKR direction. When a moderate optokinetic stimulus-speed of 40 deg/s was combined with a moderate acceleration of 0.3 G (3.0 m/s²) as in Experiment 1 (N=10), the combined-OKR velocity always increased during the agonistic condition, and the motion of the visual pattern was perceived as slow and clear in this segment. On the other hand, during the antagonistic condition, the combined-OKR velocity either remained unchanged or increased moderately, and the motion of the visual pattern was sensed as fast and unclear. Notably, in most subjects, the velocity difference in combined-OKR between the agonistic and antagonistic conditions was around the value of the tVOR velocity. In five of the ten subjects who completed an additional test session with the acceleration level increased from 0.3 to 0.5 G (4.9 m/s²), similar findings were maintained individually, suggesting independent behavior of tVOR. Therefore, we hypothesized that the interaction could be direction-selective; in other words, both tVOR and OKR are additive during the agonistic condition, but tVOR is suppressed during the antagonistic condition. To extend this hypothesis further, another group of subjects was exposed to three different optokinetic-stimulus speeds of 20, 40 and 60 deg/s combined with an acceleration of 0.3 G (Experiment 2, N=15). Combined stimulation tended to optimize the combined-OKR velocity around the given optokinetic stimulus-speed, especially in those cases where the reference-OKR velocity deviated significantly from the stimulus speed. Changes in combined-OKR velocity were small at 20 deg/s, and were likely to be linear across the agonistic and antagonistic conditions. With increasing optokinetic stimulus-speeds, the direction-selective asymmetry hypothesized above was maintained in more than half of the subjects, while in the other subjects the combined-OKR velocity difference increased remarkably, probably due to an enhancement of the OKR gain. We conclude that tVOR suppression during the antagonistic stimulus-condition and non-linearity in the tVOR-OKR interaction are characteristic of the otolith system, even under moderate-stimulus environments, in contrast to the linear eye-movement interaction in the semicircular canal system.     

Mechanisms of cutaneous vasoconstriction during upright posture

The cutaneous circulation is thought to participate in the neurocirculatory adjustments during orthostatic stress, but the underlying mechanisms mediating such reflex cutaneous vasoconstriction are poorly understood. The aim of this study was to assess the relative importance of baroreceptor (cardiopulmonary and arterial) and positional (vestibular, exercise, veno-arteriolar and myogenic) reflexes in triggering cutaneous vasoconstriction during upright posture. First, hypotensive lower body negative pressure (LBNP) was compared with actual postural changes to assess the relative contributions of baroreceptor reflexes and positional reflexes. Then changes in body position were compared with changes in limb position in the absence or presence of proximal (axillary) or distal (local cutaneous) nerve blocks, to assess the relative contributions of vestibular, exercise, veno-arteriolar and myogenic reflexes. Skin sympathetic nerve activity was determined by microneurography, and skin blood flow was determined by laser Doppler velocimetry. LBNP of –50 mmHg (cardiopulmonary + arterial baroreceptors) had no effect on skin sympathetic nerve activity or skin vascular resistance. In contrast, an upright posture with the arms dependent (baroreceptor+vestibular+exercise+veno-arteriolar reflexes) caused a two- to threefold increase in skin vascular resistance. In the supine position, passive movement of the arm into a dependent position to activate veno-arteriolar reflexes alone evoked an increase in skin vascular resistance which approximated the response to normal upright posture. Blocking central sympathetic nerve impulses by application of an axillary blockade did not influence the cutaneous vasoconstrictor response to an upright posture or changes in limb position. In contrast, application of a distal nerve block by local cutaneous surface anaesthesia completely blocked vasoconstrictor responses evoked by these manoeuvres. In conclusion, these experiments in human subjects identify a primary role for veno-arteriolar reflexes in triggering vasoconstriction in the cutaneous circulation during upright posture.     

Evidence for reduced efficacy of the Ia-pathway during shortening plantar flexions with increasing effort

To determine whether the soleus (SOL) H-reflex is modulated during shortening contractions in a manner that has been observed for isometric contractions, SOL H-reflexes and M-waves were elicited via percutaneous electrical stimulation to the tibial nerve at an intensity that evoked an H-reflex at 50% of its maximum in 11 healthy subjects. Paired electrical stimuli were delivered as the ankle angle passed through 90° at an interval of 400 ms while the subject performed shortening contractions at levels of plantar flexion torque ranging between 2 and 30% of that during a maximal voluntary contraction (MVC). H-reflexes were also recorded during the performance of isomeric contractions of plantar flexors at similar levels of plantar flexion torque and at the same joint angle (muscle length) in an additional five healthy subjects. Correlations were examined between the peak-to-peak amplitude of the first H-reflexes, M-waves and plantar flexion torques in both protocols. It was revealed that no significant correlation was found between the SOL H-reflex and increasing plantar flexion torque during shortening contractions (ρ = −0.07, P = 0.15), while a strong positive correlation was observed for the isometric conditions (ρ = 0.99, P < 0.01). No significant change was observed in the SOL M-wave for either contraction type. Furthermore, the H-reflexes elicited via paired stimuli with the same background activity in voluntary shortening contractions showed almost identical amplitudes, suggesting that the level of homosynaptic post-activation depression did not change in response to the varying levels of activation in voluntary shortening contractions. Therefore, the lack of increase in the H-reflex during shortening contractions at increasing intensities is possibly due to a centrally regulated increase in presynaptic inhibition. Such a downward modulation of the reflex suggests that Ia-excitatory input onto the SOL motoneurone pool needs to be reduced during the performance of shortening contractions.     

Loss of large-diameter spindle afferent fibres is not detrimental to the control of body sway during upright stance: evidence from neuropathy

The question of whether the final arm posture to be reached is determined in advance during prehension movements remains widely debated. To address this issue, we designed a psychophysical experiment in which human subjects were instructed to reach and grasp, with their right arm, a small sphere presented at various locations. In some trials the sphere remained stationary, while in others (the perturbed trials) it suddenly jumped, at movement onset, to a new unpredictable position. Our data indicate that the final configuration of the upper limb is highly predictable for a given location of the sphere. For movements directed at stationary objects, the variability of the final arm posture was very small in relation to the variability allowed by joint redundancy. For movements directed at "jumping" objects, the initial motor response was quickly amended, allowing an accurate grasp. The final arm posture reached at the end of the perturbed trials was neither different from nor more variable than the final arm posture reached at the end of the corresponding stationary trials (i.e. the trials sharing the same final object location). This latter result is not trivial, considering both joint redundancy and the motor reorganization imposed by the change in sphere location. In contrast to earlier observations, our data cannot be accounted for by biomechanical or functional factors. Indeed, the spherical object used in the present study did not constrain the final arm configuration or the hand trajectory. When considered together, our data support the idea that the final posture to be reached is planned in advance and used as a control variable by the central nervous system.