Stretch Reflex Lecture Notes

Introduction

• Discussion on the stretch reflex, also termed the muscle spindle reflex.

Anatomy of Skeletal Muscle

• Two types of muscle fibers:
  • Extrafusal Muscle Fibers
    • Striated fibers connected to tendons.
    • Tendons connect muscle to bone.
    • Contraction pulls tendons, generating movement.
  • Intrafusal Muscle Fibers
    • Located inside a connective tissue capsule within extrafusal fibers.
    • Function as proprioceptors, providing spatial awareness of muscles, joints, tendons, and ligaments.
    • Comprise the muscle spindle.

Functions

• Extrafusal Fibers
  • Attach to tendons and are responsible for movement.
• Intrafusal Fibers (Muscle Spindle)
  • Act as proprioceptors, detecting muscle length and velocity of stretch.

Types of Intrafusal Fibers

• Nuclear Bag Fibers
  • Larger with centrally located nuclei.
  • Sensitive to both length and velocity of stretch.
  • Two types: dynamic and static.
• Nuclear Chain Fibers
  • Nuclei arranged in a linear chain.
  • Sensitive only to length of stretch.

Sensory and Motor Innervation

• Sensory Fibers
  • Type 1a fibers (Annulo-spiral endings):
    • Found on both nuclear bag and chain fibers.
    • Detect stretch and speed.
  • Type 2 fibers (Flower-spray endings):
    • Primarily on nuclear chain fibers.
    • Detect stretch.
• Motor Fibers
  • Gamma Motor Neurons
    • Innervate polar regions of intrafusal fibers.
    • Cause contraction, maintaining fiber tension and sensitivity.

Mechanism of Action

• Stretch-induced Activation
  • Stretching fibers activates sensory fibers (Type 1a and Type 2).
  • Mechanically gated ion channels open, allowing sodium influx.
  • Leads to action potential generation in sensory neurons.
• Role of Gamma Motor Neurons
  • They stimulate contraction of polar regions of muscle spindle.
  • Maintain tension on spindle during muscle shortening.

Stretch Reflex

• Example: Patellar Reflex
  • Tapping the patellar tendon stretches quadriceps.
  • Activates intrafusal fibers, triggering sensory fiber action potentials.
  • Muscle contraction results, preventing excessive stretch.
  • Monosynaptic Reflex: Direct sensory to motor neuron interaction.
  • Reciprocal Inhibition: Inhibition of antagonist muscles.

Alpha-Gamma Coactivation

• Alpha Motor Neurons:
  • Stimulate extrafusal fibers to contract.
• Gamma Motor Neurons:
  • Simultaneously cause intrafusal fiber contraction, maintaining spindle tension.
  • Ensures continuous sensory feedback, preserving proprioception.

Clinical Relevance

• Corticospinal Tract and Gamma Motor Neurons
  • Upper motor neurons modulate gamma neurons.
  • Lesions can lead to hypertonia and spasticity due to increased gamma neuron activity.

Conclusion

• Understanding the stretch reflex is essential for clinical diagnostics and neurological examinations.

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