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.