Proprioception, also referred to as kinaesthesia, is the sense of self-movement and body position. It is sometimes described as the sixth sense.
Proprioception is mediated by proprioceptors, mechanosensory neurons located within muscles, tendons, and joints. There are multiple types of proprioceptors which are activated during distinct behaviors and encode distinct types of information: limb velocity and movement, load on a limb, and limb limits. Vertebrates and invertebrates have distinct but similar modes of encoding this information.
The central nervous system integrates proprioception and other sensory systems, such as vision and the vestibular system, to create an overall representation of body position, movement, and acceleration.
In vertebrates, limb velocity and movement (muscle length and the rate of change) are encoded by one group of sensory neurons (Type Ia sensory fiber) and another type encode static muscle length (Group II neurons). These two types of sensory neurons compose muscle spindles. There is a similar division of encoding in invertebrates; different subgroups of neurons of the Chordotonal organ encode limb position and velocity.
To determine the load on a limb, vertebrates use sensory neurons in the Golgi tendon organs: type Ib afferents. These proprioceptors are activated at given muscle forces, which indicate the resistance that muscle is experiencing. Similarly, invertebrates have a mechanism to determine limb load: the Campaniform sensilla. These proprioceptors are active when a limb experiences resistance.
A third role for proprioceptors is to determine when a joint is at a specific position. In vertebrates, this is accomplished by Ruffini endings and Pacinian corpuscles. These proprioceptors are activated when the joint is at a threshold, usually at the extremes of joint position. Invertebrates use hair plates to accomplish this; a row of bristles located along joints detect when the limb moves.
The sense of proprioception is ubiquitous across mobile animals and is essential for the motor coordination of the body. Proprioceptors can form reflex circuits with motor neurons to provide rapid feedback about body and limb position.
These mechanosensory circuits are important for flexibly maintaining posture and balance, especially during locomotion. For example, consider the stretch reflex, in which stretch across a muscle is detected by a sensory receptor (e.g., muscle spindle, chordotonal neurons), which activates a motor neuron to induce muscle contraction and oppose the stretch.
During locomotion, sensory neurons can reverse their activity when stretched, to promote rather than oppose movement.
Conscious And Non-conscious
In humans, a distinction is made between conscious proprioception and non-conscious proprioception.
Conscious proprioception is communicated by the dorsal column-medial lemniscus pathway to the cerebrum.
A non-conscious reaction is seen in the human proprioceptive reflex, or righting reflex — in the event that the body tilts in any direction, the person will cock their head back to level the eyes against the horizon.
This is seen even in infants as soon as they gain control of their neck muscles. This control comes from the cerebellum, the part of the brain affecting balance.
Mechanisms Of Proprioception
Proprioception is mediated by mechanically sensitive proprioceptor neurons distributed throughout an animal’s body. Most vertebrates possess three basic types of proprioceptors: muscle spindles, which are embedded in skeletal muscle fibers, Golgi tendon organs, which lie at the interface of muscles and tendons, and joint receptors, which are low-threshold mechanoreceptors embedded in joint capsules.
Many invertebrates, such as insects, also possess three basic proprioceptor types with analogous functional properties: chordotonal neurons, campaniform sensilla, and hair plates.
The initiation of proprioception is the activation of a proprioreceptor in the periphery. The proprioceptive sense is believed to be composed of information from sensory neurons located in the inner ear (motion and orientation) and in the stretch receptors located in the muscles and the joint-supporting ligaments (stance).
There are specific nerve receptors for this form of perception termed “proprioreceptors”, just as there are specific receptors for pressure, light, temperature, sound, and other sensory experiences. Proprioreceptors are sometimes known as adequate stimuli receptors.
TRPN, a member of the transient receptor potential family of ion channels, has been found to be responsible for proprioception in fruit flies, nematode worms, African clawed frogs, and zebrafish. PIEZO2, a nonselective cation channel, has been shown to underlie the mechanosensitivity of proprioceptors in mice. The channel mediating human proprioceptive mechanosensation has yet to be discovered.
Proprioception of the head stems from the muscles innervated by the trigeminal nerve, where the GSA fibers pass without synapsing in the trigeminal ganglion (first-order sensory neuron), reaching the mesencephalic tract and the mesencephalic nucleus of trigeminal nerve.
Although it was known that finger kinesthesia relies on skin sensation, recent research has found that kinesthesia-based haptic perception relies strongly on the forces experienced during touch. This research allows the creation of “virtual”, illusory haptic shapes with different perceived qualities.
Temporary loss or impairment of proprioception may happen periodically during growth, mostly during adolescence. Growth that might also influence this would be large increases or drops in bodyweight/size due to fluctuations of fat (liposuction, rapid fat loss or gain) and/or muscle content (bodybuilding, anabolic steroids, catabolisis/starvation).
It can also occur in those that gain new levels of flexibility, stretching, and contortion. A limb’s being in a new range of motion never experienced (or at least, not for a long time since youth perhaps) can disrupt one’s sense of location of that limb.
Possible experiences include suddenly feeling that feet or legs are missing from one’s mental self-image; needing to look down at one’s limbs to be sure they are still there; and falling down while walking, especially when attention is focused upon something other than the act of walking.
Proprioception is occasionally impaired spontaneously, especially when one is tired. Similar effects can be felt during the hypnagogic state of consciousness, during the onset of sleep. One’s body may feel too large or too small, or parts of the body may feel distorted in size.
