Lecture 04 Neuronal Signaling

Neuronal Signaling

Goals

  • Understand how neurons signal to produce adaptive movement.
  • Explore different signaling mechanisms, including neurotransmitters and ion channels.
  • Examine the underlying principles of membrane dynamics and neuronal excitability.

Overview of Neuronal Signaling

  • Purpose: Neuronal signaling enables coordinated activity among cells, supporting adaptive movement to optimize biological fitness.
  • Types of Signaling Molecules:
    • Endocrine System: Hormones.
    • Immune System: Cytokines.
    • Nervous System: Neurotransmitters and neuromodulators.
  • Key Mechanisms:
    • Endocrine (long-distance via blood).
    • Autocrine (self-signaling).
    • Paracrine (local cell-to-cell).
    • Direct signaling (via gap junctions or electrical synapses).

The Role of Membranes in Neuronal Signaling

  • Structure: Lipid bilayer embedded with receptors, channels, and pumps.
  • Functions:
    • Separates intracellular and extracellular environments.
    • Regulates ion flow and signaling molecule access.

Challenges and Adaptations

  • Osmotic Regulation:
    • Plant cells rely on thick walls to prevent osmotic bursting but limit movement.
    • Animal cells use thin membranes for dynamic regulation, supporting movement.
  • Permeability:
    • Membranes contain selective and non-selective channels.
    • Some molecules, like steroid hormones (e.g., cortisol), can diffuse directly through the membrane.

Electrochemical Gradients

  • Chemical Gradient: Molecules diffuse from high to low concentrations.
  • Electrical Gradient: Charged molecules move toward opposite charges.
  • Combined gradients influence ion movement across membranes.

Key Ions and Gradients

  • Sodium (Na⁺), Potassium (K⁺), Calcium (Ca²⁺), Chloride (Cl⁻).
  • Example:
    • K⁺ is highly concentrated inside neurons and diffuses outward.
    • Na⁺ has a high extracellular concentration and a strong drive to enter cells.

Resting Membrane Potential

  • Neurons maintain a resting potential of ~-70 mV.
  • Achieved through:
    • Selective permeability (more K⁺ channels than Na⁺ channels).
    • Sodium-Potassium Pump:
      • Uses ATP to pump 3 Na⁺ out and 2 K⁺ in.

Membrane Dynamics and Channels

Types of Channels

  1. Passive (“Leakage”) Channels:
    • Always open; contribute to resting potential.
  2. Ligand-Gated Channels:
    • Open when specific molecules (ligands) bind (e.g., neurotransmitters like glutamate or GABA).
  3. Voltage-Gated Channels:
    • Open in response to changes in membrane potential.
  4. Mechanically-Gated Channels:
    • Open in response to physical stretching.

Membrane Potential Changes

  • Depolarization:
    • Influx of positive ions (e.g., Na⁺) reduces the negative resting potential.
  • Hyperpolarization:
    • Influx of negative ions (e.g., Cl⁻) or efflux of K⁺ makes the membrane potential more negative.

Synaptic Signaling

Synapse Structure

  • Chemical Synapse:
    • Presynaptic neuron releases neurotransmitters into the synaptic cleft.
    • Postsynaptic receptors bind neurotransmitters, altering ion flow.
  • Electrical Synapse:
    • Direct ion flow through gap junctions; less common but faster.

Excitatory and Inhibitory Synapses

  • Excitatory Postsynaptic Potentials (EPSPs):
    • Glutamate binding opens ionotropic channels, allowing Na⁺ influx, causing depolarization.
  • Inhibitory Postsynaptic Potentials (IPSPs):
    • GABA binding opens Cl⁻ channels, causing hyperpolarization.

Advanced Concepts

Integration of Synaptic Inputs

  • Spatial Summation:
    • Multiple inputs from different locations combine to influence membrane potential.
  • Temporal Summation:
    • Repeated inputs from the same synapse add up over time.
  • Axon Hillock:
    • Region where summation of inputs determines action potential initiation.

Action Potential

  • Triggered when depolarization reaches a threshold.
  • Properties:
    • All-or-none response.
    • Self-propagating along the axon.
    • Refractory period prevents immediate reactivation.

Role of Astrocytes

  • Part of the tripartite synapse:
    • Regulate neurotransmitter levels.
    • Buffer ion concentrations.
    • Influence local blood flow in response to neuronal activity.

Summary and Implications

  • Neuronal signaling is fundamental to brain function, enabling rapid communication and adaptation.
  • Key mechanisms involve ion dynamics, membrane properties, and synaptic transmission.
  • These principles form the basis for understanding more complex neural circuits and behaviors.
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