Lecture Outline ◦ Setting the Stage: The cast of chemicals Fluid Composition Phospholipid Membrane Proteins Movement of Ions At rest, the cytosol (intracellular) along the inside surface of the membrane has a negative electrical charge compared to the outside ◦ Referred to as the Resting Membrane Potential Understand the Three Main Players ◦ Salty Fluids on either side of the membrane ◦ The Membrane itself ◦ The Proteins that span the membrane δ Water ◦ Polar Covalently bonds ◦ Effective solvent for other charged or polar molecules ◦ Key ingredient in intracellular and extracellular fluid ◦ Key feature = uneven charge δ δ Ions ◦ Net electrical charge ◦ Dissolve in water because the charged portions of the water molecule have a stronger attraction for the ions than they have for each other ◦ A sphere of water molecules surrounds each ion (spheres of hydration) ◦ Insulate the ions from each other ◦ Monovalent vs divalent ◦ Cations (net +’ve charge) ◦ Anions (net -’ve charge) Ions are the major charge carriers involved in the conduction of electricity in biological systems (including neurons) Ions of particular importance for cellular neurophysiology: ◦ The monovalent cation Na+ (sodium) ◦ The monovalent cation K+ (potassium) ◦ The divalent cation Ca2+ (calcium) ◦ The monovalent anion Cl- (chloride) First a review of terms ◦ Hydrophilic Dissolve in water due to uneven electrical charge (e.g., salt) Water loving ◦ Hydrophobic Does not dissolve in water due to even electrical charge (e.g., oil) Water fearing ◦ Lipids are hydrophobic Are a class of water-insoluble biolgoical molecules important to the structure of cell membranes Contribute to resting and action potentials Phospholipid bilayer is not permeable to ions Each ion has a different chemical concentration inside and outside of the cell Each ion has an electrical charge BARRIER A Few Quick Questions QUESTION: How are electrical signals generated? A Few Quick Questions QUESTION: How can a change in ion permeability occur? A Few Quick Questions QUESTION: What are the Four major types of selective ion channels in the neuron? A Few Quick Questions QUESTION: What are the 3 activation stimuli to open or close a channel? Type and Distribution of Protein Molecules distinguish neurons from other types of cells ◦ Enzymes ◦ Cytoskeleton ◦ Receptors ◦ Special transmembrane proteins Control resting and action potentials HYDROPHOBIC HYDROPHILIC Every amino acid has in common: ◦ a central alpha carbon ◦ An amino group ◦ A carboxyl group Variability for amino acids comes from the R group 20 different amino acids to make proteins Assemble into chains connected by peptide bonds Form polypeptides Primary structure is the sequence of amino acids in the polypeptide Secondary structure is the coiling of a polypeptide into a conformation such as an alpha helix Tertiary structure is the three-dimensional folding of a polypeptide Quaternary structure is when different polypeptides bond together to form a larger protein ION CHANNELS ◦ Typically requires 4-6 similar protein molecules assembled to form a pore between them ◦ Composition varies and determines their properties Diameter of pore and nature of R groups determines ION SELECTIVITY Also determines GATING properties Changes in the local microenvironment of the membrane can cause these channels to be opened or closed ION PUMPS Formed by membrane spanning proteins that use ATP to transport certain ions across the membrane Critical role in neuronal signaling by transporting Na and Ca from inside to outside neuron 1. Diffusion ◦ Dissolved ions distribute evenly ◦ Ions flow down concentration gradient ◦ Channels permeable to specific ions ◦ Concentration gradient across the membrane 2. Electricity For electrically charged particles (such as IONS), an electrical field can also be used to induce the ions to move ◦ Opposite charges attract ◦ Like charges repel Net movement of sodium toward the negative terminal (cathode) Net movement of chloride toward the positive terminal (anode) The movement of electrical charge is the ELECTRICAL CURRENT Positive current 2 Important Factors Affect Current Flow ELECTRICAL POTENTIAL (voltage, V) o The force exerted on a charged particle ◦ Reflects the difference in charge between the anode and the cathode ◦ As the difference increases, more current will flow ELECTRICAL CONDUCTANCE (g) o The relative ability of an electrical charge to migrate from one point to another ◦ Depends on: the number of particles available