Biomedical subjects

Leah R Bent

Publications and source records attributed to Leah R Bent.

9 recordsLinked to original sources

PubMed2007

Vestibular inputs do not influence the fusimotor system in relaxed muscles of the human leg.

Descending vestibular pathways have been shown to influence recruitment thresholds of alpha motoneurones in both human and cat. However, whereas parallel connections to the fusimotor system have been shown in the cat, such connections have not yet been demonstrated in humans. In the present study we investigated whether vestibular inputs can influence the firing of spontaneously active muscle spindles in the leg via activation of gamma motoneurones. Unitary recordings were made from 30 muscle spindle afferents via tungsten microelectrodes inserted percutaneously into the common peroneal nerve of seated awake human subjects. Sinusoidal bipolar binaural galvanic vestibular stimulation (GVS; frequency 0.2, 0.5, 0.8 Hz, amplitude +/-2 mA, 100 cycles) was applied to the mastoid processes. This continuous stimulation produced a sustained frequency-dependent illusion of "rocking in a boat" or "swinging in a hammock". Despite these robust illusions none of the spontaneously active muscle spindles exhibited phase-locked modulation of firing during sinusoidal GVS. We conclude that this dynamic vestibular input was not sufficient to recruit gamma motoneurones, which are known to have little spontaneous activity in relaxed human muscles.

Adult↗

PubMed2006

Visual-vestibular influences on locomotor adjustments for stepping over an obstacle.

Combined visual and vestibular influences on locomotor control, particularly in changing environments, are little understood. We studied such influences on body orientation and foot trajectory control during level walking and obstacle avoidance. Six young adults walked on the level and over an obstacle while vision was present or occluded as well as while vestibular information was intact or perturbed using galvanic vestibular stimulation (GVS). The occlusion of vision caused a slowing of gait during obstacle avoidance as well as increased clearance of the leading limb over the obstruction. GVS caused lateral deviations in head and trunk roll angles as well as in foot and trunk displacements, but these lateral deviations were the same during both level walking and obstacle avoidance. In addition, GVS had no effect at all on sagittal plane factors such as speed, foot proximity to the obstacle and vertical clearance over the obstacle. Overall, there is a complex visual control of bilateral obstacle avoidance, but the lack of differences in GVS effects between level and obstructed walking shows that vestibular information is not upregulated for obstacle avoidance. In addition, the robust indifference of anterior foot placement and body displacement to significant lateral deviations from GVS suggests an orthogonally based sensori-locomotor control.

Adult↗

PubMed2006

Modulation of muscle sympathetic bursts by sinusoidal galvanic vestibular stimulation in human subjects.

There is controversy as to whether the vestibulosympathetic reflexes demonstrated in experimental animals actually exist in human subjects. While head-down neck flexion and off-vertical axis rotation can increase muscle sympathetic nerve activity (MSNA) in awake subjects, we recently showed that bipolar galvanic vestibular stimulation (GVS) does not. However, it is possible that our stimuli (2 mA, 1 s)-although capable of causing strong postural and occulomotor responses-were too brief. To address this issue we activated vestibular afferents using continuous sinusoidal (0.5-0.8 Hz, 60-100 cycles, +/-2 mA) bipolar binaural GVS in 11 seated subjects. Sinusoidal GVS evoked robust vestibular illusions of "rocking in a boat" or "swinging from side to side." Cross-correlation analysis revealed a cyclic modulation of MSNA ranging from 31 to 86% across subjects (mean +/- SE 58 +/- 5%), with total MSNA increasing by 156 +/- 19% (P = 0.001). Furthermore, we documented de novo synthesis of sympathetic bursts that were coupled to the sinusoidal input, such that two bursts-rather than the obligatory single burst-could be generated within a cardiac interval. This demonstrates that the human vestibular apparatus exerts a potent facilitatory influence on MSNA that potentially operates independently of the baroreceptor system.

Action Potentials↗

PubMed2005

Evidence for strong synaptic coupling between single tactile afferents from the sole of the foot and motoneurons supplying leg muscles.

It has been known for some time that populations of cutaneous and muscle afferents can provide short-latency facilitation of motoneuron pools. Recently, it has been shown that the input from individual low-threshold mechanoreceptors in the glabrous skin of the hand can modulate ongoing activity in muscles acting on the fingers via spinally mediated pathways. We have extended this work to examine whether such strong synaptic coupling exists between tactile afferents in the sole of the foot and motoneurons supplying muscles that act about the ankle. We recorded from 53 low-threshold mechanoreceptors in the glabrous skin of the foot via microelectrodes inserted percutaneously into the tibial nerve of awake human subjects. Reflex modulation of ongoing whole muscle electromyography (EMG) was observed for each of the four classes of low-threshold cutaneous mechanoreceptors (17 of 21 rapidly adapting type I; 2 of 4 rapidly adapting type II; 7 of 18 slowly adapting type I; and 4 of 10 slowly adapting type II). Reflex modulation of the firing probability in single motor units (5 of 11) was also observed. These results indicate that strong synaptic coupling between tactile afferents and spinal motoneurons is not a specialization of the hand and emphasizes the potential importance of cutaneous inputs from the sole of the foot in the control of gait and posture.

Adult↗

PubMed2005

Vestibular contributions during human locomotor tasks.

This review explores vestibular contributions during dynamic tasks with the goal of identifying the underlying roles of vestibular information in task progression and balance control. Vestibular contributions to upper and lower body control during locomotor tasks were found.

