proprioception proprioception proprioception (/ˌproʊprioʊˈsɛpʃən, -priə-/ proh-pree-o-sep-shən), 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". schematics and images of limb proprioceptors in mammals and insects proprioception is mediated by proprioceptors, mechanosensory neurons located within muscles, 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, 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. 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 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 muscles, 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 proprioceptor 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 "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 channels have been found to be important 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. 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, 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. proprioception of limbs often occurs due to receptors in 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 golgi tendon organs 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 s ( s + 0.44 ) ( s + 11.3 ) ( s + 44 ) ( s + 0.04 ) ( s + 0.816 ) the following equation describes the response of secondary sensory fibers: h ( s ) = k 2 ( s + 0.44 ) ( s + 11.3 ) s + 0.816 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 [ 1 + a exp ⁡ ( − a t ) + 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 ( 1 + a s s + a + b s s + b + c s s + c ) ++\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 chronic proprioception, a sense vital for rapid and proper body coordination, can be permanently lost or impaired as a result of genetic conditions, disease, viral infections, and injuries. for instance, patients with joint hypermobility or ehlers-danlos syndrome, a genetic condition that results in weak connective tissue throughout the body, have chronic impairments to proprioception. moreover, proprioception may be chronically impaired in physiological aging (presbypropria), autism spectrum disorder, and parkinson's disease. in regards to parkinson's disease, it remains unclear whether the proprioceptive-related decline in motor function occurs due to disrupted proprioceptors in the periphery or signaling in the spinal cord or brain. in rare cases, viral infections result in a loss of proprioception. ian waterman and charles freed are two such people that lost their sense of proprioception from the neck down from supposed viral infections (i.e. gastric flu and a rare viral infection). after losing their sense of proprioception, ian and charles could move their lower body, but could not coordinate their movements. however, both individuals regained some control of their limbs and body by consciously planning their movements and relying solely on visual feedback. interestingly, both individuals can still sense pain and temperature, indicating that they specifically lost proprioceptive feedback, but not tactile and nociceptive feedback. the impact of losing the sense of proprioception on daily life is perfectly illustrated when ian waterman stated, "what is an active brain without mobility". proprioception is also permanently lost in people who lose a limb or body part through injury or amputation. after the removal of a limb, people may 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