Stretch Reflex Lecture Notes

Introduction

• Topic: Stretch Reflex (aka Muscle Spindle Reflex)
• Focus: Muscle spindles and skeletal muscle anatomy

Skeletal Muscle Anatomy

Types of Muscle Fibers

1. Extrafusal Muscle Fibers
   • Striated and attach to tendons
   • Role: Connects to bones and generates movement
   • Contraction pulls on tendon → movement
2. Intrafusal Muscle Fibers
   • Found inside a connective tissue capsule
   • Role: Proprioceptors (sense muscle position, stretch, and speed)
   • Types: Nuclear bag fibers and nuclear chain fibers

Muscle Spindles

• Comprised of multiple intrafusal muscle fibers
• Functions as proprioceptors

Functions of Muscle Fibers

Extrafusal Muscle Fibers

• Connect to tendons
• Contract to generate movement

Intrafusal Muscle Fibers (Muscle Spindles)

• Detect muscle stretch (degree and velocity)
• Two main functions:
  1. Length of muscle (degree of stretch)
  2. Velocity of muscle stretch (speed)

Types of Intrafusal Fibers

1. Nuclear Bag Fibers
   • Larger with centrally located nuclei
   • Sensitive to both length and velocity of stretch
2. Nuclear Chain Fibers
   • Linear arrangement of nuclei
   • Sensitive primarily to length

Sensory and Motor Innervation

Sensory Nerve Fibers

• Type 1a (Annulospiral Endings)
  • Found on both nuclear bag and chain fibers
  • Pick up stretch and velocity
• Type 2 (Flower Spray Endings)
  • Found mainly on nuclear chain fibers
  • Pick up degree of stretch

Motor Innervation

• Gamma Motor Neurons
  • Supply to intrafusal fibers
  • Cause contraction at the polar ends to maintain sensitivity

Mechanism of Reflex Activation

Sensory Activation

• Stretch opens mechanically gated ion channels on sensory neurons
• Sodium influx → Action potential → Sensory fiber activation

Motor Neuron Pathway

• Gamma Motor Neurons stimulate polar ends of intrafusal fibers
• Release of acetylcholine → Sodium influx → Muscle contraction

Stretch Reflex Pathway

• Example: Patellar Reflex
  1. Tap on patellar tendon → Stretch of quadriceps muscle
  2. Activation of intrafusal fibers → Sensory impulses to spinal cord
  3. Sensory neurons synapse with motor neurons → Muscle contraction

Reflex Types

• Monosynaptic Reflex: Direct synapse between sensory and motor neurons
• Reciprocal Inhibition: Contraction of quadriceps; inhibition of hamstrings

Clinical Relevance

• Upper Motor Neuron Lesions: Damage to corticospinal tracts reduces inhibition on gamma motor neurons, leading to hypertonia and spasticity

Alpha-Gamma Coactivation

• Alpha Motor Neurons: Contract extrafusal fibers
• Gamma Motor Neurons: Contract intrafusal fibers to keep sensory fibers taut
• Importance: Ensures accurate sensory input during muscle movements

Summary

• Covered stretch reflex pathway, focusing on muscle spindles
• Key aspects: Anatomy, sensory and motor pathways, reflex mechanism, clinical significance
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