Kinesthetic OrgansThe 6thsense• Proprioception: sense of position and movement of ‘self’ 6thsense (forgotten in the count of the senses) Information about position and movements of our limbs (muscle spindle; Golgi tendon organ; joint receptors) and relative to the environment (vestibular system)• Proprioception can lead to Kinesthesis= conscious sense of position and movementProprioception: The 6th senseOverview• What is the function of : muscle spindles? Golgi tendon organs? joint receptors?A simple reflex circuit, the knee-jerk responseFunction of gamma motor neuronsIntrafusal fibersIa – primaryendingsII – secondaryendingsIa and II afferentsAxons of Gamma motor neuronsAxon of alpha motor neuronextrafusalfibersNuclear chain fiberSpindle capsuleNuclear bag fiberMuscles = extrafusal fibers + muscle spindles(Top) Structure of a muscle spindle fibre. (Bottom) Structure of a nuclear bag fibre and nuclear chain fibre.II afferent fibers: respond to change in muscle length (static)Ia afferent fibers: respond to rate of change of length (dynamic) AND muscle length (static)Dynamic response is proportional to muscle lengthening velocityOverview• What is the function of : muscle spindles? Golgi tendon organs? joint receptors?Golgi tendon organ (GTO)• Tension through the muscle tendon junction will lead to membrane depolarization of receptor and AP in axon• Afferent fibre type is IbGolgi Tendon OrganUnder what conditions does the Ib afferent fire? Muscle stretch?Muscle contraction? (alpha MN, gamma MN)•Stretch will cause some firing•Contraction (generating force) causes greater activation•Greater force the greater the activationNegative feedback regulation of muscle tension by Golgi tendon organsGTO:•Maintains steady force level (fatigue)•Inhibits large forcesSchematic of the mechanisms of action of two kinds of proprioceptors in voluntary muscles:• muscle spindles that respond to muscle length (X) and rate of change of length• Golgi tendon organs that signal muscle tension (T) or forceOverview• What is the function of : muscle spindles? Golgi tendon organs? joint receptors?Joint receptors Range of receptors associated with joint (in capsule, with ligament or tendon, on fat pads) Depending on location specialized receptors respond to stimuli reflecting changes in joint movement or loads Stretch responses from ligaments/tendons Load/pressure measures Extremes/pain Mechanical stimulus leads to change in ion conductanceJoint receptors• Originally thought to only code limits of joint angle• Evidence now that specific receptors are sensitive to specific ranges (including mid-range)• Individual receptors have responses to specific range of joint angles• Therefore joint angle may be coded by combinations of receptorsJoint receptorsEvidence that muscle receptors work: tendon vibration
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