Proprioceptive system

Introduction

Body movement is an essential component of human life. In everyday activities, or competitive sports, the generation of precise and coordinated movement is essential to interact with the environment. This depends on signals coming from the body that allows it to respond to its surroundings and to react to changing circumstances . Besides the usual senses that are responsible for our interaction with the external world (sight, sound, smell, touch, and taste), there are other ones responsible for our internal functioning. The knowledge about position and movement of the limbs and trunk is provided by sensations arising in proprioceptors. This allows a person to maneuver around obstacles in the dark, or to manipulate objects out of view, for example. This internal sense, often referred to as the sixth sense, is called proprioception. It affects our lives continually, allowing the accomplishment of complex tasks that would be impossible without it. As an example to show the impact of the absence of proprioception, moving a finger would be extremely difficult. Without proprioception the brain cannot feel what the finger is doing, and the process of moving it would have to be carried in a more conscious and calculated way, compensating the loss of positional feedback with vision. Another example that shows the relevance of proprioception is driving. Drivers are able to keep their eyes on the road while adjusting their arms and hands on the wheel, and applying the correct amount of pressure to the pedals .

Proprioception is not bounded by visual cues. Even when vision is absent, there is a correct sense of limb position. At any time, a person still knows the position of the different body parts during a movement and has an accurate map of their position in space. Therefore, the proprioceptive system allows precise placements such as touching the tip of the nose with the eyes closed .

Perception “is the identification, organization, and interpretation of sensory information, in order for humans to internally represent and understand the environment .” Perceptions need signals within the nervous system. These derive from physical stimulation of the various sense organs. In the same way, proprioception requires the stimulation of mechanoreceptors via changes of body position. Specifically, proprioception pertains to the perception of body position and movements in a 3D space. The peripheral mechanoreceptors provide proprioceptive information to the brain, in order for it to integrate and use them. The physical receptors (e.g. skin, muscles, or joints) can be seen as the hardware component, and the central processing that analyses the signals, the software (1; 4; 6).

Recently, it was demonstrated that proprioception as a measure of neuromuscular response to a stimulus must involve sensory input, central processing, and motor output. Therefore, proprioception cannot be interpreted has only the afferent (hardware) part of the system, the cumulative neural input to the nervous system from the receptors located in muscles, joints and the skin. Although muscles spindles are considered the main receptors of proprioceptive information, there is a complex array of different sources and the importance of central processing in proprioception has been increasing in recent years .

A deficit in proprioception will lead to a loss of controlled movements without continuous visual feedback, a severe difficulty in maintaining force or position, and tremors could develop. It is an essential sense for the coordination of movement .

According to Proske et al. (2012), “the subject of proprioception lies at the boundary between neurophysiology and neuropsychology.” It can be considered a mysterious sense since we are largely unaware of it. In the absence of vision, the limbs positions is still known but there is no clearly defined sensation that can be identifiable. This can be explained by the predictability of proprioceptive signals. There is an awareness that a person is making a willed movement and so the sensory input that it generates is anticipated. In sensory physiology there is a concept that what we feel commonly represents the difference between what is expected and what actually occurs. Regarding proprioception, if there is no mismatch between the expected signals from a movement and those generated, there is no definable sensation, but the person still knows the location of their limbs precisely .

Besides the proprioceptive system, the vestibular system contributes to several conscious sensations as well as helping with movement and posture. Conscious sensations include the senses of limb position and movement, the sense of tension or force, the sense of effort, and the sense of balance. Kinaesthesia is a term that can be used to refer to sensations of limb position and movement. Conventionally, proprioception consists of four senses that consist of the conscious sensations described before, and it is the cumulative neural input to the central nervous system from various receptors that collect sensory information from the body .

The control of a movement is dependent on the quality of the afferent input originated from the various somatosensory systems, like the interoceptors (for the detection of a stimulus within the body) and mechanoreceptors (specialized nerve endings) that are involved in proprioception. These mechanoreceptors can be located in the joints, capsules, ligaments, muscles, tendons and skin .

Traditionally, muscle spindles have been responsible for providing the primary signals that contribute for the sense of limb position. There are several studies that used muscle tendon vibration manipulation (which stimulates preferentially muscle spindles afferent) that have produced illusions of joint position and motion. Recently, the sense of effort has been gaining ground regarding its role in joint position sense. Finally, as mentioned, beyond the peripheral aspects of proprioception, there is a significant central component to sensing body positions and movements. This was discovered through studies in the 1970s that assessed “the accuracy of reaching to proprioceptive targets that were either established through active movement of the subject or passive displacement by the experimenter” .

Proprioception and kinaesthesis

Both “proprioception” and “kinaesthesis” are terms that continue to be used in the scientific literature, sometimes with different interpretations according to the authors. Some researchers define proprioception as the sense of joint position only, while kinaesthesia as the conscious awareness of joint motion. Others consider kinaesthesia as one of the submodalities of proprioception. In this case, proprioception contains both joint position sense and the sensation of joint movement. This last definition is in accordance with the conceptualization of kinaesthesis that was originally coined by Bastian (1888): the ability to sense the position and movement of limbs and trunk. Dover and Powers (2003) include the joint position sense, kinaesthesia, and sense of tension or force as submodalities of proprioception. It has also been argued that “proprioception” and “kinaesthesis” can be synonymous .

The study of proprioception

Like any other field of knowledge, the study of proprioception has been evolving. This area has, traditionally, attracted vast interest due to the role played by proprioception in motor control. Besides this, a greater knowledge of the mechanisms of proprioception promises a better understanding of the human sensory experiences. Developments in neuroimaging, like magnetic resonance imaging (MRI), has allowed the study of the central activity patterns produced by proprioceptive inputs. This is leading to advancements in the understanding of how some proprioceptive sensations arise and how they are used to create a body image. Besides this, there is a field of study concerned with the interactions between proprioception, vision, and vestibular inputs .

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 .