Synapsis I (Physiology of Synapses)

📚 Synapse Physiology Notes

🖇 Definition & Basic Structure

Synapse = area of contact between two neurons

Components:

• Presynaptic neuron (before the cleft)
• Postsynaptic neuron (after the cleft)
• Synaptic cleft (gap between them)

Key Features:

• Unidirectional transmission (pre → post only)
• Only site in CNS where conduction can be modified
• Basis for drug action (hypnotics, stimulants)

📌 Types of Synapses

Physiological Classification:

1. Chemical Synapse (most common in CNS)
2. • Uses neurotransmitters (NTs)
   • Released from vesicles
2. Electrical Synapse
3. • Direct transmission via gap junctions
   • No chemical mediators
3. Conjunctive (Electro-chemical)
4. • Combination of both types

Histological Classification:

• Axo-dendritic: axon → dendrites
• Axo-somatic: axon → cell body
• Axo-axonic: axon → axon (fastest conduction)

🚨 Chemical Synaptic Transmission (3 Steps)

Step 1: NT Release

• Action potential arrives → Ca²⁺ channels open
• Ca²⁺ influx removes Synapsin protein
• Vesicles fuse with membrane → exocytosis

Step 2: NT Action on Postsynaptic Membrane

• NTs bind to specific receptors
• Ionotropic receptors: direct ion channel opening
• Metabotropic receptors: slower via G-proteins

Step 3: Postsynaptic Potentials

EPSP (Excitatory):

• Depolarization
• Na⁺ or Ca²⁺ influx OR prevent K⁺ efflux
• Increases internal positivity

IPSP (Inhibitory):

• Hyperpolarization
• Cl⁻ influx OR K⁺ efflux
• Increases internal negativity
• Key NTs: GABA, Glycine

🟠 Summation

Why needed? Individual potentials are localized & insufficient

Types:

1. Temporal Summation
2. • Same neuron, rapid succession (<15 ms)
2. Spatial Summation
3. • Multiple neurons simultaneously

Final response = balance of excitatory vs inhibitory inputs

🟣 Synaptic Inhibition

Presynaptic Inhibition ⭐ (High-yield topic)

Mechanism:

• Inhibitory interneuron acts on presynaptic terminal
• Releases GABA/Glycine
• Prevents Ca²⁺ channel opening
• No Ca²⁺ → no NT release

Examples:

• Pain control (stress analgesia)
• Lateral inhibition (prioritizing stronger stimuli)

📌 Signal Termination

4 Mechanisms:

1. Remove Ca²⁺ trigger
2. Enzymatic degradation (e.g., Acetylcholinesterase)
3. Active reuptake into presynaptic terminal
4. Diffusion away from cleft

🚨 Doctor's High-Yield Questions

1. What is a synapse?
2. Define chemical vs electrical synapse
3. Significance of synapse in CNS (unidirectional + modifiable)
4. Role of Ca²⁺ in NT release
5. How does depolarization occur? (Na⁺/Ca²⁺ in, K⁺ prevention)
6. How does hyperpolarization occur? (Cl⁻ in, K⁺ out)
7. Types of postsynaptic potentials (EPSP vs IPSP)
8. Why summation needed? (localized, insufficient potentials)
9. Types of summation (temporal vs spatial)
10. ⭐ Explain Presynaptic Inhibition (guaranteed exam question)
11. Signal termination mechanismsخاىونتات