Notes on Synaptic Transmission and Mechanisms

Overview of Synaptic Transmission

  • Information is digitized at the axon hillock.

  • A stream of action potentials transports neuronal output down the axon.

  • Synaptic transmission transfers information from one neuron’s axon to another neuron, causing electrical events in the receiving neuron.

  • Integration of inputs into the generator potential occurs at the axon hillock and leads to action potential generation.

  • Neural processing involves the integration of various synaptic inputs into generator potentials regionally.

Types of Synapses

A. Electrical Synapses

  • Direct, passive flow of electrical current between neurons through gap junctions that connect cell membranes.

  • Ion channels form low-resistance pathways (gap junctions) that allow ionic current to pass through freely.

  • Found in invertebrate neurons, heart muscle cells, and less commonly in mammal neurons.

  • Benefits: Synchronization of activity, fast transmission, less resistance compared to chemical synapses.

B. Chemical Synapses

  • Involves neurotransmitter release from the presynaptic neuron.

  • Neurotransmitter diffuses across synaptic cleft (20-50 nm), binding to receptors on the postsynaptic membrane.

  • Types of chemical synapses include:

    • Axosomatic (axon to cell body)

    • Axodendritic (axon to dendrite)

    • Axoaxonic (axon to another axon)

    • Dendrodendritic (dendrite to dendrite)

Molecular Structure of Electrical Synapses

  • Connexons: Special proteins create large pores at gap junctions, allowing ions and small organic molecules to pass.

  • Gap Junctions: Membranes are 3 nm apart, enabling direct ionic current flow between connected neurons.

    • Plasticity: Modify their function based on the activity in both the presynaptic and postsynaptic cells.

Chemical Synaptic Transmission Principles

  1. Neurotransmitter Synthesis: Forming neurotransmitters from precursors.

  2. Loading neurotransmitters into synaptic vesicles.

  3. Release: Action potentials trigger presynaptic terminals to release neurotransmitters into the synaptic cleft.

    • Voltage-gated calcium (Ca²⁺) channels open, allowing Ca²⁺ to flood the terminal, promoting vesicle fusion and exocytosis.

  4. Binding & Response: Neurotransmitters bind to postsynaptic receptors and elicit biochemical/electrical responses in the postsynaptic neuron.

  5. Inactivation: Removal of neurotransmitter from synaptic cleft via reuptake or degradation (e.g., acetylcholine by acetylcholinesterase).

Excitatory and Inhibitory PostSynaptic Potentials (EPSP and IPSP)

  • EPSP: Caused by depolarization of the postsynaptic membrane, moving closer to threshold for action potential generation.

  • IPSP: Hyperpolarizes the postsynaptic membrane, moving it away from threshold.

  • The membrane potential is influenced by the ionic current direction, that flows according to (E_{rev}), which is determined by the types of ion channels activated.

Synaptic Integration

  • Temporal Summation: Occurs when stimuli are sufficiently close together in time to cause a greater combined effect.

  • Spatial Summation: When multiple presynaptic inputs are active simultaneously, their combined effects can bring the postsynaptic membrane closer to threshold.

  • Length Constants ( ext{λ}): Indicates how far depolarization can spread along a neuron’s membrane before it dissipates. A larger ext{λ} means greater signal spread.

Neuromodulation & G-Protein Coupled Receptors (GPCRs)

  • G-proteins act as molecular switches triggered by extracellular signals.

    • They induce longer-lasting effects through second messengers, modifying cell metabolism and creating diverse physiological actions.

  • Key Effects: Activation of these receptors usually results in slower and smaller postsynaptic effects compared to neurotransmitter effects at classical synapses.

Experimental Techniques

  • Immunocytochemistry: To identify neurotransmitter presence in neuronal tissue.

  • In situ Hybridization: To visualize neurotransmitter mRNA localization.

  • Neuropharmacological Methods: To study how receptor pharmacology influences synaptic transmission.

These notes summarize essential mechanisms of synaptic transmission and its various forms, emphasizing the importance of neuronal communication in both excitatory and inhibitory contexts.