Bio3350 Lecture 3: The Neuronal Membrane at Rest PDF

LECTURE 3: THE NEURONAL MEMBRANE AT REST • • • • • • Chemical support: water, ions, phospholipids and proteins Movement of ions across a membrane Membrane potentials Resting membrane potential and potassium Equilibrium potential - Nerst’s equation Membrane potential, relative permeability and Goldm...

LECTURE 3: THE NEURONAL MEMBRANE AT REST • • • • • • Chemical support: water, ions, phospholipids and proteins Movement of ions across a membrane Membrane potentials Resting membrane potential and potassium Equilibrium potential - Nerst’s equation Membrane potential, relative permeability and Goldman’s equation Readings • Bear, chapter 3 Simulation Tool to explore the basis of membrane potentials • ngswin (http://www.nernstgoldman.physiology.arizona.edu/ ) 1 THIS LECTURE IN A NUTSHELL • What makes a neuron (electrically) excitable? • Hint: It is the ions that can pass the membrane AND what drives the ions to pass that membrane 2 COMMUNICATION WITHIN THE NERVOUS SYSTEM Sensory stimulus Activation of cutaneous mechanorecept ors by skin deformation Neural code Interpretation/Action Action potential Post-synaptic potential Flexor withdrawal reflex ??? 3 WATER AND IONS Extracellular and cytosolic fluid • Neurons are in an extracellular milieu that is water based • Water is a key ingredient of intra- and extracellular fluids • Key characteristic : polar solvant • Ions: atoms or molecules with a net electrical charge • Cations: positively charged (Na+, K+, Ca++) • Anions: negatively charged (Cl-) • Presence of spheres of hydratation 4 WATER AND IONS • Ionic composition • The neurons are in a milieu extracellular à base d’eau Ca•2+Ingredient cle of fluids intra- and Na extracellular + • Caracteristique principale: solvant polaire Ca2+ Na+ • Ions: atoms or molecules chargees electriquement Cl- Cl- K+ • Cations: charges positivement (Na+, K+, Ca++) • Anions: charges negativement (Cl-) • Presence d’une sphère d’hydratation K+ CONDITIONS FOR RESTING POTENTIAL • The excitability of neurons is the result of a plasma membrane that maintains a concentration gradient of different ions (Na+, K+, Ca2+, Cl-) • 1. 2. 3. This concentration gradient is the product of: The impermeability of the plasma membrane to the movement of ions the presence of proteins that maintain actively the gradient of concentraiton the presence of proteins that allow ions to passively cross the membrane 6 PLASMA MEMBRANE - PROTEINS 1. the impermeability of the plasma membrane to movement of ions 2. the presence of proteins that actively maintain the concentration gradient 3. the presence of proteins that allow ions to passively cross the membrane 1 2 3 EXTRACELLULAR SPACE CYTOSOL ATP ADP + Pi 7 PLASMA MEMBRANE - PROTEINS 1. Ion channels • Control of resting and action potentials 2. Receptors-channels • Neurotransmitters and post-synaptic potentials 3. Metabotropic receptors • Neurotransmitters and neuromodulators 4. Ion pumps • Producers of electrochemical gradients EXTRACELLULAR SPACE CYTOSOL ATP ADP + Pi 8 PROTEINS-CHANNELS • Polar R groups • Non polar R groups • Transmembrane domains • Ion selectivity • Mechanism of activation (gating) 9 ION PUMPS • Formed by transmembrane proteins • Catalyze ATP for energy • Active transport • Push ions AGAINST the concentration gradient • From more diluted to concentrated • Produces the ion concentration gradients • Neuronal signaling [ ] Gradient EXTRACELLULAR SPACE CYTOSOL ATP ADP + Pi 10 NA/K ATPASE ION EXCHANGE PUMP • Catalyze ATP into ADP • Drives 3 Na+ out • Brings in 2 K+ ions • Forms a concentration gradient of Na+ [Na+]o > [Na+]i Na+ K+ • Forms a concentration gradient of K+ [K+] o < [K+] i 11 WATER AND IONS • Ionic composition • The neurons are in a milieu extracellular à base d’eau Ca•2+Ingredient cle of fluids intra- and Na extracellular + • Caracteristique principale: solvant polaire Ca2+ Na+ • Ions: atoms or molecules chargees electriquement Cl- Cl- K+ • Cations: charges positivement (Na+, K+, Ca++) • Anions: charges negativement (Cl-) • Presence