Membrane Potentials

Introduction to Membrane Potentials

Membrane potentials refer to the electrical difference across the plasma membrane of cells, especially neurons. This difference is crucial for the transmission of electrical signals in the nervous system.

• Neurons transmit electrical signals from one part of the body to another.

• The synapse is the junction where one neuron communicates with another.

• Axon carries the electrical signal away from the cell body toward axon tips and synapses.

Electrophysiology of Neurons

Key Properties of Neurons

Neurons possess specialized properties that enable them to respond to stimuli and transmit electrical signals.

• Excitability: The ability of neurons to respond to stimuli by generating electrical changes across their membrane, known as membrane potentials.

• Conductivity: The ability of neurons to propagate electrical changes along their membrane, allowing signals to travel throughout the neuron.

• Both properties depend on the movement of charged particles (ions) across the membrane.

Resting Membrane Potential

Definition and Ionic Basis

The resting membrane potential (RMP) is the electrical potential difference across the membrane of an unstimulated (resting) neuron.

• Ion concentration differences exist across the membrane:

  • Extracellular Fluid (ECF): High Na+, low K+, high proteins for both.

  • Intracellular Fluid (ICF): Low Na+, high K+.

• Polarity: The inside of the cell is negative relative to the outside, which is positive.

Axolemma and Ion Movement

Ion Channels and Pumps

The axolemma (neuron plasma membrane) contains various ion channels and pumps that regulate ion movement.

• Ion Pumps: Move ions against their concentration gradient and require ATP.

  • Sodium-Potassium Pump: Moves 3 Na+ out and 2 K+ in per ATP molecule.

• Ion Channels: Allow ions to move down their concentration gradient without requiring ATP.

  • Leak Channels: Always open; ions leak from high to low concentration.

  • Gated Channels: Open and close in response to specific stimuli.

Ion Pumps

Sodium-Potassium Pump

The sodium-potassium pump is essential for maintaining the resting membrane potential.

• Function: Actively transports 3 Na+ ions out of the cell and 2 K+ ions into the cell per ATP hydrolyzed.

• Role: Maintains the concentration gradients of Na+ and K+ across the membrane.

Channels and Membrane Permeability

Leak Channels and Permeability

Leak channels contribute to the resting membrane potential by allowing ions to move passively.

• Leak Channels: Always open, allowing Na+ to leak in and K+ to leak out.

• Membrane Permeability: The membrane is more permeable to K+ than Na+ due to a greater number of K+ leak channels.

Ligand-Gated Channels

Mechanism and Function

Ligand-gated channels open in response to the binding of specific molecules (ligands), such as neurotransmitters.

• Structure: Channel opens when a ligand binds to its receptor site.

• Function: Allows selective ion movement, such as Na+ influx, which can initiate electrical changes in the neuron.

• Example: Acetylcholine binding to its receptor on the postsynaptic membrane opens Na+ channels.

Summary Table: Ion Channels and Pumps

The following table summarizes the main types of ion channels and pumps found in neurons:

Key Equations

Nernst Equation (for equilibrium potential)

The Nernst equation calculates the equilibrium potential for a particular ion:

• Eion: Equilibrium potential for the ion

• R: Universal gas constant

• T: Temperature in Kelvin

• z: Valence of the ion

• F: Faraday's constant

• [ion]out: Extracellular concentration

• [ion]in: Intracellular concentration

Goldman-Hodgkin-Katz Equation (for membrane potential)

This equation considers multiple ions:

• Vm: Membrane potential

• P: Permeability of the membrane to each ion

Conclusion

Understanding membrane potentials and the mechanisms of ion movement is fundamental to the study of neuronal function and communication. These principles underlie the ability of neurons to generate and propagate electrical signals throughout the nervous system.