to carry electrical charge the ease with which these particles can travel through space o Electrical Resistance (R) is the same property expressed in a different way ◦ The relative inability of an electrical charge to migrate OHM’S LAW: I = gV the amount of current equals the product of electrical conductance and electrical potential. Note I = amount of current that will flow What will happen if the conductance is zero? Remember Ohm’s Law V= IR then Iion = gion (Vm-Eion) where Iion = ionic current Vm = membrane potential Eion = equilibrium potential for the ion gion = conductance Difference Vm-Eion is the electrochemical driving force acting on the ion We have electrically charged ions in solution on either side of the neuronal membrane Movement of any ion through its channel depends on the concentration gradient and the difference in electrical potential across the membrane Let’s explore further Membrane potential: Voltage across the neuronal membrane Bear video link Let’s review K+ equilibrium Equilibrium Potentials ◦ No net movement of ions when separated by a phospholipid membrane ◦ Equilibrium reached when K+ channels inserted into the phospholipid bilayer Equilibrium Potentials (Cont’d) ◦ The Nernst Equation Box 3.2 Calculates the value of an equilibrium potential in mV Takes into consideration: Charge of the ion Temperature Ratio of the external and internal ion concentrations NOTE: you will not be asked to use the Nernst equation in this course E ion = 2.3 RT/zF log[ion]outside/[ion]inside E ion ionic equilibrium potential R gas constant T absolute temperature Z charge of the ion F faraday’s constant Log base 10 logarithm [ion] inside or outside of the cell K channels ◦ K+ channels: 4 subunits ◦ Channel selectively permeable to K+ ions ◦ MacKinnon—2003 Nobel Prize Mutations of specific K+ channels; Inherited neurological disorders eg: weaver mouse • a strain of mice that has difficulty maintaining posture and moving normally. • defect has been traced to the mutation of a single AA in the pore loop of a potassium channel found in specific neurons of the cerebellum. • mutation allows Na as well as K can pass through the channel Examples of K channels Structure of a simple bacterial K+ channel determined by crystallography The Distribution of Ions Across The Membrane Note: E ion is the membrane potential that would be achieved at body temperature if the membrane were selectively permeable to that ion The sodium-potassium pump ◦ Enzyme - breaks down ATP when Na present ◦ Calcium pump: Actively transports Ca2+ out of cytosol Characteristics of Resting Potential large proteins contribute to the negative electrical potential Na+ 10Xs more concentrated outside than inside K+ 20Xs more concentrated inside K+ and Cl- remain open allowing both ions to flow through; Na+ gates remain closed Na+-K+ pump transports Na+ outside of the cell while drawing K+ into the cell RP of a neuron provides a baseline level of polarization Relative Ion Permeabilities of the Membrane at Rest ◦ The importance of Regulating the External Potassium Concentration Depolarization What happens if cell depolarizes? Increasing extracellular potassium depolarizes neurons Relative Ion Permeabilities of the Membrane at Rest ◦ Neurons permeable to more than one type of ion ◦ Membrane permeability determines membrane potential (changes) ◦ Goldman equation Takes into account permeability of membrane to different ions WHY is the average resting membrane potential for NEURONS -65 mV? To answer this question, we need to combine all the equilibrium potentials of all the ions and take into account their permeability (their ability to move across the cell membrane) The GHK Equation (box 3.3) Vm = 61 log Pk[K+]out + PNa[Na+]out + PCl[Cl-]out z Pk[K+]in PNa[Na+]in PCl[Cl-]in The GHK equation predicts membrane potential using multiple ions Relative Ion Permeabilities of the Membrane at Rest ◦ The importance of Regulating the External Potassium Concentration Blood-Brain barrier Potassium spatial buffering 1. RP is the consequence of the differential concentrations of ions inside and outside the neuron, AND the semipermeable nature of the membrane 2. Membrane is primarily permeable to K ions. When equilibrium is reached between the diffusion pressure forcing K out of the cells, and the electrostatic pressure forcing K into the cell, RP is -60 to -70 3. Neurons must actively maintain this ion balance via the Na/K pump due to the gradual leakage of ions across the membrane