La Trobe University PSY1BNA Lecture 4: Resting Membrane Potential PDF

latrobe.edu.au PSY1BNA Lecture 4: Resting membrane potential Week 4 La Trobe University CRICOS Provider Code Number 00115M latrobe.edu.au The resting membrane potential Neural membrane Ion channels The action potential The role of ion channels in generating the action potential Action potential cond...

latrobe.edu.au PSY1BNA Lecture 4: Resting membrane potential Week 4 La Trobe University CRICOS Provider Code Number 00115M latrobe.edu.au The resting membrane potential Neural membrane Ion channels The action potential The role of ion channels in generating the action potential Action potential conduction and neural integration Neurotransmitter synthesis Synaptic transmission latrobe.edu.au Recommended readings Breedlove, S.M., & Watson, N.W. (2023). Behavioral Neuroscience (10th ed.). Sunderland, MA: Sinauer Associates, Inc. (Chapter 3; pp. 67-72). Breedlove, S.M., & Watson, N.W. (2020). Behavioral Neuroscience (9th ed.). Sunderland, MA: Sinauer Associates, Inc. (Chapter 3; pp. 63-68). Breedlove, S.M., & Watson, N.W. (2017). Behavioral Neuroscience (8th ed.). Sunderland, MA: Sinauer Associates, Inc. (Chapter 3; pp. 61-66). latrobe.edu.au Key Knowledge and Understanding Membrane potentials Electrochemical forces Diffusion Electrostatic pressure Equilibrium The ionic basis of the resting membrane potential The sodium-potassium pump Potassium channels The distribution of ions inside and outside of a neuron at rest Part 1 The Neural Membrane latrobe.edu.au Information Transmission by Neurons Neuron solves problem of conducting information over a distance by using electrical signals that sweep along the axon. Electrical charge in the cytosol of the axon is carried by electrically charged atoms → ions. However, the axon is not especially well insulated and is bathed in a salty extracellular fluid, which conducts electricity → electrical current passively conducting down the axon would not go very far before it leaks out. The axonal membrane has properties that enable it to conduct a special type of signal – the nerve impulse, or action potential (AP). latrobe.edu.au The Action Potential AP do not diminish over distance. Signals of fixed size and duration. Information is encoded by the frequency of action potentials of individual neurons. Excitable membrane → “action” When a cell with an excitable membrane is not generating impulses it is said to be at rest. At rest the cytosol along the inside of the surface of the membrane has a negative electrical charge compared with that of the outside. This difference in electrical charge across the membrane is called the resting membrane potential. AP is a reversal of this condition for very short duration. latrobe.edu.au Cytosol and Extracellular Fluid Water (H₂O) is the main ingredient of fluid inside cytosol and extracellular fluid H₂O – uneven distribution of electrical charge – sharing of electrons – covalent bond between hydrogen and oxygen atoms The oxygen has a higher affinity for electrons than the hydrogen atom → shared electrons spend more time associated with the oxygen atom → net negative charge H₂O said to be a polar molecule making it an effective solvent of other charged or polar molecules → they dissolve in water latrobe.edu.au Ions Atoms or molecules that have a net electrical charge Table salt → Na⁺ and Cl⁻ ions held together by attraction of oppositely charged atoms → ionic bond Dissolves in water because the charged portions of the water molecule have a stronger attraction for the ions than they have for each other Ions with a positive charge are called cations Ions with a negative charge are called anions Movement of charged particles is electricity For cellular neurophysiology the following ions are important: Na⁺; K⁺; Ca²⁺; Cl⁻ latrobe.edu.au Sodium Chloride in Water latrobe.edu.au Neural Membrane Phospholipid bilayer: Acts as a relatively impermeable barrier (resistor) between inside and outside the cell (intracellular and extracellular space) Membrane Proteins Can assemble to form pores in the bilayer which act as Ion Channels Can assemble to form an Ion Pump – which actively transport ions across the bilayer, utilizing energy from ATP breakdown latrobe.edu.au Phospholipid Membrane Ions and polar molecules are said to be “water loving” or hydrophilic When compounds whose atoms are bounded by nonpolar covalent bounds (shared electrons are shared evenly) have no basis for chemical interaction with water → “water fearing” of hydrophobic eg. Lipids of neuronal membrane form a barrier to water soluble ions and water. Main chemical building blocks are phospholipids latrobe.edu.au Phospholipid bilayer Contain long non polar carbon chains bounded to hydrogen atoms Also have a polar phosphate group attached to one end of the molecule Thus they have a polar “head” (containing phosphate) which is hydrophilic and a non-polar “tail” (containing carbon) that is hydrophobic Neural membrane – sheet of phospholipids two molecules thick isolating the cytosol of the neuron from the extracellular fluid. latrobe.edu.au Phospholipid bilayer latrobe.edu.au Phospholipid bilayer Saturated lipids: very dense packing Saturated + unsaturated lipids: more free space Result: membrane more permeable to water and other small molecules Part 2 Ion Channels latrobe.edu.au Protein Structure The resting and action potentials depend on special proteins that span the phospholipid bilayer The sequence in which the amino acids are linked together determines the primary structure of a protein The way in which the resulting chain bends or folds is the secondary structure of the protein Further bending and folding create the tertiary structure