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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