Stretch reflex and Muscle tone and supraspinal control

🧠 Stretch Reflex (Myotatic Reflex) — Lecture Notes

🖇 I. Structure of Reflex Arc

• Reflex = automatic, involuntary response to stimulus.
• Reflex Arc components:
• • Receptor → Sensory neuron → Center (spinal cord) → Motor neuron → Effector (muscle).
• Main Receptor: 🎯 Muscle Spindle
• • Located between Extrafusal fibers (main muscle fibers).
  • Made of Intrafusal fibers:
  • • Nuclear Bag fibers (Dynamic + Static)
    • Nuclear Chain fibers

📌 II. Nerve Supply

• Afferent (Sensory):
  1️⃣ Type Ia (Annulospiral) → wrap around bag + chain fibers
  2️⃣ Type II (Flower-spray) → only chain fibers
• Efferent (Motor):
• • Alpha (Aα) → to extrafusal fibers (main contraction)
  • Gamma (Aγ) → to intrafusal fibers (spindle sensitivity)

🟠 III. Mechanism of Stretch Reflex

• Stimulus: muscle stretch → spindle stretched → Ia afferent to spinal cord
• Center: spinal cord (spinal reflex)
• Response: activation of Alpha motor neuron → muscle contracts
• Type:
• • Spinal Reflex (center in spinal cord)
  • Monosynaptic (one synapse only)

🟣 IV. Alpha–Gamma Co-activation

• When Alpha (Aα) contracts muscle → Gamma (Aγ) also activated
• Gamma causes contraction of intrafusal ends → keeps spindle sensitive even during contraction
• 🔁 Maintains spindle tension → continuous reflex control

🟠 V. Types of Stretch Reflex Responses

1. Static (Tonic) Response

• Continuous contraction (no adaptation)
• Involves Nuclear Chain + Static Bag fibers
• Uses both Ia + II afferents
• ⚙️ Function: Maintains Muscle Tone

2. Dynamic (Phasic) Response

• Quick, short contraction (adapts rapidly)
• Involves Dynamic Bag fibers
• ⚙️ Function: Responsible for Tendon Jerk Reflex

3. Negative Stretch Reflex (Inverse)

• Very strong tension → muscle relaxes instead of contracting
• Receptor: Golgi Tendon Organ (GTO)
• ⚙️ Function: Prevents muscle damage from heavy load (Autogenic Inhibition)

📌 VI. Functions of Stretch Reflex

1. Maintain posture & muscle tone (via Static Reflex)
2. Smooth voluntary movements (Damping function)
3. • Reflex balances voluntary movement to avoid jerks
3. Servo-assist mechanism
4. • Increases contraction when load increases → keeps position steady

🖇 VII. Supra-Spinal Control

Higher brain centers can inhibit or facilitate reflexes 👇

🔻 Inhibition (↓ Reflex):

• Motor cortex (Areas 4 & 6)
• Caudate nucleus
• Red nucleus
• Reticular formation (inhibitory part)

🔺 Facilitation (↑ Reflex):

• Cerebellum (Vestibulo-cerebellum)
• Globus pallidus
• Vestibular nucleus
• Reticular formation (facilitatory part)
• Inferior olivary nucleus

🚨 VIII. Key Questions (Exam Focus)

1. Mechanism of muscle tone → Static response
2. 3 main characteristics → Spinal, Monosynaptic, Tonic
3. Explain Alpha–Gamma co-activation
4. Compare Static vs Dynamic responses
5. Functions of the Stretch Reflex (3 main roles)
6. Supra-spinal centers → which facilitate vs inhibit
7. Types of reflexes → Static, Dynamic, Inverse