Establishing Electrochemical Potentials and Action Potentials PDF

Add Portrait Photo Institute of Psychiatry, Psychology & Neuroscience 2023-2024 Dr Philip R Holland MBBS: Physiology & Anatomy of Systems Establishing electrochemical potentials and axon potentia...

Add Portrait Photo Institute of Psychiatry, Psychology & Neuroscience 2023-2024 Dr Philip R Holland MBBS: Physiology & Anatomy of Systems Establishing electrochemical potentials and axon potentials. Learning Outcomes General: Know the concentrations of Na+ and K+ ions inside and outside of cells Describe how electrical events in excitable cells are measured Describe the properties of cell membranes as applied to the lectures Explain in simple terms, potential difference, current and capacitance in relation to cell membranes 1. Electrochemical equilibria and the membrane potential Understand how the membrane separates and stores charges Understand how Na+ and K+ ion channels contribute in generating the membrane potential (MP) Understand the significance of the differential permeability of the membrane to Na+ and K+ Describe the establishment of electrochemical gradients (sodium pump, voltage dependent ion channels, membrane capacitance) 2. Action potential, propagation and nerve conduction Describe the properties of the action potential (AP) Explain the voltage and conductance changes during the AP capacitance) Explain what is meant by ‘threshold’ and ‘refractory periods’ Explain how the AP is conducted and the importance of myelin DD/Month/YYYY Professor/Dr: Topic title: How are electrical potentials measured? Extracellular Recording Intracellular Recording Patch Clamping (electrode outside cell) (electrode inside cell) (electrode sealed to cell surface) DD/Month/YYYY Professor/Dr: Topic title: Basic Principles At rest the inside of the membrane is more negatively charged than the outside (hyperpolarised). When cells become activated, the inside of the membrane becomes more positively charged (depolarised). DD/Month/YYYY Professor/Dr: Topic title: Basic Principles DD/Month/YYYY Professor/Dr: Topic title: How are electrical potentials measured? 100 mV 0.1 mV Intracellular Recording Extracellular Recording (electrode inside cell) (electrode outside cell) DD/Month/YYYY Professor/Dr: Topic title: Extracellular Recordings EMG ECG EEG DD/Month/YYYY Professor/Dr: Topic title: The Resting Membrane Potential The resting membrane potential (Vm) is typically around -70 mV. It is principally determined by Na+ and K+ ions (also Ca2+ ions). The equilibrium potential of an ion is the membrane voltage required to prevent movement of an ion down its concentration gradient. If the inside of the cell is very negative, K+ will be prevented from leaving. If the inside of the cell is very positive, Na+ will be prevented from entering. DD/Month/YYYY Professor/Dr: Topic title:  The Membrane Na+/K+-ATPase Na+ channels K+ channels The membrane surrounds the entire neuron providing a hydrophobic relatively impermeable barrier. It is composed of lipids and proteins, with ion channels and pumps providing entry and exit routes for ions. DD/Month/YYYY Professor/Dr: Topic title:  Na+/K+-ATPase pump and the Resting Membrane Potential K+ ATP Na+ The Na+/K+-ATPase pump uses energy (ATP) to actively pump three sodium (Na+) and two potassium (K+) ions out and into the cell, respectively, maintaining a more depolarised internal environment. DD/Month/YYYY Professor/Dr: Topic title:  Sodium Channels Na+ Sodium (Na+) channels permit the rapid influx of sodium into the cell upon opening, with resultant depolarisation (more positive). DD/Month/YYYY Professor/Dr: Topic title:  Potassium Channels K+ Potassium (K+) channels permit the rapid efflux of potassium out of the cell upon opening, with resultant hyperpolarisation (more negative). DD/Month/YYYY Professor/Dr: Topic title: The membrane Potential ++ + + +++ -70 mV __ _ ____ _ organic anion The specific ionic distribution across the membrane sets the resting membrane potential. DD/Month/YYYY Professor/Dr: Topic title:  The membrane Potential Try: http://www2.yvcc.edu/Biology/109Modules/Modules/RMP/RMP.htm Na+ Cl- + + + + +++ __ _ ____ _ Electrostatic force K+ organic anion Force of diffusion Ions are under two specific forces, the electrostatic force (dependent on charge) and the force of diffusion (dependent on concentration). DD/Month/YYYY Professor/Dr: Topic title: The Resting Membrane Potential Using the Nernst equation: For physiological concentrations. EK = -90 mV ENa = +50 mV Cell needs to be at -90 mV to stop K+ leaving and +60 mV to stop Na+ entering. Vm is much closer to EK than ENa because the membrane has many more (x50) K+ than Na+ channels: more permeable to K+. At constant Vm, net flow of ions is zero because the passive flow of K+ out is matched by the leak of Na+ in. DD/Month/YYYY Professor/Dr: Topic title: EK dominates the RMP +6 ENa 0 mV -70 Vm -90 EK If a cell becomes