CA

2-3 Action Potential

Action Potential Overview

  • Key terms: Action Potential, Graded Potential, Membrane Potential

  • Resting Membrane Potential: -70 mV

Action Potential Phases

  • Threshold: -55 mV

  • Depolarization: Membrane potential reaches +30 mV

  • Repolarization: Return to resting potential

  • Hyperpolarization: Temporary increase in negativity after repolarization

Membrane Potential Graph

  • Ranges:

    • Resting: -70 mV

    • Threshold: -55 mV

    • Peak: +30 mV

Channels Involved

  • Voltage-gated K+ Channels: Open during repolarization

  • Sodium Channels:

    • Activation Gate opens at -55mV

    • Inactivation Gate closes at +30mV

Actions and Objectives

Overview of Nervous System (Lecture 1)

  • Distinction between Neurons and Glial Cells

Membrane Potential (Lecture 2)

  • Ions involved: Na+ and K+

  • Graded and Action Potentials

  • Synapse and Neurotransmitters

Ionic Basis of Action Potentials

Graded vs. Action Potentials

  • Graded Potentials: Small changes in membrane potential, occur through ligand-gated channels.

  • Action Potentials: Large, rapid changes that allow impulse transmission over distances.

Conduction Mechanisms

  • Ligand-gated Channels: Allow limited ion movement, small depolarization

  • Voltage-gated Channels: Significant ion flow (e.g., Sodium influx causes depolarization)

Refractory Periods

Types

  • Absolute Refractory Period: No further action potential can occur (Na+ channels inactivated)

  • Relative Refractory Period: Increased stimulus strength required (K+ channels still open)

Consequences

  • Ensures one-way transmission of electrical signals in neurons

Factors Affecting Conduction Rate

Unmyelinated Neurons

  • Slower conduction, as every patch of membrane generates action potentials

Myelinated Neurons

  • Faster conduction via Saltatory Conduction:

    • Myelin sheaths around axons, nodes of Ranvier allow localized ion exchange

    • Increases conduction speed significantly (up to 100 m/sec)

Structural Elements

Myelination

  • Schwann Cells: Myelinate peripheral axons.

  • Oligodendrocytes: Myelinate CNS axons.

Action Potential Speed

  • Increased by axon diameter (reducing resistance to current flow)

  • Myelination improves conduction speed via reduction of charge repulsion.

Conclusion

  • Understanding the dynamics of action potentials is critical for grasping neuronal signaling and communication.

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