Muscle Spindle | Anatomy2Medicine
Muscle Spindle,NEET PG

Muscle Spindle

    • Muscle Fibers
      • Extrafusal fibers
        • innervated by a motoneurons (large cells in the ventral horn)
        • the most plentiful (MCQ)

 

  • provide the force for muscle contraction. (MCQ)

 

      • Intrafusal fibers
        • innervated by Gamma motoneurons (MCQ)
        • are encapsulated in sheaths to form muscle spindles
        • are too small to generate force for muscle contraction.
        • Nuclear bag fibers (MCQ)
          • intrafusal fibers innervated by group Ia afferents
          • that detect the rate of change in muscle length

 

  • have nuclei concentrated in a central bag-like region.

 

        • Nuclear chain fibers (MCQ)
          • intrafusal fibers innervated by group II afferents
          • detect static changes in muscle length
          • have nuclei arranged in rows.
      • Gamma motoneurons (MCQ)
        • adjust the sensitivity of the muscle spindle to provide appropriate responses during muscle contraction.
    • Function of Muscle Spindles
      • Muscle spindles act as sensory receptors in skeletal muscle stretch reflexes.
        • They detect both static and dynamic changes in muscle length.
        • The finer the movement required, the more muscle spindles a muscle contains.
      • Muscle spindle reflexes oppose increases in muscle stretching.
        • When muscle length is increased (stretched), the muscle spindle is stretched, stimulating afferent groups Ia and II. (MCQ)
        • Group Ia afferents stimulate  motoneurons in the spinal cord (MCQ)
          • cause muscle contraction and shortening of the muscle.
      • Both ends of the muscle spindle are connected in parallel with extrafusal fibers so that their length and rate of change in length can be monitored.
    • Muscle Reflexes
      • The muscle stretch (myotatic) reflex
        • is a stereotyped muscle contraction in response to a stretch of that muscle.
        • This reflex is the primary mechanism for regulating muscle tone (tension is present in all resting muscle).
        • Stretching of muscle spindles activates group Ia afferents that synapse with motoneurons in the spinal cord, causing muscle contraction.
        • The best example is the knee-jerk reflex (MCQ)

 

  • stimulated by tapping the patellar ligament

 

          • that stretches the quadriceps muscle
          • causing a sudden extension of the leg at the knee.
      • The flexor withdrawal reflex (MCQ)

 

  • is a protective
  • a usually painful stimulus causes withdrawal of a stimulated limb.

 

        • This reflex may be accompanied by a crossed extension reflex, in which
        • the contralateral limb is extended to support the body.
        • Flexor reflex afferent groups II, III, and IV synapse polysynaptically onto motoneurons in the spinal cord.

 

  • Because of the persistent neural activity in the polysynaptic circuits, an afterdischarge occurs that prevents muscle relaxation.

 

      • The inverse muscle stretch reflex
        • is associated with Golgi tendon organs arranged in series with extrafusal muscle fibers that detect muscle tension. (MCQ)
        • Golgi tendon organs respond to increases in force or tension generated by muscle contraction that increases the firing rate of group Ib afferent neurons. (MCQ)

 

  • Group Ib afferent neurons that innervate the Golgi tendon organs polysynaptically also facilitate antagonists and inhibit agonist muscles. (MCQ)

 

        • Muscle tone and reflex activity are influenced bygammamotoneurons that directly innervate muscle spindles and regulate their sensitivity to stretch.

 

  • Upper motoneurons innervate gamma motoneurons and influence the sensitivity of muscle spindles to stretch (MCQ)