of the protein Different polypeptide chains can bond together to form a larger molecule; such proteins are said to have a quaternary structure Each of the different polypeptides contributing to a protein with a quaternary structure is called a subunit latrobe.edu.au Protein Structure latrobe.edu.au Channel Proteins The exposed surface of a protein may be chemically heterogeneous Regions where non-polar R groups (the functional group of an amino acid) are exposed are hydrophobic and tend to associate readily with the lipid Regions where polar R groups are exposed are hydrophilic and tend to avoid lipid environment Protein can be suspended in phospholipid bilayer with its hydrophobic portion inside the membrane and its hydrophilic ends exposed to the watery environment on either side of the membrane latrobe.edu.au Ion Channels Hydrophilic regions of polypeptide subunits Hydrophobic surface latrobe.edu.au Ion Channels Are made from these membrane-spanning protein molecules A functional channel across the membrane requires that four to six similar protein molecules assemble to form a pore between them The composition varies from one type of channel to the next Important property: Ion selectivity is specified by Diameter of the pore Nature of the R group (eg. K⁺ channels are selectively permeable to K⁺) Gating – channels with this property can be opened and closed – gated – by changes in local microenvironment of the membrane 22 latrobe.edu.au Potassium Channels - a latrobe.edu.au Potassium Channels - b Part 3 Electrochemical Forces latrobe.edu.au The Movement of Ions The existence of an open ion channel in the membrane does not necessarily mean that there will be a net movement across the membrane Such movement also requires that external forces be applied to drive them across Ionic movement through channels are influenced by two factors: Diffusion Electricity latrobe.edu.au Diffusion Ions and molecules dissolved in water are in constant motion This temperature dependent random movement tends to distribute the particles evenly throughout the solution There is a net movement of ions from regions of high concentration to low concentrations: movement is called diffusion Such a difference in concentration is called a concentration gradient Diffusion causes ions to be pushed through channels in the membrane Driving ions across the membrane by diffusion happens when: The membrane posses channels permeable to the ions There is a concentration gradient across the membrane latrobe.edu.au Ionic Forces Underlying Electrical Signalling latrobe.edu.au Electricity An electric field can also induce a net flow of ions in a solution. Consider: Wires from two terminals of a battery are placed in a solution Opposite charges attract; like charges repel Net movement of + ions towards the negative terminal (cathode) and – ions towards the positive terminal (anode) Movement of electrical charge is called electrical current (I – measured in amps) latrobe.edu.au Electricity Two factors determine how much current (I) will flow Electrical potential or voltage Electrical conductance Electrical potential (V): force exerted on a charged particle – difference in charge between anode and the cathode (as difference increases more current flows) Electrical conductance (g): the relative ability of an electrical charge to migrate from one point to another (g – measured in units called siemens) Electrical resistance: the relative inability of an electrical charge to migrate (R – measured in units called ohms Ω) R=1/g I= gV or I=V/R latrobe.edu.au Things to Remember There are electrically charged ions in the intracellular and extracellular space Ions can cross the membrane only by way of proteins The proteins can be highly selective for specific ions Movement of any ion depends on its concentration gradient and the difference in electrical potential across the membrane Next Lecture: the ionic basis of the resting membrane potential Part 4 The Resting Membrane Potential latrobe.edu.au Ionic Basis of the Resting Membrane Potential The membrane potential is the voltage across the neural membrane at any moment – represented by Vm Vm can be measured by inserting a microelectrode into the cytosol – measurement of the potential difference between the tip of the electrode and wire placed outside the cell latrobe.edu.au Ionic Basis of the Resting Membrane Potential Electrical charge is unevenly distributed across the neural membrane – inside of the membrane is negative relative to the outside of the membrane → membrane potential The resting potential is typically -65mV (the fact that this is a negative number is very important) latrobe.edu.au Measuring the resting membrane potential latrobe.edu.au Equilibrium Potentials Consider a hypothetical cell: Inside: potassium salt solution → K⁺ and A⁻ ions (A⁻ - anion, large organic) Outside: same salt solution There is no gradient exists between the inside and outside so no net movement of ions occurs even that there are open potassium channels present in the membrane. Microelectrode records no potential difference (Vm=0) → ratio of K⁺ to A⁻ = 1 latrobe.edu.au Equilibrium Potentials Consider how this would change if potassium concentration was changed in the extracellular space: Inside: potassium salt solution → K⁺ and A⁻ ions (A⁻ - anion, large organic) Outside: same salt solution but 10x less concentrated Initially K+ pass through the channels out of the cell (moves down the concentration gradient – from high concentration area to low concentration area) Because A- is left behind, the inside of the membrane begins to acquire a net negative charge → electrical potential difference is established across the membrane latrobe.edu.au