Reflex action & its properties

📚 Human Reflexes

🖇 مراجعة سريعة (CNS concepts)

• Sensitization: Persistent pain after burn → interneuron excites motor neuron.
• STP: Short memory (phone number) → ↑ Ca²⁺ in presynaptic neuron.
• LTP: Long memory → ↑ Ca²⁺ in postsynaptic neuron + ↑ AMPA receptors + ↑ Glutamate release.
• LTD: Forgetting unused memory → ↓ AMPA receptors (endocytosis).

📌 Reflex Basics

• Definition: Involuntary & rapid response to changes.
• Functional unit = Reflex action.
• Structural unit = Reflex arc.

🟠 Reflex Arc – Components (5)

1. Receptor → detects change.
2. Afferent neuron → to CNS (cell body in DRG).
3. Center → processes and decides.
4. Efferent neuron → delivers the command.
5. Effector → executes the response.

🟣 Classification of Reflexes

A. By Synapses

• Monosynaptic → only 1 synapse (Stretch reflex = only one in humans).
• Bisynaptic → 2 synapses + 1 interneuron.
• Polysynaptic → more than 1 interneuron.

B. By Center Location

• Peripheral reflexes:
  • Local axon reflex (Triple response flare).
  • Local enteric reflex (peristalsis).
  • Ganglionic reflex (Enterogastric).
• Central reflexes:
  • Spinal → spinal cord.
  • Medullary → BP, Respiration, GI.
  • Midbrain → Pupillary light, Accommodation.
  • Hypothalamic → Temperature, Feeding, Osmoregulation.

C. By Acquisition

• Conditioned (acquired) → requires Cortex.
• Unconditioned (inborn).

D. By Receptor Site

• Superficial (skin).
• Deep (muscle, joint).
• Visceral.

🚨 Properties of Reflex Action

• Plantar reflex: Normal = plantar flexion.
  • Babinski sign: dorsiflexion + fanning (in deep coma, anesthesia, infants <1 yr).
• Crossed extensor reflex: Withdrawal on one side + extension on the other (balance).
• Positive supporting reaction: Deep sole pressure → flexor + extensor contract (weight support).
• Localization: Site of stimulus decides response type.
• Nature of stimulus: Different responses at same site depending on type.
• Occlusion: 2 adjacent inputs → weaker response than sum.
• Facilitation: 2 distant inputs → stronger response (subliminal fringe).
• Irradiation: Stronger stimulus → wider + stronger response.

📌 Key Exam Questions

1. Burn + persistent pain → Sensitization (via interneuron).
2. Difference STP vs LTP → Ca²⁺ presynaptic vs postsynaptic.
3. Forgetting unused memories → LTD (↓ AMPA).
4. Only monosynaptic reflex? Stretch reflex.
5. Reflex arc components (5).
6. When is Babinski sign physiological? (Deep coma – Deep anesthesia – Infants <1 yr).