5. Ion Channels

Membrane Potential & Ion Channels

• Membrane potential is a key aspect of neuronal function
• Ion concentrations (intracellular and extracellular) differ significantly
• Nernst equation calculates the equilibrium potential for a single ion
• Goldman-Huxley-Katz equation calculates membrane potential considering multiple ions
• Ion channel impact on resistance influences the rate of ion flow
• Various ion channel types exist
• Ion channels are regulated by a number of processes

Electrical Excitability

• Neurons are excitable cells driven by electrical signals
• Membrane properties are crucial to neuronal function
• Electrical signalling shows a variety of forms desirable for activity
• Integration of electrical signals is crucial for complex neuronal function, including inhibitory and excitatory signals, spatial and temporal summation. Signalling occurs along dendrites, soma, and axon.

Integration

• Neurons receive inputs from hundreds of thousands of synapses and have inherent activity
• All of this input and intrinsic activity is processed to form the neuronal output

Membrane Potential

• Resting membrane potential is approximately -70mV. This level can vary between cells and also according to the cell condition, and so it is more accurate to describe potential from -50mV to -80mV.
• Neuronal resting potential can vary depending on cell type and cellular condition eg sleep/wake
• Resting membrane potential and most active conductances are mediated by ion channels

Ion Concentrations & Charges

• Extra- and intracellular concentrations of K+, Na+, Ca2+, and Cl- differ significantly.
• Ion movement is influenced by concentration gradients

Permeability

• An impermeable membrane prevents ions from crossing, resulting in no membrane potential
• Permeability allows ions to move and create a membrane potential

Potassium Selectivity

• Leak channels are specific for K+, so only K+ can flow across the membrane
• The concentration gradient of K+ drives the movement out of the cell.
• Membrane potential is primarily determined by K+

Potassium Charge

• Equal positive and negative charges are present on both sides of the membrane
• K+ ions flow down the concentration gradient
• The efflux of K+ ions inside the cell, makes the inside more negative

Few Ions Required

• A small number of K+ ions moving out of the cell are enough to generate a significant membrane potential
• The thin membrane and opposite charges quickly build a potential
• This is sufficient to trigger action potentials, needing a low amount of ion movement

Equilibrium Potential

• K+ ions exit the cell down its concentration gradient and create a negative charge inside the cell
• This negative charge pulls K+ ions back into the cell
• Equilibrium potential is reached when the inward and outward forces are balanced

Nernst Equation

• The Nernst equation calculates equilibrium potential for a single ion
• The equation takes into account the gas constant, temperature, Faraday constant and ion valence and concentration

Nernst Equation (Simplified)

• Simplified Nernst equation for potential calculation for a single valent ion
• Values for room and body temperature are provided

Nernst in Action

• Specific numerical examples of calculating equilibrium potentials for various ions (K+, Na+, Cl-, Ca2+)

Equilibrium Potential

• Equilibrium potential (reversal potential) is used to predict ion flow direction based on membrane potential
• Ions flow down their gradients depending on potential difference
• Chloride movement depends on whether the membrane potential is hyperpolarized or depolarised

Goldman-Huxley-Katz Equation

• Goldman-Huxley-Katz equation is extended Nernst equation to account multiple ions simultaneously.
• The equation includes permeabilities of multiple ions in addition to concentrations
• Example calculation of resting membrane potential is included

Goldman-Huxley-Katz Equation (Calculation Example)

• Numerical example calculation of resting membrane potential using Goldman-Huxley-Katz equation at 37°C
• Example calculation of membrane potential in an A.P.

Membrane Potential (Vm)

• Membrane potential is determined by ion permeabilities and equilibrium potentials of multiple ions. This can range from -70mV to 0mV causing depolarization.
• Ligand gated ion channels can also influence membrane potential

Resting Membrane Potential (Em)

• Resting membrane potential values (approximately -70 mV) are often seen.
• This potential is mostly influenced by K+ and is maintained through passive transport or through ion channels present in the membrane.
• Equilibrium Potential (Ek) greatly influences the membrane resting potential.

Passive Conduction - Dendrites

• Dendrites have a low number of voltage-gated ion channels (Na+ & Ca2+)
• No reinforcement mechanism (all or nothing); unlike axonal signalling
• No myelination; signal weakens with propagation distance
• Current/potential can propagate in either direction by passive conduction
• Signal weakens along dendrite due to leakage ("leaky hose" analogy)

Current Leak

• Current is lost through holes; the larger the diameter, the less resistance or leakage
• Fewer holes in the membrane lead to less leak

Passive Conductance

• Electrical signal spreads along a dendrite but the amplitude becomes smaller as the distance increase. The signal starts with a certain amplitude at the stimulation point, decreasing significantly by the time it propagates to the end of a dendrite.