Equilibrium Potentials As the fluid acquires more and more net negative charge, the electrical force starts to pull positively charged K+ ions back through the channels and into the cell When a certain potential difference is reached the electrical force pulling K+ ions inside exactly counterbalances the force of diffusion pushing them out Thus, equilibrium state is reached in which diffusional and electrical forces are equal and opposite Electrical potential that exactly balances an ionic concentration gradient is called equilibrium potential → Eion (EK+ ≈ -80mV) latrobe.edu.au Equilibrium Potentials – Important Points Large changes in membrane potential are caused by miniscule changes in ionic concentrations The net difference in electrical charge occurs at the inside and outside surface of the membrane The phospholipid bilayer is extremely thin (< 5 nm) – electrostatic interaction of net negative charge inside cell and net positive charge outside the neuron mutually attracted → membrane said to store electrical charge → capacitance latrobe.edu.au Equilibrium Potentials – Important Points Ions are driven across the membrane at a rate proportional to the difference between the membrane potential (Vm) and the equilibrium potential (Eion) Net movement of ions only occurs if the electrical membrane potential differs from the equilibrium potential. The ionic driving force for a particular ion is Vm - Eion If the concentration difference across the membrane is known for an ion, an equilibrium potential can be calculated for that in – using the Nernst Equation latrobe.edu.au The Nernst Equation Each ion has its own equilibrium potential (EP) – the steady electrical potential that would be achieved if the membrane were permeable only to that ion: Potassium equilibrium potential: EK Sodium equilibrium potential: ENa Calcium equilibrium potential: ECa EP in mV (millivolt) can be calculated using the Nernst Equation: The charge of the ion, The temperature, Ratio of the external and internal ion concentration, Eg. K+ for twenty fold difference in concentration: EK = -80 mV latrobe.edu.au The Nernst Equation Eion= 2.303 RT/zF log [ion]o/[ion]I Eion →ionic equilibrium potential R → universal gas constant T → absolute temperature (in Kelvins) z → charge of ion F → Faraday constant [ion]o → ionic concentration outside cell (molar concentration→ M or mM) [ion]i → ionic concentration inside cell latrobe.edu.au The Nernst Equation Ionic equilibrium potentials at body temperature (37oC or 310K): latrobe.edu.au The distribution of ions across the membrane 44 latrobe.edu.au The distribution of ions across the membrane Neuronal membrane potential depends on the ionic concentrations on either side of the membrane Importantly K+ is more concentrated on the inside than the outside, and Na+ and Ca2+ ions are more concentrated on the outside of the neuron Ionic concentration gradients are established by the actions of ion pumps in the neuronal membrane 45 latrobe.edu.au Ion pumps Membrane spanning proteins come together to form ion pumps Ion pumps are enzymes that use energy released by the breakdown of ATP to transport certain ions across the membrane Critical role in neuronal signalling by transporting Na+ and Ca2+ ions from the inside to the outside of the neuron The sodium-potassium pump is a membrane protein which transports ions across the membrane against their concentration gradient, using energy in the form of ATP breakdown 46 latrobe.edu.au Sodium/Potassium pump 47 latrobe.edu.au The ionic basis of the resting potential 48 latrobe.edu.au The ionic basis of the resting potential Relative ion permeability of the membrane at rest: Resting membrane potential is close to the EK because the membrane is mostly permeable to K+ Membrane potential is sensitive to extracellular K+ concentration Increased extracellular K+ depolarises the membrane (membrane potential will be less negative) 49 latrobe.edu.au Relative ion permeabilities of the membrane at rest The pumps establish ionic concentration gradients across the membrane Neurons are not permeable to only a single type of ion The resting potential can be calculated using the Goldman equation → takes into account the relative permeability of the membrane to different ions latrobe.edu.au K+ channel Most potassium channels have four subunits Pore-loop: contributes to the selective filter that makes the channel permeable mostly to K+ MacKinnon—2003 Nobel Prize Mutations of specific K+ channels; inherited neurological disorders; epilepsy latrobe.edu.au Regulating K+ Cell is sensitive to changes in the concentration of extracellular potassium (Kalium → K+) A change of membrane potential from normal resting (-65 mV) to a less negative value is called depolarisation of the membrane → increasing extracellular K+ depolarizes neurons Importance of the blood-brain barrier Role of glia – potassium spatial buffering latrobe.edu.au Things to remember Activity of the sodium-potassium pump Large K+ concentration gradient Electrical potential difference across the membrane Similar to a battery Potassium channels Contribute to resting potential latrobe.edu.au Concluding remarks The resting potential is the consequence of the differential concentrations of ions inside and outside the neuron and the semipermeable nature of the membrane. The membrane is most permeable to potassium ions. When equilibrium is reached between the diffusion pressure forcing potassium out of the cell and the electrostatic pressure forcing potassium into the cell, the resting membrane potential is about -6570 mV. Because Na+ ions slowly diffuse across the membrane into the cell, neurons must constantly run the Na+–K+ pump to maintain this ion balance.