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Muscle Spindles/Golgi Tendon Organs

Topic updated on 09/20/16 6:36am

Overview
 
 
 
Snapshot
  • A 67-year-old male is seen by his physician for a health-maintenance examination. He is currently doing well, but is distressed about poor muscle movement of his right arm. Approximately 6 months ago the patient was hospitalized for the management of an acute stroke. On physical examination, his right arm is hypertonic. When attempting to passively flex the arm, the physician noted resistance. Eventually, the joint rapidly closed. (Clasp-knife reflex - an exaggerated golgi-tendon reflex)
Introduction
  • Skeletal muscle contraction and relaxation plays an important role in maintaining posture and movement
  • Motor unit = single motor neuron + muscle fibers
    • motor neuron can innervate few to many muscle fibers
  • Two types of motor neurons:
    • α-motoneurons
      • innervates extrafusal muscle fibers (fibers that cause contraction)
    • γ-motoneurons
      • innervates intrafusal muscle fibers, a component of the muscle spindle
      •  adjusts muscle spindle sensitivity
    • α-motoneurons and γ-motoneurons co-activation
      • ensures the muscle spindle remains sensitive to changes in muscle length (eg, contraction)
 

Function
Physiology
  • Muscle spindle
  • Returns muscle to its resting length when it is either stretched or contracted
    • muscle spindle reflex
      • muscle stretched
        • extrafusal muscle lengthened
      • intrafusal muscle lengthens
        • it is in parallel with extrafusal muscle
      • activation of sensory fibers
      • co-activation of α-motoneurons and γ-motoneurons
      • contraction of extrafusal fibers, and maintains sensitivity of muscle spindle 
  • Are in parallel with extrafusal skeletal muscle
  • Innervated by sensory and motor neurons
    • sensory: group Ia and II afferent neurons
    • motor: γ-motoneurons
  • Golgi tendon
  • Senses contractile force
    • golgi tendon reflex
      • extrafusal muscle contraction
      • activation of golgi tendon organ
      • activation of group Ib afferent (sensory) nerves
      • activation of inhibitor interneurons
      • inhibits α-motoneurons 
      • muscle relaxes
        • synergistic muscle relax, antagonistic muscles contract
  • Are in series with the extrafusal skeletal muscle
    • mostly located between the extrafusal muscle and tendon
  • Innervated by group Ib afferent neurons



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