Membrane Potentials

Introduction to Membrane Potentials

Membrane potentials are essential electrical properties of neurons that enable them to communicate and process information. These potentials arise due to the distribution of ions across the neuronal membrane and are fundamental to the function of the nervous system.

• Membrane potential: The voltage difference across a cell's plasma membrane.

• Neurons use changes in membrane potential to transmit electrical signals along their axons and communicate at synapses.

Electrophysiology of Neurons

Key Properties of Neurons

Neurons possess two critical properties that underlie their function in the nervous system:

• Excitability: The ability of a neuron to respond to stimuli by generating electrical changes (membrane potentials).

• Conductivity: The ability to propagate electrical changes along the length of the neuron.

• Both properties depend on the movement of charged particles (ions) across the neuronal 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. It is primarily established by differences in ion concentrations inside and outside the cell.

• Ion concentration differences exist across the membrane:

  • Extracellular fluid (ECF): High Na+, low K+

  • Intracellular fluid (ICF): Low Na+, high K+, high proteins

• Polarity: The inside of the neuron is negative relative to the outside (typically -70 mV).

• Charge difference is maintained by selective permeability and active transport mechanisms.

Axolemma and Ion Movement

Ion Channels and Pumps

The axolemma (neuron plasma membrane) contains specialized proteins that regulate ion movement:

• Ion pumps (e.g., Na+/K+ ATPase): Move ions against their concentration gradients using ATP.

• Ion channels: Allow ions to move down their concentration gradients. Types include:

  • Leak channels: Always open; allow passive movement of ions (e.g., K+ leak channels).

  • Gated channels: Open or close in response to specific stimuli (ligand, voltage, or mechanical).

Ion Pumps

Types and Functions

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

• This pump is essential for maintaining the resting membrane potential and restoring ion gradients after action potentials.

• Other pumps (e.g., Ca2+ pumps) help regulate intracellular ion storage and return ions to storage sites such as the sarcoplasmic reticulum.

Equation:

Channels and Membrane Permeability

Leak Channels and Selective Permeability

• Leak channels are always open, allowing ions to move passively down their concentration gradients.

• Na+ leaks into the cell, while K+ leaks out.

• There are more K+ leak channels than Na+ leak channels, making the membrane more permeable to K+.

• This differential permeability is a major factor in establishing the negative resting membrane potential.

Ligand-Gated Channels

Mechanism and Function

• Ligand-gated channels open in response to the binding of a specific molecule (ligand), such as a neurotransmitter.

• When the ligand binds to the channel, it changes shape and opens, allowing specific ions (e.g., Na+) to flow into or out of the cell.

• Removal of the ligand or cessation of binding causes the channel to close.

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

Summary Table: Types of Ion Channels

Additional info: Later sections would cover voltage-gated and mechanically-gated channels, action potentials, and synaptic transmission in more detail.