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.