Proprioception

Proprioception (ˌproʊpri.oʊˈsɛpʃən,_-ə- ), also called kinaesthesia (or kinesthesia), 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. In vertebrates, limb movement and velocity (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 position, usually at the extremes of joint position.