Proprioceptive system: Difference between revisions - VR & AR Wiki

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Studies that observed motor cortical neurons concluded that the brain is not concerned with information about muscle length changes from individual afferents, but with the population of muscles afferent input signals that arises in groups of muscles. Another area that has been explored is the relation between proprioception and fatigue from exercise. Some of the clumsiness in movements felt after intense exercise could have an origin in proprioception. An important point is age and proprioception. Evidence shows that a decline in proprioception due to age is responsible for an increase in falls in the elderly <ref name=”2”></ref>.

==Brief historical background==

There have been speculations about a muscle sense that date back at least to the 17th century. Von Helmholtz proposed the theory of “sensation of innervation”, in which sensations that would apparently arise within the muscles had origin in the brain, in association with motor commands. The discovery of the muscle sense is attributed to Charles Bell, in 1826. He also speculated about whether the signals were of central or peripheral origin. He questioned, “(do) muscles have any other purpose to serve than merely to contract under the impulse of their motor nerves?” He then wrote that “between the brain and the muscles there is a circle of nerve; one nerve (ventral roots) conveys the influence from the brain to the muscle, another (dorsal roots) gives the sense of the condition of the muscle to the brain.” <ref name=”1”></ref> <ref name=”2”></ref> <ref name=”12”> Winter, J. A., Allen, T. J. and Proske, U. (2005). Muscle spindle signals combine with the sense of effort to indicate limb position. Journal of Physiology, 568(3): 1035-1046</ref>

Two schools of thought developed: one that supported that the muscle sense had an entirely central origin, and another that believed that the main responsible was a peripheral signal (2). Henry Bastian, the originator of the term kinaesthesis, was at the time the only who proposed a hybrid theory that encompassed both central and peripheral components. He abandoned this idea in favor of a purely peripheral mechanism <ref name=”1”></ref> <ref name=”2”></ref>.

For much of the 20th century, the prevailing view regarding the limb position sense was that the joints where the main receptors responsible for kinaesthetic sensations. This changed after the experiments of Goodwin and colleagues on the sensory effects of muscle vibration. This study provided evidence for the role of muscle spindles in conscious sensation and currently they are considered the principal proprioceptors <ref name=”2”></ref> <ref name=”9”></ref> <ref name=”12”></ref>.

The term “proprioception” was introduced by Sherrington in 1906. The term is a combination of the Latin “propius” (one’s own) and “perception”. He described it as a type of feedback from the limbs to the central nervous system. He referred to proprioceptors as: “‘In muscular receptivity we see the body itself acting as a stimulus to its own receptors – the proprioceptors.” <ref name=”1”></ref> <ref name=”10”></ref> <ref name=”11”></ref>

==The proprioceptive senses==

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The central nervous system distinguishes between muscle spindle impulses generated by muscle stretch and by fusimotor activity, which may further indicate the prominent role of these receptors in kinaesthesia <ref name=”5”></ref>.

==The sense of effort==

Traditionally, the sense of effort, force, and heaviness have been regarded as to be generated by signals of central origin associated with motor commands. The senses of effort, force, and heaviness are distinct, has proprioceptive sensations, since they are always associated with motor commands. Kinaesthetic sensations can arise in passive limbs. Recent developments have given the centrally generated sense of effort a greater contribution to position sense. Fortier and Basset (2012) mention that the current hypothesis is that when “spindles are activated through the fusimotor system, they no longer contribute to position sense and the effort signal generated by the motor command provides the additional positional information”, and that “future experiments should concentrate on identifying the central sites of origin of the effort sensation, determining the effect of sense of effort on position sense, and assessing the interaction between peripheral and central systems.” <ref name=”2”></ref> <ref name=”5”></ref> <ref name=”7”></ref>

==The central nervous system==

The central nervous system receives input from several sources like the somatosensory system, the vestibular system, and the visual system. The somatosensory system is composed by several types of receptors, like mechanoreceptors, thermoreceptors, pain receptors, and proprioceptors. The integration of proprioceptive signals by the central nervous system is still not completely understood, despite several studies on the subject matter. The traditional views are changing and some propose that the sense of position and movement results from integrating multiple sources of proprioceptive information within the central nervous system, and not just from muscle spindles <ref name=”5”></ref>.

==Effect of exercise==

The effect of exercise in the sense of position remains controversial to explain, and further research is necessary. Some studies suggest a decline of the sense of velocity and position sense after an isometric contraction exercise protocol. Indeed, there is a common feeling of awkwardness and clumsiness after intense exercise that is not only muscle weakness, but also a lesser certainty about the placement of the fatigued limbs in the absence of vision <ref name=”2”></ref> <ref name=”5”></ref>.

There is also the suggestion that the effort that is required to maintain the position of a limb against the force of gravity is the element that provides the positional cue, although some studies contradict this sense of effort hypothesis. Moreover, data supports the view that muscle spindle would be responsible for the sense of movement, not being sensitive to exercise disruption <ref name=”5”></ref>.

==Proprioception and virtual reality==

Proprioception allows the formation of a mental model, describing the spatial and relational dispositional of the body and its parts. A virtual reality system needs that the normal proprioceptive data that is used to form a mental model of the body be overlaid with sensory data that is supplied by the computer-generated displays. For an effective virtual reality, it is fundamental that there is consistency between proprioceptive information and sensory feedback. This is done by the correct capturing of the movement of the user, and simulating it in the virtual environment, in order to increase a sense of immersion <ref> Slater, M., Usoh, M. and Steed, A. (1995). Taking steps: the influence of a walking technique on presence in virtual reality. ACM Transactions on Computer_Human Interaction, 2(3): 201-219</ref>. Also, according to Mine (1997), “providing a real-world frame of reference in which to operate and a more direct and precise sense of control, proprioception helps to compensate for the lack of haptic feedback in virtual-environment interaction.” <ref> Mine, M. R. (1997). Exploiting proprioception in virtual-environment interaction. PhD thesis, University of North Carolina</ref>

==References==