Synapsis II (properties of synapsis)

🖇 Synaptic Transmission – Properties & Plasticity

📌 Basic Properties

• Uni-directional Conduction:
• من Pre → Post
• (بسبب وجود الـ NT Vesicles في Pre والـ Receptors في Post).
• Synaptic Delay: = 0.5 ms ⏱ (Ca++ entry → NT release → diffusion → receptor binding → PSP → AP).
• Synaptic Fatigue: ↓ تدريجي في الاستجابة مع تكرار التنبيه.
• • 🔹 أسباب: NT depletion – metabolites accumulation – abnormal ion distribution.
  • 🔹 أهمية: Protective (terminates convulsions).
• Synaptic After-discharge: استمرار الاستجابة بعد توقف المنبه.
• • 🔹 بسبب NT طويل المفعول (Substance P) أو بطء تعطيل NT.

📌 Synaptic Plasticity (Modifiability)

🟠 Potentiation (↑ Response)

• Short-Term Potentiation (STP):
• • Repeated low-frequency stim.
  • Lasts ثوانٍ–دقائق.
  • ↑ Pre-synaptic Ca++.
  • Example: Short-term memory.
• Long-Term Potentiation (LTP):
• • Repeated high-frequency stim.
  • Lasts أيام–شهور.
  • 🔹 ↑ Post-synaptic Ca++ via Glutamate → AMPA depolarization → remove Mg++ block → Ca++ entry.
  • Effects: ↑ AMPA receptors, ↑ dendritic spines, release of NO (retrograde messenger).
  • Example: Long-term memory.

🟣 Depression (↓ Response)

• Habituation: ↓ response to harmless repeated stimulus (e.g. background noise).
• • Mechanism: ↓ Ca++ entry (pre-synaptic).
• Long-Term Depression (LTD):
• • Weak, slow stim.
  • ↓ Post-synaptic Ca++ → AMPA receptor removal (endocytosis).
  • Example: Removal of old memories.

🚨 Sensitization

• ↑ Response when painful stimulus added with habituation.
• Mechanism = Pre-synaptic facilitation:
• • Pain neuron releases Serotonin → activates cAMP → blocks K+ channels → prolonged depolarization → ↑ Ca++ influx → ↑ NT release.

📌 Factors Affecting Transmission

• pH:
• • Acidosis (↓ pH) → depression → coma at pH 7.
  • Alkalosis (↑ pH) → facilitation → convulsions at pH 7.8.
  • Mechanism: Ca++ level changes affect Na+ depolarization.
• Hypoxia: 3–5 sec → unconsciousness.
• Drugs:
• • Caffeine/Theophylline → ↑ transmission.
  • Strychnine/Tetanus toxin → block Glycine (inhibition ↓ → excitability ↑).
  • Tranquilizers/Anesthetics → ↑ GABA (inhibition ↑).

📌 Key Questions (Important Points)

• Only monosynaptic reflex: Stretch reflex → balance/equilibrium.
• Calculation of synapses: Central delay ÷ 0.5 ms → # synapses; neurons = synapses + 1.
• Fatigue: 3 causes + protective role.
• Potentiation: Distinguish STP (Pre Ca++) vs LTP (Post Ca++).
• LTP details: Glutamate → AMPA → Mg++ block removal → Ca++ entry → NO retrograde messenger.
• Sensitization: Serotonin → block K+ channels → ↑ depolarization & Ca++.
• pH effects: Acidosis → coma; Alkalosis → convulsions.