d’une sphère d’hydratation K+ CONCENTRATION GRADIENT 13 CONCENTRATION GRADIENT RESTING MEMBRANE POTENTIAL Cl- Na+ K+ Ca2+ EXTRACELLULAR SPACE CYTOSOL ATP Cl- ADP + Pi K+ Na+ Ca2+ 14 ION MOVEMENT ACROSS THE MEMBRANE • Diffusion • Dissolved ions redistribute in a homogeneous manner • Ions diffuse along their concentration gradient… • … when • the channels are permeables to ions • There is a concentration gradient across the membrane • • Impermeable membrane No diffusion • • Permeable membrane Diffusion according to the concentration gradients • • • Permeable membrane Diffusion Equilibrium 15 A LITTLE REMINDER ABOUT ELECTRICITY  Electrical potential (V)  Pressure exerted on ions  Influences the movement of ions + ? + ? - 16 A LITTLE REMINDER ABOUT ELECTRICITY  Electrical potential (V)  Pressure exerted on ions  Influences the movement of ions + ? + ? + 17 A LITTLE REMINDER ABOUT ELECTRICITY  Electrical potential (V)  Pressure exerted on ions  Influences the movement of ions + ? + ? + 18 A LITTLE REMINDER ABOUT ELECTRICITY  Electric current (I) Is R high or low? G? I?  Movement of ions according to Ohm’s law: V=RxI  or I = V x G +  Electric conductance (G)  = ease by which I can flow through  Resistance (R)  = capacity to block I  R=1/G + + + 19 19 A LITTLE REMINDER ABOUT ELECTRICITY  Electric current (I) Is R high or low? G? I?  Movement of ions according to Ohm’s law: V=RxI  or I = V x G +  Electric conductance (G)  = ease by which I can flow through  Resistance (R)  = capacity to block I  R=1/G + + + + + + + 20 20 A LITTLE REMINDER ABOUT ELECTRICITY  Electric current (I)  Movement of ions  Electric conductance (G) Ohm’s Law V=RxI or I=GxV  = ease by which I can flow through  Resistance (R)  = capacity to block I  R=1/G 21 MOVEMENT OF IONS ACROSS THE MEMBRANE • According to the membrane potential (V), ions can cross the membrane (I > 0) • IF the membrane is permeable + - I=0 + + I>0 + 22 - 22 EQUILIBRIUM POTENTIAL (EION) • Each ion has a potential at which the net ionic flow is 0 • I.e. membrane potential at which the movement of the inside (i) to the outside (o) is the same as in the other direction • Iion=0 or Io->i = Ii->o • E.g. The equilibrium potential for K+ would be the membrane potential at which IK = 0 23 EQUILIBRIUM POTENTIAL (EION) Difference in electrical potential that counters the diffusive force due to the concentration gradient The membrane is impermeable to K+ 24 The membrane becomes permeable to K+ and to K+ only When iK = 0, the membrane potential is EK EQUILIBRIUM POTENTIAL (EION) For example, consider the EK , i.e., the membrane potential at which the K+ current is 0, i.e., IK = 0 Gradient of K+ established by of membrane proteins Anions are present to counterbalance the positive K+ K+ A+ EXTRACELLULAR SPACE CYTOSOL 25 K K+ A- EQUILIBRIUM POTENTIAL (EION) • The ion pumps maintain a higher concentration of K+ in the inside of the cell. • Therefore concentration gradient pushes ions inside towards the outside of the neuron A+ K+ EXTRACELLULAR SPACE CYTOSOL 26 K+ K Gradient of K+ pushes the K+ A- EQUILIBRIUM POTENTIAL (EION) The K+ ions that accumulate at the outside have 2 effects: • They diminish the concentration gradient • They turn the inside more electrically negative than the outside A+ K+ EXTRACELLULAR SPACE K K K K CYTOSOL 27 K+ K Gradient of K+ pushes the K+ A- EQUILIBRIUM POTENTIAL (EION) The K+ ions that accumulate at the outside have 2 effects: • They diminish the concentration gradient • They turn the inside more electrically negative than the outside Electric gradient pushes the K+ ions towards the inside K+ EXTRACELLULAR SPACE K K K K K A+ K K K CYTOSOL 28 K+ K Gradient of K+ pushes the K+ A- K EQUILIBRIUM POTENTIAL (EION) The membrane potential when IK+, o->i = IK+, i->o Electric gradient pushes the K+ ions towards the inside IK+, o->i K+ K EXTRACELLULAR SPACE K K K K K K A+ K K CYTOSOL IK+, i->o 29 K+ K Gradient of K+ pushes the K+ A- K K K EQUILIBRIUM POTENTIAL OF SODIUM (ENA) • Gradient of Na+ Na+outside> Na+inside Scenario: Membrane is impermeable to