Scientists have discovered the part of the brain that tracks the position of our limbs as we move through space.
When a mosquito lands on your hand, you can rapidly and effortlessly make a movement of the other hand to brush it away, even in darkness. But performing this seemingly simple action involves a surprisingly complex coordination of different types of sensory information in order for your brain to construct a constantly updated 'map' of the body in space.
Now, scientists from UCL [University College London] and Barcelona [Pompeu Fabra University, ICREA and University of Barcelona] have identified an area of the human brain called the parietal cortex that constructs this body model from the combination of tactile information from your skin [for example, where the mosquito is on your hand] with "proprioceptive" information about the position of your hand relative to your body.
In an experiment they found that impairing the parietal cortex, using a brief pulse of magnetic stimulation, significantly impaired volunteers' judgements about the spatial relationship between their face and arms, but not their perception of touch or location alone. The research is published in the journal Current Biology, and was funded by the Biotechnology and Biological Sciences Research Council [BBSRC].
Professor Patrick Haggard, UCL Institute of Cognitive Neuroscience, who led the UCL team, said "Our brain constantly keeps track of the movements of the limbs, so that we always know the posture of our body, even with our eyes closed. Our results show, for the first time, how the brain updates this 'body space'.
"Our findings may be particularly relevant to children with developmental coordination disorder: these children have difficulty in coordinating their movements, but recent evidence suggests that one underlying problem is their poor sense of where their limbs are in space. Our result identifies the specific part of the parietal cortex needed to construct this map of body space."
In the experiment volunteers' forearms were placed in a sling which could be raised and lowered. Researchers then applied a brief tap at one of many locations on the forearm of volunteers, shortly followed by a brief tap at one of many locations on the face. Participants were asked to judge if the location of the arm tap was above or below the face tap, a judgement that could only be done by combining information about the tap location on the skin, and about the position of the arm relative to the body.
Researchers then disrupted the activity of the posterior parietal cortex [PPC] in the right hemisphere of the brain by delivering a brief pulse of magnetic stimulation after the arm tap and before the face tap. The stimulation was delivered through a coil placed on the scalp just over the location of the posterior parietal cortex indicated on a brain scan taken of each participant.
The stimulation significantly impaired volunteers' judgements about the spatial relation between the arm tap and the face tap. Crucially, when the volunteers just judged arm position alone, or touch location alone, the same stimulation had no effect. The scientists concluded that the posterior parietal cortex is the key brain area that combines touch and limb position, to produce a map of where the tap is in egocentric space.
The proprioception is the sense of positioning and movement. It is mediate by proprioceptors, a small subset of mechanosensory neurons localized in the dorsal root ganglia that convey information about the stretch and tension of muscles, tendons, and joints. These neurons supply of afferent innervation to specialized sensory organs in muscles [muscle spindles] and tendons [Golgi tendon organs]. Thereafter, the information originated in the proprioceptors travels throughout two main nerve pathways reaching the central nervous system at the level of the spinal cord and the cerebellum [unconscious] and the cerebral cortex [conscious] for processing. On the other hand, since the stimuli for proprioceptors are mechanical [stretch, tension] proprioception can be regarded as a modality of mechanosensitivity and the putative mechanotransducers proprioceptors begins to be known now. The mechanogated ion channels acid-sensing ion channel 2 [ASIC2], transient receptor potential vanilloid 4 [TRPV4] and PIEZO2 are among candidates. Impairment or poor proprioception is proper of aging and some neurological diseases. Future research should focus on treating these defects. This chapter intends provide a comprehensive update an overview of the anatomical, structural and molecular basis of proprioception as well as of the main causes of proprioception impairment, including aging, and possible treatments.
Proprioception [sense of body positioning in space] is an important bodily neuromuscular sense. It falls under our "sixth sense", more commonly known as somatosensation. The term somatosensation [or somatosensory senses] is an all encompassing term which includes the sub-categories of mechanoreception [vibration, pressure, discriminatory touch], thermoreception [temperature], nociception [pain], equilibrioception [balance] and proprioception [sense of positioning and movement].[1] The feedback from all these different sensory components arise from our peripheral nervous system [PNS], and feed information to our central nervous system [CNS], both at the level of the spinal cord [reflexive] and sent to the cerebral cortex for higher processing.[2]
Proprioception itself can be understood as including various sub-modalities:
Proprioception [Joint Position Sense]: Proprioception is our sense of joint / limb positioning. It is often measured through joint position sense - active joint position sense [AJPS] and passive joint position sense [PJPS]. Joint position sense determines the ability of a person to perceive a presented joint angle and then, after the limb has been moved, to actively or passively reproduces the same joint angle[3] [Clinically measured as a joint matching task].
