Membrane Potential Fundamentals

Membrane Potential Fundamentals

• Cells have a membrane potential (RMP) typically between -70 and 40 mV
• This is due to ion concentration gradients and ion permeability across the membrane
• Extracellular concentrations of Na+, Cl-, and other ions differ from intracellular concentrations
• The RMP is primarily determined by the K+ concentration gradient
• The K+ concentration is much higher inside the cells than outside
• Ion channels such as voltage-gated channels control ion movement across the membrane
• Changes in ion movement across the membrane can be exploited by the cell to effect changes in cellular events

Equilibrium Potential

• Equilibrium potential is the membrane potential at which there is no net movement of an ion across the membrane
• Equilibrium potential is determined by the Nernst equation
• It considers the concentration gradient and the electrical gradient of an ion
• The Nernst equation calculates the equilibrium potential and gives the result in mV
• The different ions can be described as being at equilibrium under different conditions/states determined by the Nernst equation

Potassium Equilibrium

• For a typical neuron, extracellular K+ concentration is 5 mM and intracellular K+ concentration is 100 mM
• The resting membrane potential (RMP) is -70 mV
• The concentration gradient would cause K+ efflux
• The electrical gradient would cause K+ influx, which happens at rest
• The equilibrium potential (Ek) for K+ is usually about -80 mV
• The fact that the RMP is -70 mV and not -80 mV implies membrane permeability to other ions
• The RMP is a balance between permeability to these other ions (Na+, Cl-)

How the RMP is established

• Most mammalian cells have RMPs that are slightly more positive than the K+ equilibrium potential (as there is always some leakage pathway)
• Na+ and Cl- permeabilities contribute to the RMP
• The extent to which an ion contributes to the RMP is relative to the permeability of the membrane to that ion
• Even large concentration gradients can have little impact on the RMP when the ion permeability is low

Importance of RMP

• Rapid changes in the ratios of ion permeability are the basis for bioelectric phenomena
• These changes underlie neuronal action potentials
• Alterations in permeability underlie subthreshold and suprathreshold potentials.

Physiology of Voltage-Gated Ion Channels

• Voltage-gated channels open/close depending on the membrane potential
• Ion (e.g., Na+, Ca^2+, K+) flows into or out of the cell
• The initial membrane potential change causes a cascade of effects, which involves the movement of charge in the channel structures

Single Channel Recording

• Patch clamp technique allows the study of individual ion channel activity
• Nobel Laureates Erwin Neher and Bert Sakmann received the Nobel Prize for Medicine in 1991 for this work

Methods for Recording Ionic Currents

• Two-electrode voltage clamp is a method for measuring ionic currents
• Different patch-clamp configurations exist (e.g., cell-attached, whole-cell, inside-out, outside-out)

Properties of Ion Channels - Selectivity, Gating, and Inactivation

• Ion channels select which ions they permit to pass
• Channel selectivity is determined by the structure of the pore
• Gating describes how the channels' open and close states are regulated
• Inactivation is a process, in which, once an ion channel is open there are various routes back to a non-conducting state; one mechanism is deactivation (driven by repolarization) and another is inactivation (driven by sustained depolarization)

Features of Voltage-Gated Ion Channels

• Voltage-gated ion channels (e.g., Na+, K+, Ca2+) have a specific selectivity filter and a voltage sensor

Structures of Voltage-Activated Ion Channels

• Channels composed of multiple subunits (e.g. α-subunit) have specific transmembrane structures (e.g. S4 helix) that participate in ion channel activation and inactivation

Selectivity Filter: The Pore Loop

• Pore loop plays a role in ion selectivity; ion selectivity is determined by carbonyl backbone groups of the TVGYG motif in the pore loop
• For example, Nav channels have specific DEKA side chains.

Ion Channel Structure - Ion Selectivity

• Details about the channel structure in terms of folds and how selectivity related to location in water vs within the pore
• Figures show the differences in how Na+ and K+ ions interact within the channel
• Specific structural components in ion channel structure relate to ion selectivity

Gating - The Voltage Sensor

• Detailed structure of the S4 transmembrane helix including its positively charged amino acids
• These positively charged amino acids are responsible for the voltage-sensing properties of the channel
• This mechanism of voltage sensing relates to the opening of the channel allowing ions within the membrane to carry current

Voltage-Activated Sodium Channels - Gating

• Sodium channel activation is associated with charge movement within the channel
• The movement through the structures is known as the gating current
• This charge movement precedes ion flow

Inactivation - The IFM Motif for NaV Channels

• Fast inactivation of Na+ channels is mediated by an intracellular 'gate' that binds to the intracellular mouth of the pore
• The IFM motif is a hydrophobic triad that forms the inactivation gate and is part of a tethered pore blocker
• The IFM peptide occludes the Na+ channel pore to stop ion flow