Neuronal Membrane Potentials: Resting, Graded, and Action Potentials

Introduction

• Main Topics: Resting Membrane Potentials, Graded Potentials, and Action Potentials of Neurons
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Resting Membrane Potential (RMP)

Definition

• Voltage difference across the cell membrane when the cell is at rest
• Applies to: All cells, but focused on neurons here

Voltage Range

• Typical values: -70mV to -90mV (common average: -70mV)

Mechanisms Establishing RMP

Sodium-Potassium ATPases

• Pumps 3 Na+ out & 2 K+ in
• Slightly negative inside the cell (e.g., from 0mV to -5mV)
• Establishes Na+ out (high) & K+ in (high)

Leaky Potassium Channels

• Channels always open for K+ to move freely down its concentration gradient
• Loss of K+ leaves behind anions (e.g., phosphates, proteins) → more negative inside
• Could make the inside as negative as -90mV

Leaky Sodium Channels

• Allows Na+ to enter the cell down its gradient
• Makes inside slightly positive, mitigating K+-induced negativity to around -70mV

Important Factors

• Permeability: Potassium >> Sodium
• Additional channels: Calcium and Chloride (less emphasis)
• Nernst Potential: Equations for Na+ and K+ to find resting potentials based on electrochemical gradients

Graded Potentials

Purpose

• Modify the RMP either to reach threshold potential or move away
• Threshold voltage: ~-55mV (for opening voltage-gated Na+ channels)
• Depolarization: Moving towards -55mV
• Hyperpolarization: Moving away from threshold (e.g., to -90mV)

Types

Excitatory Postsynaptic Potential (EPSP)

• Goal: Bring RMP towards threshold
• Mechanism: Stimulatory neurotransmitter (e.g., Glutamate), opens ligand-gated ion channels for Na+ or Ca2+, making inside more positive

Inhibitory Postsynaptic Potential (IPSP)

• Goal: Move RMP away from threshold
• Mechanism: Inhibitory neurotransmitter (e.g., GABA), opens channels allowing Cl- in or K+ out, making inside more negative

Spatial and Temporal Summation

Temporal Summation

• One presynaptic neuron repeatedly stimulates a postsynaptic neuron

Spatial Summation

• Multiple presynaptic neurons stimulate one postsynaptic neuron simultaneously

Action Potential (AP)

Steps from RMP to AP

1. RMP: -70mV (established by Na+/K+ ATPases, leaky K+ and Na+ channels)
2. Threshold Potential: -55mV (achieved via EPSPs and summation)
3. Depolarization: Voltage-gated Na+ channels open → Na+ rushes in → cell becomes +30mV
4. Initiation at Axon Hillock: Voltage-gated Na+ channels are highly concentrated

Phases of Action Potential

Depolarization

• Na+ influx due to voltage-gated Na+ channel opening
• Voltage change: -55mV (activation gates open) to +30mV (inactivation gates close)

Repolarization

• Voltage-gated K+ channels open
• K+ efflux bringing voltage from +30mV to -90mV (initial hyperpolarization) then to -70mV (RMP)

Propagation of Action Potential

• Positive ions: Move along the axon, opening subsequent voltage-gated Na+ channels
• Depolarization: Wave of positive charge moving down the axon
• Repolarization: Follows, ensuring proper sequence and resting state recovery

Refractory Periods

Absolute Refractory Period

• Na+ channels inactivated (inactivation gates closed)
• No new AP possible irrespective of stimulus strength

Relative Refractory Period

• K+ efflux, cell hyperpolarized at -90mV
• Higher than normal stimulus can initiate another AP

Summary

• Key Elements: RMP, EPSP/IPSP for graded potentials, threshold achievement, AP phases, and refractory periods