Proprioception

Proprioception (/ˌproʊprioʊˈsɛpʃən, -priə-/ PROH-pree-o-SEP-shən) , also referred to as kinaesthesia (or kinesthesia, in American English), is the sense of self-movement and body position. It is sometimes described as the 'sixth sense'. Proprioception (/ˌproʊprioʊˈsɛpʃən, -priə-/ PROH-pree-o-SEP-shən) , also referred to as kinaesthesia (or kinesthesia, in American English), is the sense of self-movement and body position. It is sometimes described as the 'sixth sense'. Proprioception occurs when proprioceptors, located in the periphery throughout the body, are activated. Proprioceptors are mechanosensory neurons within muscles, tendons, and joints. There are multiple types of proprioceptors which are activated during distinct behaviors and encode distinct types of information: limb velocity and movement, load on a limb, and limb limits. Vertebrates and invertebrates have distinct but similar modes of encoding this information. The central nervous system integrates proprioception and other sensory systems, such as vision and the vestibular system, to create an overall representation of body position, movement, and acceleration. More recently proprioception has also been described in flowering land plants (angiosperms). 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. 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.