Stretch Reflex Lecture Notes

Introduction

• Focus on stretch reflex and muscle spindles.
• Understanding basic anatomy of skeletal muscle is crucial.

Anatomy of Skeletal Muscle

Types of Muscle Fibers

1. Extrafusal Muscle Fibers
   • Striated fibers (red color).
   • Connection: Attach to tendons, which connect muscle to bone.
   • Function: Generate movement when contracted.
2. Intrafusal Muscle Fibers
   • Located inside a connective tissue capsule.
   • Do not connect to tendons.
   • Function: Serve as proprioceptors to inform about the position of muscles, joints, etc.
   • Types of intrafusal fibers:
     • Nuclear Bag Fibers
     • Nuclear Chain Fibers
   • Muscle Spindles: Groups of intrafusal fibers.

Functions of Muscle Fibers

• Extrafusal Fibers:
  • Connect to tendons and generate movement.
• Intrafusal Fibers:
  • Proprioceptors that detect stretch (degree and velocity).

Types of Intrafusal Fibers

1. Nuclear Bag Fibers
   • Larger; nuclei centrally located.
   • Sensitive to both length (degree of stretch) and velocity of stretch.
2. Nuclear Chain Fibers
   • Nuclei arranged in a linear fashion.
   • Sensitive primarily to length.

Mechanism of Action in Stretch Reflex

Activation of Sensory Fibers

• Stretching the muscle activates sensory fibers (Type 1A and Type 2):
  • Type 1A Fibers: Annulo spiral endings; wrap around central portion.
  • Type 2 Fibers: Flower spray endings; primarily on nuclear chain fibers.
• Stretching activates mechanically gated ion channels, causing sodium influx and action potential generation.

Role of Gamma Motor Neurons

• Gamma Motor Neurons stimulate contraction of intrafusal fibers, maintaining sensitivity during muscle contractions.
• They prevent slackness of intrafusal fibers, ensuring continuous sensory feedback.

Reflex Pathway Overview

• Deep Tendon Reflexes: Involve muscle spindles rather than Golgi tendon organs.
• Example: Patellar Reflex
  • Tapping the patellar tendon stretches quadriceps muscles (extrafusal and intrafusal fibers).
  • Sensory impulses travel to spinal cord, synapsing directly onto motor neurons (monosynaptic reflex) or through interneurons (polysynaptic reflex).
  • Reciprocal Inhibition: Activation of quadriceps (agonist) while inhibiting hamstrings (antagonist).

Alpha-Gamma Co-activation

• Alpha Motor Neurons: Innervate extrafusal fibers, causing muscle contraction.
• Gamma Motor Neurons: Innervate intrafusal fibers to keep them taut and responsive.
• Both types of neurons allow for proper proprioceptive feedback during muscle contractions.

Clinical Relevance

• Damage to corticospinal tracts leads to increased gamma motor neuron activation, resulting in hypertonia and spasticity.

Conclusion

• Summary of key points on the stretch reflex, muscle spindles, and their clinical implications.
• Encouragement for audience engagement and support.