Notes on Stretch Reflex Lecture

Introduction to Stretch Reflex

• Also known as muscle spindle reflex.
• Discussion of muscle spindles requires understanding skeletal muscle anatomy.

Anatomy of Skeletal Muscle

Types of Muscle Fibers

1. Extrafusal Muscle Fibers

   • Striated fibers (red color).
   • Attach to tendons, connecting muscles to bones.
   • Generate movement when they contract.

2. Intrafusal Muscle Fibers

   • Encased in connective tissue capsule.
   • Do not connect to tendons; act as proprioceptors.
   • Provide information about muscle positions and movements in three-dimensional space.
   • Types of intrafusal fibers:
     • Nuclear Bag Fibers: Larger and more sensitive to length and velocity of stretch.
     • Nuclear Chain Fibers: Sensitive primarily to length (degree of stretch).

Muscle Spindles

• Composed of a bundle of intrafusal fibers (muscle spindles).
• Function: detect stretch and maintain muscle tone.

Function of Muscle Fibers

• Extrafusal Fibers:
  • Connect to tendons and generate movement.
• Intrafusal Fibers:
  • Proprioceptors that inform about muscle position and stretch.
  • Detect:
    • Degree of stretch
    • Velocity of stretch

Types of Intrafusal Fibers

1. Nuclear Bag Fibers

   • Sensitivity: both length and velocity.
   • Nuclei are centrally located.

2. Nuclear Chain Fibers

   • Sensitivity: primarily length.
   • Nuclei arranged in a chain-like fashion.

Sensory Innervation of Intrafusal Fibers

• Type 1A Fibers (anulo spiral endings):
  • Wrapped around intrafusal fibers; detect degree and speed of stretch.
• Type 2 Fibers (flower spray endings):
  • Primarily found on nuclear chain fibers; less sensitive than type 1A.

Motor Innervation of Intrafusal Fibers

• Gamma Motor Neurons:
  • Supply intrafusal fibers.
  • Trigger contraction of intrafusal fibers, thus maintaining tension and sensitivity.

Mechanism of Stretch Reflex

• Stretching muscle activates sensory fibers (type 1A and type 2).
• Sensory fibers generate action potentials that travel to the spinal cord.
• Synapse either directly onto motor neurons (monosynaptic reflex) or via interneurons (polysynaptic).

Patellar Reflex Example

1. Tapping the patellar tendon stretches the quadriceps muscle.
2. Activates type 1A and type 2 sensory fibers.
3. Sensory information travels to the spinal cord's dorsal root ganglia.
4. Sensory neuron synapses:
   • Directly onto alpha motor neurons:
     • Stimulates quadriceps contraction.
   • On an interneuron:
     • Inhibits hamstring muscle contraction (reciprocal inhibition).

Alpha-Gamma Co-Activation

• Both alpha motor neurons (for extrafusal fibers) and gamma motor neurons (for intrafusal fibers) are activated during muscle movement.
• Ensures that both types of fibers contract, maintaining muscle spindle sensitivity and proprioception.

Clinical Relevance

• Damage to the corticospinal tract leads to upper motor neuron lesions, affecting gamma motor neuron activity, resulting in hypertonia and spasticity.

Conclusion

• Overview of the stretch reflex mechanism and its importance in maintaining muscle tone and preventing overstretch injury.