Kinaesthesia: Kinaesthesia [kinaesthesis] is the awareness of motion of the human body [motion sense].[4] Sense of movement refers to the ability to appreciate joint movement, including the duration, direction, amplitude, speed, acceleration and timing of movements.[3]
Sense of Force: Sense of Force [SoF] is also known as sense of effort / heaviness / tension or the force matching sense. It is the ability to reproduce [or match] a desired level of force one or more times. Sense of force is thought to stem from the afferent feedback of the Golgi Tendon Organs [GTOs] embedded within our tendons, the muscle spindles within our muscles and proprioceptions within our skin.[5]
Sense of Change in Velocity [SoV]: SoV is our ability to detect vibration, derived from oscillating objects placed against the skin.[6] It is believed to travel through the same type of large afferent nerve fibers [Aαβ] as proprioception.[7]
Somatosensation. Retrieved from: Ager, A.L., Borms, D., Deschepper, L., Dhooghe, R., Dijkhuis, J., Roy, J.S., & Cools, A.Proprioception and shoulder pain: A Systematic Review. J Hand Ther. 2019 Aug 31. pii: S0894-1130[19]30094-8. doi: 10.1016/j.jht.2019.06.002.
Globally, all sub-modalities of proprioception arise from the sum of neural inputs from the joint capsules, ligaments, muscles, tendons, and skin, in a multifaceted system, which influences behavior regulation and motor control of the body.[8]
Proprioception is critical for meaningful interactions with our surrounding environment. Proprioception helps with the planing of movements, sport performance, playing a musical instrument and ultimately helping us avoid an injury.
The neurological basis of proprioception comes primarily from sensory receptors [mechanoreceptors and proprioceptors] located in your skin, joints, and muscles [muscle spindles with a smaller component from tendon organ afferents, cutaneous receptors and minimal input from joint receptors]. These muscle afferents receptors allow for the identification of limb position and movement via neural signalling of a change in muscle, skin or joint stretch[9]. Hence, proprioception is basically a continuous loop of feedforward and feedback inputs between sensory receptors throughout your body and your nervous system.
A mechanoreceptor is a sensory receptor in your body that responds to mechanical changes of tissues. Different types of mechanoreceptors include
- Pacinian Corpuscles
- Meissner's Corpuscles
- Merkel's Discs
- Ruffini Corpuscles
- Golgi Tendon Organs [GTO]
- Free nerve endings
There are also mechanoreceptors within the hair and skin.
There are four types of mechanoreceptors found within ligamentous tissues. As all the types of mechanoreceptors are myelinated and rapidly transmit sensory information to the CNS.[10]
- Type I: [small] Low threshold, slow adapting in both static and dynamic settings;
- Type II: [medium] Low threshold, rapidly adapting in dynamic settings;
- Type III: [large] High threshold, slowly adapting in dynamic settings;
- Type IV: [very small] High threshold pain receptors that communicate injury.
Type II and Type III mechanoreceptors in particular are believed to be linked to one's sense of proprioception.[10]
The short video below gives a good insight into the complexities of proprioception.
[11]
Causes of Proprioception Impairment[edit | edit source]
Poor proprioception at a joint may result in the increased likelihood of an injury.[12] The reason for proprioception impairments are not clear at this time. A decreased sense of proprioception can be caused by localized tissue damage, the presence of edema [swelling] or competitive nociceptive inputs [presence off pain].
Proprioception can be affected by the following factors:
- Temporary impairment from a compromised state [for example the consumption of alcohol].
- Age-related changes also affect proprioception. The risk of proprioception loss increases as we age due to a combination of natural age-related changes to the nerves, joints, and muscles.
- Injuries or medical conditions that affect the neuromuscular system [muscles, nerves, and the cerebellum, CNS] which can cause long-term or permanent proprioception impairment.
Proprioception impairments have been noted among the following neurological conditions:
The measurement of proprioception is presently not well developed. Proprioception can only be confidently measured in a laboratory setting, using complex computer-interfaced equipment. There is presently a lack of valid, reliable and responsive tools and outcome measures to quantify proprioception deficits, in a clinical setting.[13]
If you suspect a proprioception deficit, focus on the following clinical aspects.
Subjective assessment should include questions regarding the following:
- Balance issues, such as having trouble standing on one foot or frequent falls while walking or sitting;
- Uncoordinated movement [ataxia], such as not being able to walk in a straight line, or difficulty reaching for an object;
- The avoidance of certain activities, such as climbing stairs or walking on uneven surfaces because of a fear of falling.
