Muscle Spindle and Stretch Reflex

Introduction

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• Focus: Muscle spindle and stretch reflex

Muscle Spindle

• Sensory structure distributed throughout muscle belly
• Sends information about muscle length and changes in length
• Length: 3 to 10 mm

Components

1. Intra-fusal muscle fibers
   • Modified muscle fibers
   • Ends attached to extrafusal fibers
   • Ends innervated by gamma motor neurons
   • Two types based on nuclei arrangement:
     • Nuclear bag fibers: 1-3 per spindle, nuclei concentrated in central portion
     • Nuclear chain fibers: 3-9 per spindle, nuclei aligned in chain
2. Nerve supply
   • Sensory innervation:
     • Primary endings: Type Ia fibers, encircles central portion of fibers
     • Secondary endings: Type II fibers, innervate sides of primary ending
   • Motor innervation:
     • Gamma motor neurons innervate contractile ends

Function of Muscle Spindle

• Main role in stretch reflex

Stretch Reflex

• Sudden muscle stretch -> Reflex contraction of the same muscle
• Components: Muscle spindle, sensory neuron, motor neuron, same muscle
• One synapse in the circuit, shortest reflex pathway
• Mechanism:
  • Muscle stretch -> central region of spindle stretches -> firing rate of sensory neurons increases
  • Type Ia fibers carry signal to spinal cord -> direct synapse with alpha motor neuron of the same muscle
  • Alpha motor neuron stimulates extrafusal fibers -> muscle contraction
  • Prevents further stretching, opposes sudden length change

Reciprocal Innervation

• Sensory neuron also innervates inhibitory interneuron
• Inhibitory interneuron inhibits motor neurons of antagonistic muscle
• Antagonistic muscle relaxes, no resistance to main muscle movement
• Increases effectiveness of stretch reflex

Summary

• Muscle spindle senses muscle length changes
• Made of intrafusal fibers (nuclear bag and chain fibers)
• Sensory innervation by type Ia and II fibers
• Motor innervation by gamma motor neurons
• Stretch reflex prevents sudden changes in muscle length

Final Notes

• Motor nerve supply to the muscle spindle to be covered in a future video
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