Na+ inside outside We permeabilize the membrane to Na+ inside outside Eventually, the membrane potential will be at ENa inside outside 30 EQUILIBRIUM POTENTIAL (EION) • 5 important points: inside outside 1. Larges changes of Vm • But minuscule changes in ion concentrations 2. Net difference in electric charge • There is actually an equal number of positive and negative ions on each side of the membrane • But there is more cations on the immediate outside of the plasma membrane and more anions on the immediate inside of the plasma membrane 31 inside outside EQUILIBRIUM POTENTIAL (EION) • 4 important points : 3. Ion current (Iion) • Proportional to Vm-Eion and to the conductivity of the membrane to that ion (gion) • Iion = gion (Vm – Eion) • Vm – Eion : the electric force that drives the ions in a direction • gion : the permeability of the membrane to that ion 32 inside outside inside outside EQUILIBRIUM POTENTIAL : NERNST’S EQUATION 4. Difference of concentration allow us to calculate the equilibrium potential of each ion, Eion • Eion (exact value of the equilibrium potential for the ion A in mV) for a membrane can be calculated using the Nernst equation • Takes into account: • Charge of the ion A (z) EA= RT ln [A]o zF [A]i • Temperature (T) • Ratio of outside and inside concentrations of the ion or EA= 0,0615 log[A]o [A]i R= Ideal gas constant (joules / kelvin . mole) F= Faraday constant 33 EQUILIBRIUM POTENTIAL : NERNST’S EQUATION 5. Eion are essentially constants. In the lifetime of a neuron, the values of Eion does not change very much Ca2+ Na+ Ca2 + Na+ Cl- K+ K+ Cl34 34 EQUILIBRIUM POTENTIAL What happens to the equilibrium potential EK … 1. if [K+ ]o equals [K+ ]i? 2. if [K+ ]i quadruples? 3. if [K+ ]o quadruples? 35 Is the membrane potential determined solely by the concentrations of K+? 36 EQUILIBRIUM POTENTIAL, NERST’S EQUATION Nernst’s equation EA= RT ln [A]ex zF [A]in or EA= 0,0615 log[A]ex [A]in 37 RESTING POTENTIAL • Membrane potential : Vm = voltage across the membrane • Resting potential Membrane potential when the neuron is inactive = -65mV (approx.) 38 EK AND ENA SET THE UPPER AND LOWER BOUNDS OF VM Physiological range of Vm -80mV 0mV +40mV ∝ m ∝ I MEMBRANE POTENTIAL (VM): GOLDMAN’S EQUATION • • • • In reality, the neurons are permeable to many ions present Vm depends on the relative permeabilities Goldman's eq calculates the Vm in the presence of many ions Goldman's eq calculates the instantaneous Vm according to the relative permeability at a given time • Goldman's eq for the principal ions Na+, K+, Cl- is: Vm= RT F ln [K+]ex + PNa/PK [Na+]ex + PCl/PK [Cl-]in [K+]in + PNa/PK [Na+]in + PCl/PK [Cl-]ex • Note 1: at 37oc, RT/F= 26,4mV ; at 20oc, RT/F= 25mV • Note 2: the gradient of Cl- is inverse due to its negative charge 40 RESTING POTENTIAL • We can conclude, based upon the proximity of Vm at rest and EK that… • the membrane is predominatly permeable to K+ ions • K+ channel of type Shaker is the « base model » • But there are many K+ channels 41 + STRUCTURE OF K CHANNELS • 4 transmembrane subunits • Hairpin pore endows selectivity and permeability to K+ ions • Mutations of K+ channels • Hereditary neurological disorders • epilepsy, etc. 42 RESTING POTENTIAL AND RELATIVE PERMEABILITY • Vm at rest is near to EK because of the large permeability to K+ • Vm is sensitive to extracellular concentrations of K+ ions • Increase of extracellular K+ ions depolarizes the Vm 43 RESTING POTENTIAL • Since resting Vm is very dependant to the [K+]o, [K+]o is regulated by • Blood brain barrier • Potassium spatial buffering • Astrocyte 44 MEMBRANE POTENTIAL  What happens to the membrane potential Vm … 1. if [K+ ] outside equals [K+ ] inside? 2. if intracellular [K+ ] quadruples? 3. if the permeability of K+ is increased? 4. if the permeability of Na+ is increased? 45 are EIONS AND VM +100 mV ENa ges o CaCa Ca Ca e of 2+ 2+ 2+ Cl- Cl- 2+ K+ Cl- K+ Cl- K+ K+ Na+ Na+ Na+ Na+ ECa EXTRACELLULAR SPACE 0 mV CYTOSOL Ca2+ Ca2+ Ca2+ Ca2+ ATP ClCl- Cl- K+ Cl- EK ADP + Pi K+ K+ K+ ECl Na+ Na+ Na+ Na+ -100 mV 46