Dendrites - Soma

• CNS synaptic potentials are usually small and are insufficient to create an action potential for transmission.
• Signals must travel along dendrites to the soma, but decrease in voltage as they transmit.
• To generate an action potential, multiple, coincident (occurring at the same time) and depolarising signals are required.
• Ion channels play a crucial role in modulating signals in dendrites and soma by influencing excitability.
• Neurotransmitter release in synapses is also regulated by ion channels.

Considerations

• Ion channel influence on cell excitability
• Generating electrical circuits. This is influenced by channel activity
• More channels open, lower resistance, greater leak
• A current generates lower electrical potential across a dendrite due to channel resistance and leakage
• Neurotransmitter release and intracellular messenger systems regulate ion channels

Ion Currents

• Ionic currents mostly involve Na+, K+, Ca2+, and Cl-
• Ca2+ currents are too small to majorly influence membrane potential yet are critical for signalling
• Mammals have different types of Na+, Ca2+, and K+ currents

Ion Channels

• Leak channels passively allow ion flow regardless of the membrane potential
• Gated channels open or close in response to stimuli (voltage, ligand, etc.)
• Some channels rectify, preferring certain currents at certain voltages
• Ion channel activity can be modulated by various factors

Leak Channels

• Individual leak channels randomly open and close, unlike constant-open aqueous pores.
• Recording of individual channel activity reveals their fluctuating nature
• Multiple channels contribute to overall current related to membrane potential

Gated Channels

• Voltage gated channels respond to changes in the membrane potential
• Ligand-gated channels open or close in response to binding of specific molecules (ligands)
• Ligands may be extracellular transmitters, intracellular ions or nucleotides
• Other gated channels respond mechanically to changes.

I/V Relationships

• Channels often show direct relationships between current (I) and voltage (V)
• Different ions (Na+, K+) may show different relationships

Rectification

• Channels may rectify, favouring certain current direction/amplitude at given voltages as current may flow more easily in one direction than the other, due to the ion channel itself.
• Na+ and K+ channels may respond differently

How do we selectively record ion currents

• Drugs selectively block ion channels
• These drugs, which are usually ion channel blockers, allow the isolation of individual ion currents. Techniques such as single-channel recordings are also used to study channels at a more detailed level.

Structure & Function of Ion Channels

• Overview of ion channel function, structure, and the factors that determine ion selectivity in a channel. This is crucial for membrane signalling and its function.

Potassium Channels

• Found in most species
• High structural and functional similarity
• Have various types; classified into 4 major groups
• Includes voltage-gated, inwardly rectifying, two-pore domain, and ligand-gated K+ channels.

Basic Structure

• Ion channels have numerous subunits, that vary across different types. Some channels can have other auxiliary subunits.
• Each subunit consists of multiple transmembrane domains & includes a pore domain, formed due to an internal loop, which contains the selectivity filter amino acid sequence.

Pore

• Pore structure highly conserved within different types of channels
• Pore's selectivity filter formed by an intramembrane loop, located between two transmembrane domains
• This filter contains specific amino acid sequences regulating selectivity.

K+ channel Gating

• Gating is the opening/closing of the channel.
• Channels generally have three states: resting (usually closed), activated (open), and inactivated (closed)
• Channels have gates, located intra- and extracellularly.

Gates

• Extracellular gate usually formed by the selectivity filter
• Intracellular gate may form by voltage sensors.
• Inactivation (closed -> open -> inactivated -> closed) can occur and may happen by different mechanisms depending on the channel.

VGKC - Kv

• Channels with 6 transmembrane segments, including a pore loop for selectivity
• Channels have a voltage sensor
• Essential for cell repolarisation
• Opening with cell depolarisation, closing during hyperpolarisation

Kv (Potassium Channel)

• Structural details like transmembrane (TM) domains, the voltage sensor, and the location of the pore and associated subunits

Other Potassium Channels

• Potassium channels show diversity in types, especially the two-pore domain or inwardly rectifying channels and ligand gated K+ channels.
• Types like K2P or Kir have structures distinct from voltage-gated channels.
• Specific roles in hyperpolarisation or upon specific ligand binding

Sodium Channels (Nav)

• Sodium channels are voltage-gated, highly similar to voltage-gated potassium channels. They consist of a huge monomeric protein with 4 6TM segment domains.
• The pore is formed by loops in the segments 5 and 6. A voltage sensor occurs in segments 1 to 4.
• Additional regulatory β proteins exist.
• Inactivation occurs in the loop between domains 3 and 4.
• Sodium leak currents may also occur through sodium channels.