Objective assessment should include observation of the above and the points below:
- Overal coordination [Reaching tests, Star Excurtion Balance Test, Upper extremity coordination tests];
- Clumsiness, such as dropping or bumping into things;
- Poor postural control, such as slouching or having to place extra weight on a table for balance while sitting;
- Trouble recognizing the appropriate level of muscular strength for a task [for example: pressing on a pen too hard when writing or not being able to gauge the force needed to pick up an object].
There are a few clinical tests Physiotherapists can use to assess proprioception, depending on the body part being assessed. The include:
- Romberg test;
- Heel-shin. The patient is asked to touch the heel of one foot to the opposite knee and then to drag their heel in a straight line all the way down the front of their shin and back up again. In order to eliminate the effect of gravity in moving the heel down the shin, this test should always be done in the supine position.
- Ataxia. Best revealed if the examiner's finger is held at the extreme of the patient's reach, and if the examiner's finger is occasionally moved suddenly to a different location.
- Finger—nose—finger test. The patient is asked to alternately touch their nose and the examiner's finger as quickly as possible
- Distal proprioception test. The tester will move the joints of the hip, knee ankle and big toe up and down while you watch. You then ask the client to repeat the same movement with your eyes closed.
- A contralateral joint matching task. Asking the patient to match a demonstrated joint angle, and measuring the difference between the actual joint angle, and the reproduced joint angle [the difference representing the proprioception error].
An intact sense of proprioception is crucial to learning a new skill. During the learning of any new skill, [sport performance or an artistic activity, for example] it is usually necessary to become familiar with some proprioceptive tasks specific to that activity. Without the appropriate integration of proprioceptive input, an artist would not be able to brush paint onto a canvas without looking at the hand as it moved the brush over the canvas; it would be impossible to drive an automobile because a motorist would not be able to steer or use the foot pedals while looking at the road ahead. A person could not touch-type [typing without looking at the keys] or perform a ballet dance. The bottom line remains that our sense of proprioception is important to train and develop, as it allows us to interact with our environments without the dependence on visual feedback [for example, reaching for a cup on the top shelve, without looking at the cup].
Physiotherapy - Training Proprioception[edit | edit source]
No matter the underlying cause of a proprioceptive deficit, clinicians can rehabilitate patients with tasks and activities to improve motor skills, strength,balance and coordination. They can also help patients learn how to manage daily tasks [ADLs] while living with a proprioception dysfunction.
There is converging evidence that proprioceptive training can yield meaningful improvements in somatosensory and sensorimotor function.[14] Retraining of a somatosensory function includes any interventions that addresses the remediation of the somatosensory modalities. Intervention methods include:
- Education;
- Repetitive practice and feedback in detecting, localising, discriminating, or recognising different sensory stimuli, pressure, or objects; PRACTICE, PRACTICE, PRACTICE!
- Proprioceptive training;
- Balance training [unstable surfaces, unpredictable situations with an external stimuli];
- Dual task training [in the absence of visual feedback for example]
- Somatosensory stimulation,[15]
A 2019 review on sensory retraining of the leg following a stroke, concluded that interventions used for retraining leg somatosensory impairment significantly improved somatosensory function and balance, but not gait suggesting a specificity of training effect.[15]
A 2005 systematic review of the effect of proprioceptive and balance exercises on people with an injured or reconstructed anterior cruciate ligaments, reported that proprioceptive and balance exercise improves outcomes in individuals with ACL-deficient knees.[16] Similarly a 2015 review on ankle sprains amongst a sporting population, concluded that proprioceptive training programmes are effective at reducing the rate of re-injury, particularly amongst those with a history of ankle sprain.[17]
The effectiveness of physiotherapy treatment on balance dysfunction and postural instability in persons with Parkinson’s disease: a systematic review and meta-analysis in 2016 reported that physiotherapy interventions like balance training combined with muscle strengthening, range of movement and walking training exercise is effective in improving balance in patients with Parkinson’s disease. As proprioception can also be improved with balance training, this could possibly advocate for proprioceptive retraining as well, amongst this population.[18]
Proprioception rehabilitation often include:[edit | edit source]
- 1. Balance exercises. Standing on a Balance board is often used to retrain or increase proprioception abilities, particularly as physical therapy for ankle or knee injuries. You can also perform weight bearing exercises on an unstable surface [such as a Bosu Ball or stability disc] for the upper extremities [such as push ups, or simply weight bearing on elbows or an outstretched arm position]. 2. Tai Chi, which improves lower limb proprioception and Yoga, which improves balance and muscle strength. The slow, focused movements of Tai Chi practice provide an environment whereby the proprioceptive information being fed back to the brain stimulates an intense, dynamic "listening environment" to further enhance mind / body integration.
3. somatosensory stimulation training, such as vibration therapy, different textures [cotton ball vs. velcro];