Similar effects can sometimes occur during epilepsy or migraine auras. These effects are presumed to arise from abnormal stimulation of the part of the parietal cortex of the brain involved with integrating information from different parts of the body.
Proprioceptive illusions can also be induced, such as the Pinocchio illusion.
The proprioceptive sense is often unnoticed because humans will adapt to a continuously present stimulus; this is called habituation, desensitization, or adaptation. The effect is that proprioceptive sensory impressions disappear, just as a scent can disappear over time.
One practical advantage of this is that unnoticed actions or sensation continue in the background while an individual’s attention can move to another concern.
The Alexander Technique addresses these unconscious elements by bringing attention to them and practicing a new movement with focus on how it feels to move in the new way.
People who have a limb amputated may still have a confused sense of that limb’s existence on their body, known as phantom limb syndrome. Phantom sensations can occur as passive proprioceptive sensations of the limb’s presence, or more active sensations such as perceived movement, pressure, pain, itching, or temperature. There are a variety of theories concerning the etiology of phantom limb sensations and experience.
One is the concept of “proprioceptive memory”, which argues that the brain retains a memory of specific limb positions and that after amputation there is a conflict between the visual system, which actually sees that the limb is missing, and the memory system which remembers the limb as a functioning part of the body. Phantom sensations and phantom pain may also occur after the removal of body parts other than the limbs, such as after amputation of the breast, extraction of a tooth (phantom tooth pain), or removal of an eye (phantom eye syndrome).
Temporary impairment of proprioception has also been known to occur from an overdose of vitamin B6 (pyridoxine and pyridoxamine). Most of the impaired function returns to normal shortly after the amount of the vitamin in the body returns to a level that is closer to that of the physiological norm. Impairment can also be caused by cytotoxic factors such as chemotherapy.
It has been proposed that even common tinnitus and the attendant hearing frequency-gaps masked by the perceived sounds may cause erroneous proprioceptive information to the balance and comprehension centers of the brain, precipitating mild confusion.
Proprioception is permanently impaired in patients that suffer from joint hypermobility or Ehlers-Danlos syndrome (a genetic condition that results in weak connective tissue throughout the body). It can also be permanently impaired from viral infections. The catastrophic effect of major proprioceptive loss is reviewed by Robles-De-La-Torre (2006).
Proprioception is also permanently impaired in physiological aging (presbypropria).
Parkinson’s Disease is characterized by a decline motor function as a result of neurodegeneration. It is likely that some of the symptoms of Parkinson’s Disease are in part related to disrupted proprioception. Whether this symptom is caused by degeneration of proprioceptors in the periphery or disrupted signaling in the brain or spinal cord is an open question.
Proprioception is what allows someone to learn to walk in complete darkness without losing balance. During the learning of any new skill, sport, or art, 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 pedals while looking at the road ahead; a person could not touch type or perform ballet; and people would not even be able to walk without watching where they put their feet.
Oliver Sacks reported the case of a young woman who lost her proprioception due to a viral infection of her spinal cord. At first she could not move properly at all or even control her tone of voice (as voice modulation is primarily proprioceptive). Later she relearned by using her sight (watching her feet) and inner ear only for movement while using hearing to judge voice modulation.
She eventually acquired a stiff and slow movement and nearly normal speech, which is believed to be the best possible in the absence of this sense. She could not judge effort involved in picking up objects and would grip them painfully to be sure she did not drop them.
The proprioceptive sense can be sharpened through study of many disciplines. Examples are the Feldenkrais method and the Alexander Technique. Standing on a wobble board or balance board is often used to retrain or increase proprioception abilities, particularly as physical therapy for ankle or knee injuries. Slacklining is another method to increase proprioception.
Standing on one leg (stork standing) and various other body-position challenges are also used in such disciplines as yoga, Wing Chun and tai chi. Moreover, there are specific devices designed for proprioception training, such as the exercise ball, which works on balancing the abdominal and back muscles. Tuthill, John C.; Azim, Eiman (March 2018). Proprioception. Current Biology. 28 (5): R194–R203. doi:10.1016/j.cub.2018.01.064 6  Lundberg, Malmgren, & Schomburg (Nov 1978). Role of joint afferents in motor control exemplified by effects on reflex pathways from Ib afferents. The Journal of Physiology. 284: 327–343  Woo SH, Lukacs V, de-Nooij JC, Zaytseva D, Criddle CR, Francisco A, Jessell TM, Wilkinson KA, Patapounian A (2015). Piezo2 is the principal mechanotransduction channel for proprioception. Nature Neuroscience. 18 (12): 1756–1762. doi:10.1038/nn.4162  Robles-De-La-Torre G, Hayward V (2001). Force can overcome object geometry in the perception of shape through active touch. Nature. 412 (6845): 445–8. Bibcode:2001Natur.412..445R. doi:10.1038/35086588  Ehrsson HH, Kito T, Sadato N, Passingham RE, Naito E (2005) Neural Substrate of Body Size: Illusory Feeling of Shrinking of the Waist. PLoS Biol 3(12): e412. https://doi.org/10.1371/journal.pbio.0030412  Weeks, S.R.; Anderson-Barnes, V.C.; Tsao, J. (2010). Phantom limb pain: Theories and therapies. The Neurologist. 16 (5): 277–286. doi:10.1097/nrl.0b013e3181edf128  Sacks, O. The Man Who Mistook His Wife For A Hat: And Other Clinical Tales. Touchstone (April 2, 1998) ISBN: 978-0684853949