Sodium Channels (NaV)

• Structural details like transmembrane (TM) domains, the voltage sensor, regulatory beta subunit, and the inactivation gate.

Ca2+ Channels

• Ca2+ channels can be either voltage-gated or ligand-gated.
• Includes various types like L-, P/Q-, N-, R-, and T-type, each with specific properties.
• Many ligand-gated or other types are present. Some are slightly permeable to sodium.

Cav Channels

• Heteromeric channel with α1, α2δ, β, and γ subunits crucial for function.
• α1-pore subunit similar to that of Na+ channels

Cl- Channels

• Chloride channels are poorly understood and may have varying properties.
• CFTR is a notable example. A wide variety of types are known.

Ion Channel Selectivity

• Ion size and charge influence ion channel selectivity
• Negative charges repel negatively charged ions; positive ions are attracted.
• Mechanisms for selecting among different positive ion sizes will be examined in detail.

Potassium (K+) Channel Selectivity Filter

• TVGYG selectivity filter in all K+ channel subunits
• 4 binding sites (S1-S4) on carbonyl groups

Energetics

• Ions treated as point charges
• Interactions with negative charges are favoured energetically due to ion stabilisation
• Ions have solvation shells in the solution, and can form bonds or interactions with water molecules

Movement of K+ Ions

• K+ is strongly positive; attracts water molecules from solution
• Amino acids have carbonyl groups that have specific distances relative to K+ as in water
• Carbonyl groups enhance K+ stability by replacing the water molecules
• K+ ions pass through the channel by alternating with water molecules

Selectivity K+ > Na+

• Na+ atoms also attract water molecules (hydration)
• Na+ ions too small to interact with all carbonyl groups
• Energetically unfavourable to remove water molecules around Na+ ions. This prevents Na+ ions from passing through the channel

Na+ Channel Selectivity Filter

• Sodium channels have two selectivity rings, termed outer and inner.
• The inner ring comprises DEKA as its amino acid residues, while outer rings have less impact in determining ion selectivity
• Amino acid side chains present in outer rings do not usually influence ion binding or movement.

Selectivity Na+ > K+

• K+ ion size with its hydration shell is too large to pass through the channel
• Energetically unfavourable to remove water around the K+ compared to Na+
• This difference influences ion conduction through the channels

Ca2+ Channels

• Ca2+ size similar to that of Na+, making selection complex
• Ca2+ bears a double positive charge
• Increase in negative charges in the selectivity filter is required

Selectivity Na+/Ca2+ Channels

• Stabilization of positive ions is crucial for energy favouring. Na+ is not stabilised, while Ca2+ is well stabilised

Ca2+ Channel Selectivity Filter

• Na+ channels have DEKA selectivity ring
• Ca2+ channels have 4 COO - groups
• Ca2+ channels are favoured by a smaller pore diameter

Fun With Molecular Biology

• Modifying ion channels for different purposes is feasible
• No ion channel is entirely ion-specific
• Selectivity varies substantially across different ion channels
• Concentration gradients also influence ion flux significantly.

Receptor Ion Channels

• Overview of receptor channels and their key properties; also related to the different superfamilies.
• Properties like ion selectivity and pore size are key

Main Ionotropic Receptors

• Key ionotropic receptors in CNS, including their main neurotransmitters and roles

Receptor Superfamilies

• Different types of receptor superfamilies; examples include Cys-loop or Glutamatergic receptors based on their structures and the ligands they bind, and related receptor mechanisms

Receptor Channel Opening (Basic Concept)

• Receptor subunits arrange; hydrophobic side chains prevent ion entry in a closed configuration
• Binding of a specific ligand changes the protein conformation, allowing ions to pass

Receptor Pores

• Receptor pores are generally larger compared to other ion channels
• Minimum pore size varies, lower than that of a potassium channel
• Ion selectivity in receptor pores is lower because of the physical size of the pores.
• Exception to this is AMPA or Kainate receptors which do not conduct calcium.

CNS Ionotropic Receptors

• Fast control of excitability via ion flux
• Excitatory receptors (AMPA, Rs, NMDAR) for the brain
• Inhibitory receptors like GABAAR and glycine receptors
• Other types such as 5-HT3, nAChR have more fast modulatory roles, as it is less likely for them to trigger an AP

Summary

• Ion channel selectivity: K+, Na+, and Ca2+ selectivity varies substantially among channels
• Wide differences in ion pore structures and properties; particularly in voltage-gated channels
• Selectivity filters and energetics of ion hydration determine ion permeability

Test your knowledge on the pivotal role of ion channels in neuronal function, including selectivity and membrane potential. This quiz covers essential concepts related to K+ channels, Na+ channels, and ligand-gated receptors. Strengthen your understanding of how neurons communicate and process synaptic inputs.