Topic 2 Module 2 - Resting Membrane and Action Potentials Notes

Overview of the Resting Membrane and Action Potentials

• Resting Membrane Potential (RMP): Electrical difference across nerve membrane, with the inside more negative than the outside, typically around -70 mV.
• Enables neurons to signal changes in input by altering this potential difference.

Objectives

1. Understanding Charge Differences: Learn about polarization, depolarization, and hyperpolarization.
2. Ion Channels: Definition, types, and regulatory factors affecting ion movement through channels.
3. Semi-Permeable Membranes: Mechanism of charge difference creation in nerve cells.
4. Equilibrium Potentials: Understanding equilibrium potentials for Na+ and K+ in neurons.
5. Goldman-Hodgkin-Katz Equation: Importance of the equation for calculating nerve resting membrane potentials.
6. Na+/K+ Pump: Role in maintaining ionic concentrations and resting membrane potential.
7. Changes in RMP: How stimuli lead to changes in RMP and the characteristics of action potentials.
8. Ionic Events of Action Potential: Understanding depolarization, repolarization, and hyperpolarization phases.
9. Return to Resting Level: Mechanism for returning membrane potential post-action potential.

Electrical Polarization in Neurons

• Electrical Charge Difference: The neuron has a resting membrane potential (RMP) characterized by the inside being negatively charged relative to the outside.
• Measurement is made with electrodes and voltmeters showing zero voltage when outside the cell and a negative value once inside.
• Polarization, Depolarization, Hyperpolarization:
• Polarized: Membrane at RMP.
• Depolarized: Membrane becomes less negative.
• Hyperpolarized: Membrane becomes more negative than RMP.

Generation and Maintenance of RMP

• Charge Carriers: In biological systems, ions (charged atoms/molecules) carry charge instead of electrons.
• Membrane Permeability: By allowing selective passage of certain ions, membranes create potential differences.
• Equilibrium Potential: Calculated using the Nernst equation for specific ions demonstrates the balance of concentration and electrical gradients.
• Larger concentration creates a stronger equilibrium potential.
• Example: Ek+ is -85 mV, Ena+ can be +60 mV depending on the concentration gradients.

Action Potential (AP)

• Definition: Sudden and brief change in membrane potential due to significant input causing depolarization, culminating in an action potential.
• Stages:

1. Initial Depolarization: Following stimulus to threshold.
2. Rapid Depolarization: Triggered by Na+ influx after opening voltage-gated Na+ channels.
3. Repolarization: K+ channels open allowing K+ to exit, returning potential to negative values.
4. Hyperpolarization: Membrane potential exceeds resting value due to K+ outflow.
5. Return to RMP: Gradual return facilitated by the Na+/K+ pump and closing of K+ channels.

Significance of Action Potential

• Neurons communicate across synapses by generating action potentials, which rely on the fast and transient influx of sodium and subsequently the efflux of potassium ions.
• The threshold potential is crucial; only sufficient stimuli (threshold) will generate an AP, while sub-threshold stimuli will not.

Ionic Movements During Action Potential

• Na+ Influx leads to rapid depolarization, driving the membrane potential to +30 mV.
• K+ Efflux during repolarization leads to membrane potential hyperpolarizing below the resting level (~-85 mV) before returning.
• The Na+/K+ Pump actively restores the balance of ions post-AP, ensuring readiness for future signals.

Summary of the Action Potential Events

• Threshold: Voltage-gated Na+ channels open leading to depolarization.
• Peak Em: Na+ influx peaks at +30 mV due to saturation of Na+ channels.
• Repolarization: K+ exits to restore a negative intracellular environment.
• Return to Rest: Na+/K+ pump readjusts ionic concentrations to set RMP.