Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons located within

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

s, tendons, and joints. Most animals possess multiple subtypes of proprioceptors, which detect distinct kinematic parameters, such as joint position, movement, and load. Although all mobile animals possess proprioceptors, the structure of the sensory organs can vary across species. Proprioceptive signals are transmitted to the central nervous system, where they are integrated with information from other sensory systems, such as the visual system and the

vestibular system The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the purpose of coordinating movement with balance. Together with the cochlea, a part of the auditory system, it constitutes ...

, to create an overall representation of body position, movement, and acceleration. In many animals, sensory feedback from proprioceptors is essential for stabilizing body posture and coordinating body movement.

System overview

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.

Reflexes

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. * Non-conscious proprioception is communicated primarily via the dorsal spinocerebellar tract and ventral spinocerebellar tract, to the

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...

. * 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

, the part of the brain affecting balance.

Mechanisms

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 Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of m ...

Golgi tendon organ The Golgi tendon organ (GTO) (also called Golgi organ, tendon organ, neurotendinous organ or neurotendinous spindle) is a proprioceptor – a type of sensory receptor that senses changes in muscle tension. It lies at the interface between a mu ...

s, which lie at the interface of muscles and tendons, and joint receptors, which are low-threshold mechanoreceptors embedded in

joint capsule In anatomy, a joint capsule or articular capsule is an envelope surrounding a synovial joint.chordotonal neurons, campaniform sensilla, and hair plates. The initiation of proprioception is the activation of a proprioceptor in the periphery. The proprioceptive sense is believed to be composed of information from

sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The ...

s located in the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in t ...

(motion and orientation) and in the stretch receptors located in the

muscles Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

and the joint-supporting ligaments (stance). There are specific nerve receptors for this form of perception termed "proprioceptors", just as there are specific receptors for pressure, light, temperature, sound, and other sensory experiences. Proprioceptors are sometimes known as adequate stimuli receptors. Members of the transient receptor potential family of

ion channel Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of ...

s have been found to be important for proprioception in

fruit flies Fruit fly may refer to: Organisms * Drosophilidae, a family of small flies, including: ** ''Drosophila'', the genus of small fruit flies and vinegar flies ** ''Drosophila melanogaster'' or common fruit fly ** '' Drosophila suzukii'' or Asian fruit ...

, nematode worms, African clawed frogs, and

zebrafish The zebrafish (''Danio rerio'') is a freshwater fish belonging to the minnow family (Cyprinidae) of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often c ...

. PIEZO2, a nonselective cation channel, has been shown to underlie the mechanosensitivity of proprioceptors in mice. Humans with loss-of-function mutations in the PIEZO2 gene exhibit specific deficits in joint proprioception, as well as vibration and touch discrimination, suggesting that the PIEZO2 channel is essential for mechanosensitivity in some proprioceptors and low-threshold mechanoreceptors. 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.

Anatomy

Proprioception of the head stems from the muscles innervated by the

trigeminal nerve In neuroanatomy, the trigeminal nerve (literal translation, lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for Sense, sensation in the face and motor functions ...

, 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 The mesencephalic nucleus of trigeminal nerve is involved with reflex proprioception of the periodontium and of the muscles of mastication in the jaw that functions to prevent biting down hard enough to lose a tooth. To subserve this reflex protect ...

. Proprioception of limbs often occurs due to receptors in

connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tissue ...

near joints.

Function

Stability

An important role for proprioception is to allow an animal to stabilize itself against perturbations. For instance, for a person to walk or stand upright, they must continuously monitor their posture and adjust muscle activity as needed to provide balance. Similarly, when walking on unfamiliar terrain or even tripping, the person must adjust the output of their muscles quickly based on estimated limb position and velocity. Proprioceptor reflex circuits are thought to play an important role to allow fast and unconscious execution of these behaviors, To make control of these behaviors efficient, proprioceptors are also thought to regulate reciprocal inhibition in muscles, leading to agonist-antagonist muscle pairs.

Planning and refining movements

When planning complex movements such as reaching or grooming, animals must consider the current position and velocity of their limb and use it to adjust dynamics to target a final position. If the animal's estimate of their limb's initial position is wrong, this can lead to a deficiency in the movement. Furthermore, proprioception is crucial in refining the movement if it deviates from the trajectory.

Development

In adult fruit flies, each proprioceptor class arises from a specific cell lineage (i.e. each chordotonal neuron is from the chordotonal neuron lineage, although multiple lineages give rise to sensory bristles). After the last cell division, proprioceptors send out axons toward the central nervous system and are guided by hormonal gradients to reach stereotyped synapses. The mechanisms underlying axon guidance are similar across invertebrates and vertebrates. In mammals with longer gestation periods, muscle spindles are fully formed at birth. Muscle spindles continue to grow throughout post-natal development as muscles grow.

