Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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This study investigated the phenomenon of temporal summation in response to repetitive focused ultrasound stimulation of skin, muscle and joint in human volunteers. Stimulation was carried out using a custom-designed, focused ultrasonic stimulator with a resonant frequency of 1.66 MHz. A series of stand-off attachments were used to ensure that the focal region of the ultrasound beam projected either cutaneously, within the distal interphalangeal joint of the index finger, or within the first dorsal interosseous muscle. ⋯ Focused ultrasound is a potent, noninvasive technique with which to investigate temporal summation from somatic structures. A number of factors may account for the higher intensities required to elicit pain in muscle and the increased rate of temporal summation. It is clear, however, that if temporal summation is more pronounced in muscle than other tissues then this may be an important factor contributing to pain in musculoskeletal syndromes.
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Different sensory systems (e.g. proprioception and vision) have a combined influence on the perception of body orientation, but the timescale over which they can be integrated remains unknown. Here we examined how visual information and neck proprioception interact in perception of the "subjective straight ahead" (SSA), as a function of time since initial stimulation. In complete darkness, healthy subjects directed a laser spot to the point felt subjectively to be exactly straight ahead of the trunk. ⋯ To examine the role of vision, one group of subjects fixated a central visual target at the start of each block of continuous neck vibration, with SSA then measured at successive intervals in darkness. The illusory deviation of SSA was eliminated whenever visual input was provided, but returned as a linear function of time when visual information was eliminated. These results reveal: the persistent effects of neck proprioception on the SSA, both during and after vibration; the influence of vision; and integration between incoming proprioceptive information and working memory traces of visual information.
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Anticipatory control of upright posture is the focus of this study that combines experimental and modeling work. Individuals were asked to raise or lower their arms from two initial postures such that the final posture of the arm was at 90 degrees with respect to the body. Holding different weights in the hand varied the magnitude of perturbation to postural stability generated by the arm movement. ⋯ The posture COM profile from the model simulation was calculated. Results show that simulated posture COM profile and measured posture COM profile are identical for about 200 ms following the onset of arm movement and then they deviate. Therefore, the initial control of COM is passive in nature and the observed joint moment response is for joint stabilization and not for the control of COM.
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This study investigated the contribution of ankle muscle proprioception to the control of dynamic stability and lower limb kinematics during adaptive locomotion, by using mechanical vibration to alter the muscle spindle output of individuals' stance limbs. It was hypothesised that muscle length information from the ankle of the stance limb provides information describing location as well as acceleration of the centre of mass (COM) with respect to the support foot during the swing phase of locomotion. Our prediction, based on this hypothesis was that ankle muscle vibration would cause changes to the position and acceleration of the COM and/or compensatory postural responses. ⋯ There were also significant task-specific changes in stepping behaviour associated with COM control (measured as peak M/L acceleration, M/L foot displacement and COP position under the stance foot during the step over the obstacle). The results provide strong evidence that the primary endings of ankle muscle spindles play a significant role in the control of posture and balance during the swing phase of locomotion by providing information describing the movement of the body's COM with respect to the support foot. Our results also provide supporting evidence for the proposal that there are context-dependent changes in muscle spindle sensitivity during human locomotion.
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Low levels of central serotonin (5-HT) have been related to the state of depression, and 5-HT is the major target of the newer antidepressant drugs such as selective serotonin reuptake inhibitors (SSRIs). Neurons and platelets display structural and functional similarities, so that the latter have been proposed as a peripheral model of central functions. In particular, in blood more than 99% of 5-HT is contained in platelets, so that one could consider changes in 5-HT levels in platelets as a mirror of changes in central 5-HT. ⋯ In conclusion this work supports the literature in proposing platelets as a peripheral model of central functions. In particular, the present data support the idea that peripheral 5-HT platelet levels can reflect the state of the central 5-HT system in conditions of depression. Furthermore, the main outcome of this study is that PRP may mirror central extracellular 5-HT levels, whilst IPs mirror neuronal 5-HT changes.