The membrane Potential

Membrane Potential

• The membrane potential is a measure of electric charge distribution across the cell membrane, expressed in millivolts (mV).

  • It is defined based on the inside of the cell relative to the outside, with the outside set at 0 mV.

  • Key Measurement:

    • Cytosol: typically around -70 mV at resting state.

    • This value can vary by cell type but -70 mV is commonly accepted.

• The membrane potential influences the ability of neurons and muscle cells to generate electrical signals, known as action potentials.

Resting Membrane Potential

• At rest, most ion channels are closed except for leakage channels (unpredictable).

• Ion Concentrations:

  • Na+ concentration is much higher outside the cell (10x).

  • K+ concentration is higher inside the cell.

  • The cytosol contains anions (e.g., phosphate ions, proteins) that contribute to the negatively charged environment.

• Membrane potential of -70 mV results from this ion distribution, stabilized by leakage channels and the Na+/K+ pump.

The Action Potential

• An electrical signal that occurs when the membrane potential changes significantly.

Phases of Action Potential

1. Depolarization:

   • Triggered by a neurotransmitter binding to a receptor, causing Na+ channels to open.

   • Na+ influx reduces the negativity inside the cell, moving towards 0 mV, ultimately reaching +30 mV.

   • The membrane potential becomes less negative, transitioning from -70 mV to +30 mV.

2. Repolarization:

   • Occurs as K+ channels open, allowing K+ to flow out.

   • Membrane potential decreases, returning toward -70 mV.

3. Hyperpolarization:

   • The membrane briefly becomes more negative than -70 mV as K+ continues to exit.

   • This phase is due to delayed closing of K+ channels, leading to an overshoot in potential.

Characteristics of Action Potentials

• It's an all-or-nothing event; if the membrane does not reach threshold (-55 mV), no action potential occurs.

• Action potentials have consistent amplitude; the size does not change regardless of stimulus strength.

  • Stronger stimuli can generate multiple action potentials in rapid succession, but each remains the same size (e.g., pain sensation).

Types of Ion Channels

Voltage-Gated Na+ Channel

• Has two gates: activation and inactivation.

  • Activation gate opens at -55 mV, allowing Na+ influx.

  • Inactivation gate closes at the peak of depolarization, preventing Na+ entry after stimulation.

Voltage-Gated K+ Channel

• Responds by opening at around -50 mV but does so more slowly than Na+ channels.

  • Opens during peak Na+ influx and closes when membrane repolarizes past -50 mV, contributing to hyperpolarization due to continued K+ exit.

Propagation of the Action Potential

• Initiated at the axon's initial segment (high density of Na+ channels).

• Depolarization triggers adjacent channels, propagating the action potential down the axon.

• Continuous Conduction: Occurs in unmyelinated axons.

• Saltatory Conduction: In myelinated axons, action potential jumps between nodes of Ranvier for rapid propagation.

Factors Affecting Conduction Speed

• Axon diameter plays a role – wider axons allow faster conduction due to reduced resistance.

Homeostatic Imbalances

• Glial cells, particularly astrocytes, regulate extracellular K+ concentrations.

• Imbalances (e.g., after a stroke) can impair astrocyte function, affecting membrane potential regulation and cellular function.

Terms to Know

• Refractory Period: Phases (absolute and relative) where a neuron is less responsive to new stimuli due to ion channel states.

• Cytosol: Fluid component inside cells where metabolic processes happen.