Mathematical models

Proprioceptors transfer the mechanical state of the body into patterns of neural activity. This transfer can be modeled mathematically, for example to better understand the internal workings of a proprioceptor or to provide more realistic feedback in neuromechanical simulations. Various proprioceptor models of complexity have been developed. They range from simple phenomenological models to complex structural models, in which the mathematical elements correspond to anatomical features of the proprioceptor. The focus has been on muscle spindles, but

s and insects hair plates have been modeled too.

Muscle spindles

Poppelle and Bowman used linear system theory to model mammalian muscle spindles Ia and II afferents. They obtained a set of de-afferented muscle spindles, measured their response to a series of sinusoidal and step function stretches, and fit a transfer function to the spike rate. They found that the following Laplace transfer function describes the firing rate responses of the primary sensory fibers for a change in length:

H(s) = K_1 \frac

The following equation describes the response of secondary sensory fibers:

H(s) = K_2 \frac

More recently, Blum et al. showed that the muscle spindle firing rate is modeled better as tracking the force of the muscle, rather than the length. Furthermore, muscle spindle firing rates show history dependence which cannot be modeled by a linear time-invariant system model.

Golgi tendon organs

Houk and Simon provided one of the first mathematical models of a Golgi tendon organ receptor, modeling the firing rate of the receptor as a function of the muscle tension force. Just as for muscle spindles, they find that, as the receptors respond linearly to sine waves of different frequencies and has little variance in response over time to the same stimulus, Golgi tendon organ receptors may be modeled as linear time-invariant systems. Specifically, they find that the firing rate of a Golgi tendon organ receptor may be modeled as a sum of 3 decaying exponentials:

r(t) = K + A\exp(-at) + B \exp(-b t) + C \exp(-c t) u(t)

where

r(t)

is the firing rate and

u(t)

is a step function of force. The corresponding Laplace transfer function for this system is:

H(s) = K\left(1 + \frac  + \frac + \frac \right)

For a soleus receptor, Houk and Simon obtain average values of K=57 pulses/sec/kg, A=0.31, a=0.22 sec⁻¹, B=0.4, b=2.17 sec⁻¹, C=2.5, c=36 sec⁻¹ . When modeling a stretch reflex, Lin and Crago improved upon this model by adding a logarithmic nonlinearity before the Houk and Simon model and a threshold nonlinearity after.

Impairment

Permanent

Proprioception is permanently impaired in patients with 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 as reported by Sacks. The catastrophic effect of major proprioceptive loss is reviewed by Robles-De-La-Torre (2006). Proprioception is also permanently impaired in physiological aging (presbypropria) and

autism spectrum disorder The autism spectrum, often referred to as just autism or in the context of a professional diagnosis autism spectrum disorder (ASD) or autism spectrum condition (ASC), is a neurodevelopmental condition (or conditions) characterized by difficulti ...

Parkinson's disease Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...

is characterized by a decline in 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. 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

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 Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrical ...

migraine Migraine (, ) is a common neurological disorder characterized by recurrent headaches. Typically, the associated headache affects one side of the head, is pulsating in nature, may be moderate to severe in intensity, and could last from a few ho ...

auras. These effects are presumed to arise from abnormal stimulation of the part of the parietal cortex of the

brain The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head ( cephalization), usually near organs for special ...

involved with integrating information from different parts of the body. Proprioceptive illusions can also be induced, such as the

Pinocchio illusion Pinocchio ( , ) is a fictional character and the protagonist of the children's novel ''The Adventures of Pinocchio'' (1883) by Italian writer Carlo Collodi of Florence, Tuscany. Pinocchio was carved by a woodcarver named Geppetto in a Tuscan vil ...

. Temporary impairment of proprioception has also been known to occur from an overdose of

vitamin B6 Vitamin B6 is one of the B vitamins, and thus an essential nutrient. The term refers to a group of six chemically similar compounds, i.e., "vitamers", which can be interconverted in biological systems. Its active form, pyridoxal 5′-phosphat ...

(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 Cytotoxicity is the quality of being toxic to cells. Examples of toxic agents are an immune cell or some types of venom, e.g. from the puff adder (''Bitis arietans'') or brown recluse spider (''Loxosceles reclusa''). Cell physiology Treating ...

factors such as

chemotherapy Chemotherapy (often abbreviated to chemo and sometimes CTX or CTx) is a type of cancer treatment that uses one or more anti-cancer drugs (chemotherapeutic agents or alkylating agents) as part of a standardized chemotherapy regimen. Chemothe ...

. It has been proposed that even common

tinnitus Tinnitus is the perception of sound when no corresponding external sound is present. Nearly everyone experiences a faint "normal tinnitus" in a completely quiet room; but it is of concern only if it is bothersome, interferes with normal hearin ...

