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Proprioception is the sense that gives us our awareness of the relative positions of the different parts of our bodies. The system has sensors within joints, muscles and skin which relays information to the brain about joint angles and skin and muscle stretch. Its role is to act as the start point of any sequence of movement as without information about where we are now we could not format the instructions for moving to where we want to move to. Rather like asking a GPS system to guide you to London but you are unable to tell it where you are now... an impossible request. The loss of proprioception is devastating causing an inability to control limbs in space and therefore an inability to instruct them to move under any level of control.

Proprioception is a vital part of our own feeling of self, although we are not physically aware of “knowing “the position of our limbs etc. we are unwittingly always working at maintaining the relative positions of parts of our bodies. This is done by a combination of proprioceptor located in the joints, muscles and to a lesser ex tent in the skin. In the joints we have four different receptors, two in the joint capsule itself and two in the ligament. They record and integrate information about the joint angle, the speed of movement and warn of the limitation of movement to avoid damage to the joint. This information is supported by information from the muscles; Golgi tendon organs detect muscle contraction levels and muscle spindle receptors detect muscle stretch. Golgi tendon organs innervate the collagen fibres that lie in a capsule between the end of a muscle and the tendon that attaches to the joint. As muscle contract the amount of collagen being active increases and the neuron branches detect this.

Muscle spindles lie encased in the muscle sheath and consist of two bobbin like strictures which are separately innervated by free nerve endings. There are two types of afferent neurons here with varying actions.

|Type |Properties |role | |1a |Large diameter myelinated axons, fast axon |Detect initial stretch by increasing | | |potential conduction, |firing rate at onset of stretch and as | | | |stretch develops but then reducing the | | | |rate as stretch plateaus | |Group II |Smaller diameter myelinated axons, slower |Slower response to initial stretch but | | |conduction speeds |maintain an increased firing rate | | | |throughout entire period of stretching |

These receptors provide the brain with information about the amount and the duration of stretch. There are also motor afferents in the spindle which contribute to proprioception in adjusting the whole systems sensitivity. This system is the fusimotor system and also has two types of neurons.

|Type |role | |Gamma motor neurons |Act on the spindle inside which if left | | |unaided would become too long for the new | | |shortened length of the muscle. | |Alpha motor neurons |Controls the muscle itself – so firing | | |results in contraction of the muscle and | | |shortening length |

This tandem working of alpha and gamma motor neurons is called alpha-gamma co-activation and Prochazka and Hulliger found that the level of activity in the fusimotor system was tuned into the type of activity the body was engaged in. They determined there was both dynamic and static gamma fibres, dynamic...
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