Q6:

Give a brief account of Mechanism of synaptic transmission.

1. Theoretical Foundation of Synaptic Transmission

Synaptic transmission is the electrochemical process by which a neuron (the presynaptic cell) communicates with a target cell (the postsynaptic cell) across a microscopic gap known as the synaptic cleft. This process is governed by the conversion of an electrical signal (action potential) into a chemical signal (neurotransmitter release), and back into an electrical signal (postsynaptic potential).

2. Schematic Representation of the Synapse

3. Sequential Stages of Transmission

Step 1: Arrival of the Action Potential
The process initiates when an action potential reaches the axon terminal of the presynaptic neuron. [Per the All-or-None Law of nerve impulse propagation].

Step 2: Voltage-Gated Calcium Influx
The depolarization of the presynaptic membrane triggers the opening of voltage-gated $Ca^{2+}$ channels. Calcium ions flow down their electrochemical gradient from the extracellular fluid into the synaptic knob. [Ref: Nernst Equation for equilibrium potentials].

Step 3: Exocytosis of Neurotransmitters
The rise in intracellular $Ca^{2+}$ concentration promotes the docking and fusion of synaptic vesicles with the presynaptic membrane. Neurotransmitters (e.g., Acetylcholine, GABA) are released into the synaptic cleft via exocytosis.

Step 4: Diffusion and Binding
Neurotransmitters diffuse across the synaptic cleft (approx. 20-40 nm). Upon reaching the postsynaptic membrane, they bind to specific ligand-gated ion channels (receptors). [By Lock-and-Key hypothesis of receptor-ligand interaction].

Step 5: Postsynaptic Potential Generation
Binding causes the opening of ion channels, leading to either:

• Excitatory Postsynaptic Potential (EPSP): Influx of $Na^+$ ions, leading to depolarization.
• Inhibitory Postsynaptic Potential (IPSP): Influx of $Cl^-$ or efflux of $K^+$ ions, leading to hyperpolarization.

4. Comparison of Synaptic Types

5. Termination of Signal

To prevent continuous stimulation, the neurotransmitter is removed from the cleft via:

1. Enzymatic Degradation: (e.g., Acetylcholinesterase breaking down Acetylcholine).
2. Reuptake: Active transport back into the presynaptic neuron.
3. Diffusion: Simple physical movement away from the cleft.

Final Solution: Synaptic transmission is a highly regulated, calcium-dependent process that converts an electrical action potential into a chemical signal via neurotransmitter exocytosis, followed by receptor binding on the postsynaptic membrane to initiate or inhibit a new electrical impulse.