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. 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 Bodybuilding is the use of progressive resistance exercise to control and develop one's muscles (muscle building) by muscle hypertrophy for aesthetic purposes. It is distinct from similar activities such as powerlifting because it focuses ...

anabolic steroid Anabolic steroids, also known more properly as anabolic–androgenic steroids (AAS), are steroidal androgens that include natural androgens like testosterone as well as synthetic androgens that are structurally related and have similar effects t ...

s, catabolisis/

starvation Starvation is a severe deficiency in caloric energy intake, below the level needed to maintain an organism's life. It is the most extreme form of malnutrition. In humans, prolonged starvation can cause permanent organ damage and eventually, dea ...

). It can also occur in those that gain new levels of flexibility,

stretching Stretching is a form of physical exercise in which a specific muscle or tendon (or muscle group) is deliberately flexed or stretched in order to improve the muscle's felt elasticity and achieve comfortable muscle tone. The result is a feelin ...

, 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.

Diagnosis

Impaired proprioception may be diagnosed through a series of tests, each focusing on a different functional aspect of proprioception. The

Romberg's test Romberg's test, Romberg's sign, or the Romberg maneuver is a test used in an exam of neurological function for balance, and also as a test for driving under the influence of an intoxicant. The exam is based on the premise that a person requires ...

is often used to assess balance. The subject must stand with feet together and eyes closed without support for 30 seconds. If the subject loses balance and falls, it is an indicator for impaired proprioception. For evaluating proprioception's contribution to motor control, a common protocol is joint position matching. The patient is blindfolded while a joint is moved to a specific angle for a given period of time and then returned to neutral. The subject is then asked to move the joint back to the specified angle. Recent investigations have shown that hand dominance, participant age, active versus passive matching, and presentation time of the angle can all affect performance on joint position matching tasks. For passive sensing of joint angles, recent studies have found that experiments to probe psychophysical thresholds produce more precise estimates of proprioceptive discrimination than the joint position matching task. In these experiments, the subject holds on to an object (such as an armrest) that moves and stops at different positions. The subject must discriminate whether one position is closer to the body than another. From the subject's choices, the tester may determine the subject's discrimination thresholds. Proprioception is tested by American

police officer A police officer (also called a policeman and, less commonly, a policewoman) is a warranted law employee of a police force. In most countries, "police officer" is a generic term not specifying a particular rank. In some, the use of the ...

s using the field sobriety testing to check for

alcohol intoxication Alcohol intoxication, also known as alcohol poisoning, commonly described as drunkenness or inebriation, is the negative behavior and physical effects caused by a recent consumption of alcohol. In addition to the toxicity of ethanol, the main ...

. The subject is required to touch his or her nose with eyes closed; people with normal proprioception may make an error of no more than , while people with impaired proprioception (a symptom of moderate to severe alcohol intoxication) fail this test due to difficulty locating their limbs in space relative to their noses.

Training

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 Touch typing (also called blind typing, or touch keyboarding) is a style of typing. Although the phrase refers to typing without using the sense of sight to find the keys—specifically, a touch typist will know their location on the keyboard thro ...

or perform ballet; and people would not even be able to walk without watching where they put their feet.

Oliver Sacks Oliver Wolf Sacks, (9 July 1933 – 30 August 2015) was a British neurologist, naturalist, historian of science, and writer. Born in Britain, Sacks received his medical degree in 1958 from The Queen's College, Oxford, before moving to the Uni ...

reported the case of a young woman who lost her proprioception due to a viral infection of her

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spin ...

. 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

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.

Juggling Juggling is a physical skill, performed by a juggler, involving the manipulation of objects for recreation, entertainment, art or sport. The most recognizable form of juggling is toss juggling. Juggling can be the manipulation of one object ...

trains reaction time, spatial location, and efficient movement. Standing on a wobble board or balance board is often used to retrain or increase proprioceptive abilities, particularly as

physical therapy Physical therapy (PT), also known as physiotherapy, is one of the allied health professions. It is provided by physical therapists who promote, maintain, or restore health through physical examination, diagnosis, management, prognosis, patie ...

for ankle or knee injuries.

Slacklining Slacklining refers to the act of walking, running or balancing along a suspended length of flat webbing that is tensioned between two anchors. Slacklining is similar to slack rope walking and tightrope walking. Slacklines differ from tightw ...

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 Yoga (; sa, योग, lit=yoke' or 'union ) is a group of physical, mental, and spiritual practices or disciplines which originated in ancient India and aim to control (yoke) and still the mind, recognizing a detached witness-conscio ...

Wing Chun Wing Chun (Chinese: 詠春 or 咏春, lit. "singing spring"), sometimes spelled Ving Tsun, is a concept-based fighting art, form of Southern Chinese kung fu and close-quarters system of self-defense. In Mandarin, it is pronounced "Yong Chun. ...

and tai chi. The vestibular system of the inner ear, vision and proprioception are the main three requirements for balance. Moreover, there are specific devices designed for proprioception training, such as the