System Overview and Mechanisms
- Limb movement and velocity are encoded by type Ia sensory fibers in vertebrates.
- Static muscle length is encoded by group II neurons in vertebrates.
- Muscle spindles are composed of these two types of sensory neurons.
- In invertebrates, limb position and velocity are encoded by different subgroups of neurons in the Chordotonal organ.
- Vertebrates use Golgi tendon organs to determine limb load, while invertebrates use Campaniform sensilla.
- Proprioception is initiated by the activation of proprioceptors in the periphery.
- Proprioception involves information from sensory neurons in the inner ear and stretch receptors in muscles and joint-supporting ligaments.
- Most vertebrates have three basic types of proprioceptors: muscle spindles, Golgi tendon organs, and joint receptors.
- Many invertebrates also have three basic proprioceptor types: chordotonal neurons, campaniform sensilla, and hair plates.
- The transient receptor potential family of ion channels is important for proprioception in various animals.
- PIEZO2, a nonselective cation channel, is essential for mechanosensitivity in some proprioceptors and low-threshold mechanoreceptors.

Reflexes and Conscious/Nonconscious Proprioception
- Proprioception is essential for motor coordination and maintaining posture and balance.
- Proprioceptors can form reflex circuits with motor neurons to provide rapid feedback about body and limb position.
- The stretch reflex is an example of a reflex circuit where stretch across a muscle activates a motor neuron to induce muscle contraction.
- Sensory neurons can reverse their activity when stretched during locomotion to promote movement.
- Reflex circuits are important for flexibly maintaining posture and balance, especially during locomotion.
- Humans have conscious and nonconscious proprioception.
- Conscious proprioception is communicated to the cerebrum via the dorsal column-medial lemniscus pathway.
- Nonconscious proprioception is communicated to the cerebellum via the dorsal spinocerebellar and ventral spinocerebellar tracts.
- Nonconscious proprioception is involved in the proprioceptive reflex or righting reflex.
- The cerebellum plays a role in balance and control of neck muscles.

Anatomy and Impairment
- Proprioception of the head is mediated by muscles innervated by the trigeminal nerve.
- Proprioception of limbs often occurs due to receptors in connective tissue near joints.
- Specific nerve receptors called proprioceptors are responsible for proprioception.
- The impact of losing the sense of proprioception on daily life can be significant.
- People who lose a limb or body part through injury or amputation may experience permanent loss of proprioception.
- Proprioception can be permanently lost or impaired due to genetic conditions, diseases, viral infections, and injuries.
- Conditions such as joint hypermobility or Ehlers-Danlos syndromes can result in chronic impairments to proprioception.
- Physiological aging, autism spectrum disorder, and Parkinson's disease can also cause chronic impairments to proprioception.
- Proprioception can be impaired spontaneously, especially when one is tired.
- Temporary impairment of proprioception can occur from an overdose of vitamin B6 or cytotoxic factors such as chemotherapy.

Muscle Spindles and Golgi Tendon Organs
- 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 and measured their response to sinusoidal and step function stretches.
- They fit a transfer function to the spike rate and found that a specific Laplace transfer function describes the firing rate responses of the primary sensory fibers for a change in length.
- The firing rate responses of secondary sensory fibers are described by a different equation.
- Muscle spindle firing rates show history dependence which cannot be modeled by a linear time-invariant system model.
- Houk and Simon provided one of the first mathematical models of a Golgi tendon organ receptor.
- They modeled the firing rate of the receptor as a function of the muscle tension force.
- Golgi tendon organ receptors may be modeled as linear time-invariant systems.
- The firing rate of a Golgi tendon organ receptor may be modeled as a sum of 3 decaying exponentials.
- The corresponding Laplace transfer function for this system is derived.

Diagnosis, Training, and History
- Impaired proprioception can be diagnosed through various tests.
- The Rombergs test is often used to assess balance and can indicate impaired proprioception if the subject loses balance and falls.
- Joint position matching is a common protocol for evaluating proprioception's contribution to motor control.
- The patient is blindfolded and a joint is moved to a specific angle, testing their ability to match the position.
- Other tests may focus on different functional aspects of proprioception.
- Proprioception is tested by American police officers using field sobriety testing to check for alcohol intoxication.
- Field sobriety testing assesses proprioceptive abilities in individuals.
- Proprioception is essential for learning new skills, sports, or arts.
- Familiarity with proprioceptive tasks specific to the activity is necessary for skill development.
- Without appropriate proprioceptive integration, tasks such as painting, driving, typing, and ballet would be challenging.
- In 1557, Julius Caesar Scaliger described the position-movement sensation as a sense of locomotion.
- Charles Bell, in 1826, proposed the concept of muscle sense as one of the first descriptions of physiologic feedback mechanisms.
- Robert Todd, in 1847, highlighted differences in the anterolateral and posterior columns of the spinal cord and their involvement in movement coordination and balance.
- Moritz Heinrich Romberg described unsteadiness associated with proprioceptive disorders of the legs.
- The term 'proprioception' was introduced by Charles Scott Sherrington in 1906, referring to receptors that transmit mechanical data on joint capsule, tendon, and muscle tension.

Proprioception (Wikipedia)

Proprioception (/ˌproʊpri.oʊˈsɛpʃən, -ə-/ PROH-pree-oh-SEP-shən, -⁠ə-) is the sense of self-movement, force, and body position.

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.