Study Notes on Neuronal Communication Part 2

Neuronal Communication - Part 2

Overall Overview

  • The transmission of information from one neuron to another occurs across a synapse, which is a small gap consisting of:

    • Presynaptic Ending: A synaptic knob that houses:

    • Synaptic vesicles filled with neurotransmitters

    • Mitochondria and other organelles

    • Postsynaptic Ending: Contains receptor sites for neurotransmitters

    • Synaptic Cleft: The space between presynaptic and postsynaptic endings.

Chemical Synapse Structure

  • Components:

    • Presynaptic Neuron: Contains:

    • Synaptic vesicles with neurotransmitter.

    • Microtubules of cytoskeleton.

    • Mitochondria.

    • Postsynaptic Neuron: Contains:

    • Receptor for neurotransmitters.

    • Synaptic Cleft: The gap where neurotransmitter diffusion occurs.

Functioning of Synapses

  • Action Potential (AP) Mechanism:

    • The action potential travels along the axon to the synaptic bulb.

    • It triggers Voltage-Gated Ca²⁺ channels to open, allowing Ca²⁺ ions to diffuse into the neuron.

    • Ca²⁺ ions cause:

    • Synaptic vesicles to move to the end of the presynaptic membrane and fuse with it.

    • Release of neurotransmitters (NT) via exocytosis into the synapse.

    • Neurotransmitters diffuse across the synapse and bind to receptors on the postsynaptic neuron, causing:

    • Ligand-Gated Ion Channels to open, permitting ion passage in or out of the postsynaptic neuron.

Types of Neurotransmitter Receptors

General Overview

  • Two types of neurotransmitter receptors:

    • Ionotropic Receptors (Direct):

    • Also called ligand-gated receptors.

    • Operate without second messengers.

    • Metabotropic Receptors (Indirect):

    • Utilize second messenger systems for signaling, acting as a middleman.

Ionotropic Receptor Properties

  • Initiate direct effects on ion channels.

  • Example substances include ACh (Acetylcholine), glutamate, and aspartate.

  • Mechanism:

    1. Neurotransmitter binds to the receptor.

    2. Activation of the channel occurs, allowing ion flux across the membrane, leading to immediate cellular responses.

Metabotropic Receptor Properties

  1. Direct Coupling: Slow ion channel regulation.

  2. Second Messenger System:

    • Neurotransmitter binding activates a G protein that interacts with intracellular messengers, potentially affecting multiple cellular responses.

    • Phases of Activation:

      • Neurotransmitter binds

      • G protein activation

      • Effect on ion channels or other intracellular processes.

Sensory Receptors and Their Mechanism

Different Types of Sensory Receptors:

  • Mechanoreceptors:

    • Sensitivity to pressure opens an ion channel.

  • Thermoreceptors:

    • Temperature affects membrane proteins related to ion channels.

  • Electroreceptors:

    • Respond to electric charges, influencing the opening of ion channels.

  • Chemoreceptors:

    • A molecule binding to a receptor initiates signal transduction affecting ion channels through second messenger cascades.

  • Photoreceptors:

    • Light alters a signaling cascade affecting ion channels.

Neurotransmitter Classes

  1. Acetylcholine (ACh): Formed from acetyl-CoA and choline.

  2. Amino Acid Neurotransmitters: Such as glutamate and glycine.

  3. Monoamines: Include dopamine, norepinephrine, serotonin.

  4. Neuropeptides: Chains of amino acids functioning as neurotransmitters.

Types of Neurotransmitter Activity

Excitatory vs. Inhibitory Neurotransmitters:

  • Excitatory Neurotransmitters:

    • Cause depolarization of postsynaptic membranes, promoting action potentials.

  • Inhibitory Neurotransmitters:

    • Cause hyperpolarization, inhibiting action potentials. Main examples:

    • GABA and glycine.

Synaptic Transmission Examples

  1. Excitatory Cholinergic Synapse: Mediated by ACh.

  2. Inhibitory GABA-ergic Synapse: Mediated by GABA.

  3. Excitatory Adrenergic Synapse: Mediated by Norepinephrine (NE).

Mechanisms of Synaptic Action

Excitatory Cholinergic Synapse

  1. AP Arrival: Triggers Ca²⁺ influx.

  2. Neurotransmitter Release: Vesicles fuse and release ACh.

  3. Channel Opening: Ion channels open, influencing postsynaptic potential.

  4. AChE Activity: Acetylcholinesterase clears ACh from receptors.

Inhibitory GABA-ergic Synapse

  • GABA released, Cl⁻ channels open, leading to hyperpolarization making the postsynaptic neuron less likely to reach the threshold for an action potential.

Adrenergic Synapse

  • NE release triggers a second-messenger system that can cause prolonged effects, such as ion channel opening.

Postsynaptic Potentials

  • EPSP (Excitatory Postsynaptic Potential):

    • Involves opening sodium channels, making it more likely for an action potential.

  • IPSP (Inhibitory Postsynaptic Potential):

    • Involves increased permeability to potassium (K⁺) or chloride (Cl⁻), inhibiting action potential generation.

Summation of Potentials

  • Summation among EPSPs and IPSPs determines the postsynaptic neuron's ability to reach threshold:

    • Temporal Summation: Adding EPSPs over time from a single neuron.

    • Spatial Summation: Combining EPSPs from different neurons simultaneously.

Synaptic Delay

  • A delay of 0.2–0.5 milliseconds occurs between action potential arrival and the initiation of an action potential in the postsynaptic neuron. More synapses in action chains result in longer delays.

Neuromodulators and Their Effects

  • Neuromodulators: Chemical signals affecting groups of neurons over longer terms than neurotransmitters.

    • Roles include:

    • Modulating neurotransmitter release rates.

    • Altering postsynaptic sensitivity.

    • Affecting reuptake or breakdown speeds of neurotransmitters.

Mechanisms of Presynaptic Inhibition and Facilitation

  • Inhibition: A presynaptic neuron suppresses neurotransmitter release through mechanisms like blocking calcium channels, reducing postsynaptic activity.

  • Facilitation: Enhancements in neurotransmitter release due to prolonged calcium channel activity increase postsynaptic excitation.

Terminology: Agonist vs. Antagonist

  • Agonist: Activates receptor - produces an effect.

  • Antagonist: Blocks receptor function - produces no effect.

Opioid Receptors

  • Endogenous Opioids (e.g., Dynorphins, Endorphins): Provide pain relief.

  • Agonists: Morphine, fentanyl, etc., activate opioid receptors.

  • Antagonists: Naloxone, naltrexone, block opioid receptor activation without eliciting an effect.

Conclusion

  • Neuronal communication involves complex physiological processes that regulate synaptic transmission, integrate signals through summation, and modulate overall neural network activity. Understanding these concepts is crucial for comprehending brain function and the implications for pharmacological interventions in various neurological conditions.