Gait↗

PubMed2004

When is vestibular information important during walking?

Locomotion relies on vision, somatosensory input, and vestibular information. Both vision and somatosensory signals have been shown to be phase dependently modulated during locomotion; however, the regulation of vestibular information has not been investigated in humans. By delivering galvanic vestibular stimulation (GVS) to subjects at either heel contact, mid-stance, or toe-off, it was possible to investigate when vestibular information was important during the gait cycle. The results indicated a difference in the vestibular regulation of upper versus lower body control. Upper body responses to GVS applied at different times did not differ in magnitude for the head (P = 0.2383), trunk (P = 0.1473), or pelvis (P = 0.1732) showing a similar dependence on vestibular information for upper body alignment across the gait cycle. In contrast, foot placement was dependent on the time when stimulation was delivered. Changes in foot placement were significantly larger at heel contact (during the double support phase) than when stimulation was delivered at mid-stance (in the single support phase of the gait cycle; P = 0.0193). These latter results demonstrate, for the first time, evidence of phase-dependent modulation of vestibular information during human walking.

Adolescent↗

PubMed2004

Is the use of vestibular information weighted differently across the initiation of walking?

The purpose of this experiment was to examine vestibular contributions at specific times during the initiation of walking in human subjects. Subjects began walking forward at the sound of an auditory tone, with vision present or occluded. Galvanic vestibular stimulation (GVS) was delivered with the anode electrode on the right or left side at either: (1) onset of the anticipatory postural adjustment (APA), (2) toe-off of the first swing limb (TO) or (3) heel contact of the first swing limb (HC). Ground reaction forces and kinematic data were collected. Upper body (roll angles from head, trunk and pelvis) and lower body (foot placement) data were analysed to determine whether the timing and magnitude of the response to GVS, and therefore the level of vestibular contribution, was modulated at different points during the initiation of gait. With vision present and occluded, the magnitude of the lower body response varied depending on the event in the gait cycle at which the stimulation was delivered. These novel results demonstrate evidence that vestibular weighting during gait initiation is dependent upon the specific gait initiation events. Upper body roll also exhibited magnitude differences between events. However, these changes are proposed to occur due to the transition from a stationary position into a dynamic state, prompting the increased weighting of vestibular information. With vision present no significant changes were seen in the segment roll response. The observations suggest a distinction in vestibular regulation of upper body roll versus foot placement for successful completion of the gait initiation task. Changes in upper body roll are influenced by the dynamic nature of the task, whereas foot placement changes are modulated based on the event during gait initiation at which GVS is delivered.

Adult↗

PubMed2002

Visual-vestibular interactions in postural control during the execution of a dynamic task.

The purpose of this experiment was to determine the interaction between visual and vestibular information during the transition from quiet standing to the completion of a forward step. Six subjects were asked to take one step forward at the sound of an audio tone, with their eyes open or closed, and terminate the step in a standing position. During stimulation trials, galvanic vestibular stimulation (GVS) was delivered 1500 ms before the auditory cue. GVS was delivered at an intensity three-fold that of each subject's quiet stance threshold with either stimulus right, left or no stimulation. Force data were collected from three forceplates for the calculation of centre of pressure (CoP), and kinematic data were used to calculate centre of mass (CoM) and body trajectories. In quiet stance all subjects responded to the GVS perturbation by demonstrating upper body segment roll and whole body sway towards the anode electrode. Unexpectedly, in the presence of vision during quiet stance, the upper body roll response was not attenuated, even though the CoP sway patterns were reduced when vision was available. During the initiation phase of the step, despite ongoing GVS stimulation, there were no significant effects seen in CoM, CoP or upper body roll responses. During step execution, however, both CoM displacement and upper body roll demonstrated significant effects and both responses were significantly reduced when subjects' eyes were open. Analysis of the medio-lateral CoP integrals also indicated a strong stimulation effect between conditions late in the execution phase, which were largely attenuated with vision. The results suggest that the importance of visual and vestibular information varies depending on the phase of the task. In addition, the different integration between visual and vestibular input during quiet standing suggests a dual role for vestibular information. We propose that vestibular information in quiet standing has a role in maintaining whole body postural stability, as well as playing an integral role in the alignment of the body segments in preparation for proper movement execution. Vision was demonstrated to differentially attenuate these responses based on the phase of the task. Thus, visual and vestibular information appear to be integrated differently across the different phases of a forward-stepping task.

Acoustic Stimulation↗

PubMed2002

Vestibular contributions across the execution of a voluntary forward step.

This work addressed the influence of information arising from the vestibular system on the dynamic control of a forward step. Six subjects performed the stepping task with their eyes closed under three conditions of bipolar, binaural galvanic vestibular stimulation (GVS), including (1) no GVS, (2) GVS with the anode electrode on the side of the swing limb, and (3) GVS with the anode electrode on the side of the stance limb. GVS was delivered 1,500 ms prior to a cue to step. Ground reaction forces were collected from three force platforms and movement was recorded from IRED markers placed bilaterally on the body. The results showed that, following slight deviations caused by GVS onset, the step initiation behaviour was unaffected, but lateral deviations were found during the latter, more dynamic, phases of stepping for centre of mass trajectories, time integrals of the centre of pressure displacement and upper body roll. These findings showed that vestibular information is used differently across the execution of a step without vision. While the initiation phase is run in a feedforward manner without vestibular influence, vestibular information appears to be upregulated during the more dynamic phases. Also, the level of up-regulation may be different across step execution.

Adaptation, Physiological↗