116. Synaptic Transmission in Central Nervous System – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Synapses are specialized junctions that transmit information between neurons. They are essential for information processing, learning, memory, and neural integration. Synapses also exhibit plasticity, allowing learning and adaptation throughout life.
• Synapses are classified as:
  • By location: Axodendritic (most common), axosomatic, axoaxonic, and dendrodendritic.
  • By transmission: Chemical (most common), electrical, and mixed.
  • By neuronal connections: One-to-one, many-to-one (convergence), and one-to-many (divergence).
• A chemical synapse consists of a presynaptic terminal, synaptic cleft, and postsynaptic membrane. The presynaptic terminal contains synaptic vesicles, active zones, voltage-gated calcium channels, and mitochondria.
• Steps of synaptic transmission:
  • Action potential reaches the presynaptic terminal.
  • Voltage-gated calcium channels open and calcium enters the terminal.
  • Calcium triggers vesicle fusion through SNARE proteins (synaptobrevin, syntaxin, SNAP-25).
  • Neurotransmitter is released by calcium-dependent exocytosis, diffuses across the synaptic cleft, binds to postsynaptic receptors, and produces a postsynaptic potential.
  • Neurotransmitter action ends by enzymatic degradation, reuptake, or diffusion.
• Botulinum toxin blocks acetylcholine release by destroying SNARE proteins, producing flaccid paralysis. Tetanus toxin blocks release of inhibitory neurotransmitters (gamma-aminobutyric acid and glycine), causing spastic paralysis.
• Postsynaptic potentials are graded responses:
  • Excitatory postsynaptic potential (EPSP): Depolarization caused mainly by sodium or calcium entry; increases neuronal excitability.
  • Inhibitory postsynaptic potential (IPSP): Hyperpolarization caused mainly by chloride entry or potassium exit; decreases neuronal excitability.
  • EPSPs and IPSPs undergo temporal and spatial summation. If threshold is reached at the axon initial segment, an action potential is generated.
• Electrical synapses transmit impulses directly through gap junctions. They are extremely fast, often bidirectional, and important for synchronized neuronal activity.
• Important properties of synapses include:
  • Law of forward conduction (chemical synapses conduct only from presynaptic to postsynaptic neuron).
  • Synaptic delay of about 0.5 ms.
  • Convergence, divergence, facilitation, summation, occlusion, habituation, sensitization, potentiation, and synaptic plasticity, which forms the cellular basis of learning and memory.
• Synaptic inhibition may be:
  • Postsynaptic (direct): GABA or glycine produces IPSPs.
  • Presynaptic (indirect): Reduces neurotransmitter release, often through GABA acting on presynaptic terminals.
  • Feedback inhibition is mediated by Renshaw cells, while feedforward inhibition is important in the cerebellum.
• Major neurotransmitters:
  • Acetylcholine: Neuromuscular junction, autonomic ganglia, parasympathetic nerves; acts through nicotinic and muscarinic receptors.
  • Catecholamines: Norepinephrine, epinephrine, and dopamine.
  • Dopamine: Motor control and reward; deficiency causes Parkinson disease.
  • Gamma-aminobutyric acid (GABA): Principal inhibitory neurotransmitter in the brain.
  • Glycine: Major inhibitory neurotransmitter in the spinal cord.
  • Glutamate: Principal excitatory neurotransmitter; excess causes excitotoxicity.