permeable to an ion then that ion will move down its electrochemical gradient and will drive Vm towards the equilibrium potential for that ion. DD/Month/YYYY Professor/Dr: Topic title: Driving Force on Ion = Vm -Eeq For K+ = -70mV – (-90 mV) = +20 mV Inside Outside -70 mV 0 mV +20 mV Forcing K+ OUT K + K+ DD/Month/YYYY Professor/Dr: Topic title: Driving Force on Ion = Vm -Eeq For Na+= -70mV – (+50mV) = -120 mV Inside Outside -70 mV 0 mV -120 mV forcing Na+ IN Na+ Na+ DD/Month/YYYY Professor/Dr: Topic title: Driving Force on Ion = Vm -Eeq IN OUT K + K+ 20 mV K+ OUT + - 120 mV Na+ IN Na Na+ + DD/Month/YYYY Professor/Dr: Topic title: Permeability & Conductance High conductance K+ K+ K+ K+ K+ K+ Low conductance DD/Month/YYYY Professor/Dr: Topic title: Permeability & Conductance High conductance K+ K+ K+ K+ X X K+ K+ K+ K+ Low conductance DD/Month/YYYY Professor/Dr: Topic title: The Resting Membrane Potential The Nernst equation only deals with one ion at a time. Given the different permeability of different ions, we need to use the Goldman Hodgkin Katz equation to calculate Vm based on different ions. Vm = 58 mV x log [PK[K+]out + PNa[Na+]out [PK[K+]in + PNa[Na+]in DD/Month/YYYY Professor/Dr: Topic title: The Resting Membrane Potential In mM OUT mM Na 10 140 K 140 4 Vm = 58 mV x log [PK[K+]out + PNa[Na+]out Permeability of membrane to K+ is 50 fold [PK[K+]in + PNa[Na+]in greater than Na+ Ignoring permeability Considering permeability Vm = 58 x log [4 + 140] Vm = 58 x log [50x4] + 140 [140 + 10] [50x140] + 10] = -1.0 mV = -76 mV DD/Month/YYYY Professor/Dr: Topic title: The Action Potential: Basic Principles 1. Triggered by a depolarising stimuli. 2. There is a specific threshold of depolarisation required to trigger an action potential. 3. It is an all or nothing event, you don’t get ½ an action potential. 4. Propagates without decrement (faster in larger myelinated axons). 5. At its peak: Vm approached ENa. 6. After an action potential the membrane is inexcitable during its refractory periods. DD/Month/YYYY Professor/Dr: Topic title: The Action Potential 5 4 3 1 2 1. Resting membrane potential 2. Depolarizing stimuli (more +) 3. Depolarization reaches threshold: voltage-gated sodium channels (NaV) open and sodium ions (Na+) enter neuron 4. Rapid Na+ entry depolarizes the neuron further 5. NaV channels inactivate and slower (0.5 mS) potassium channels (Kv) open DD/Month/YYYY Professor/Dr: Topic title:  The Action Potential 5 4 6 3 1 2 7 9 8 6. Potassium ions (K+) moves out of the neuron (repolarizing, more -) 7. Kv channels remain open and more K+ leaves the neuron, hyperpolarizing it 8. Kv channels close, some K+ enters cell through leak channels 9. Normal membrane potential DD/Month/YYYY Professor/Dr: Topic title: Refractory Periods Absolute Relative Absolute results from the inactivation of Na+ channels, and lasts until the resting membrane potential is restored Relative results from the hyperpolarization phase, during which a greater stimuli is needed to reach threshold DD/Month/YYYY Professor/Dr: Topic title:  0 0 mV mV Functional States of Ion Channels - - - -- -- ---- -55 -70 - - - -- -- ---- -55 -70 Na + Na + Na + Na + 1. 1. Na + 30 30 30 30 30 ++ ++ + +++ + ++ + + - +- +- + +- -+ -0 - -- - - - - - - - --- -- -- - - -- ---- 0 0 0 0 mV mV mV mV mV -55 -55 -55 -55 - - - -- - - --- -- -- - -55 - -70 ++ -- ---- ++ -70 ++ + +++++ + -70 + + + ++++ + -70 + +++ + + -70 Na + 2. 2. Closed (resting) Na + Na + Open (active) Inactive (refractory) Na + 30 30 30 30 ++ ++ + +++ + ++ + + +++ + + +0 + ++ + +++ + ++ + + +++ + + 0 0 0 mV mV mV mV -Not - - -all - channels - - --- -- -- will have- -all - - -3 - -55 - - states - - -- -55 - -70 - - --- -- -- --55 - -- ---- -55 -70 -70 -70 V-gated Na channels Na + have all 3 Na + 30 30 - - - -- - - --- -- -- - - -- ---- V-gated K channels 0 have no mV 0 mV ++ ++ inactivation + +++ + state -55 + + + -70 + -55 + + + + + -70 DD/Month/YYYY Professor/Dr: Topic title: The Action Potential Vm +30mV gNa Depolarisation 0mV gK Threshold -90mV Hyperpolarisation After-hyperpolarisation Threshold to open voltage-dependent, transient Na+ channels Delayed, persistent K+ channel DD/Month/YYYY Professor/Dr: Topic title: Action Potential Conduction: Non-Myelinated _Na __ _ + + + + + +++ + ++ ++ + + + + + + + _ __ _ _ _ _ __ _ __ _ __ + _ Na + _+_+ _ + + + + + ++ + + + + + + ++ _ __ _ __ _ __ _ _ _ _ __ + K+ + + + + + + + + _ _+ _ + ++ + + + + + Na+ _ _ _ _ _ _ _ _ __ _ __ _ __ _ __ K+ DD/Month/YYYY Professor/Dr: Topic title: Action Potential Conduction: Myelination DD/Month/YYYY Professor/Dr: Topic title: Action Potential Conduction: Myelinated Na+ ___ + + + + ++ + + + + + + _ _ _ _ + ___ _ + ___ _ ++ + + Na+ ++ + ++ + + + + + + + + + + ___ _ ___ _ _ _ + ___ _ _ ___ _ +++ DD/Month/YYYY Professor/Dr: Topic title: X Conduction in Non-Myelinated & Myelinated Axons Non-myelinated Myelinated DD/Month/YYYY Professor/Dr: Topic title: Learning Outcomes General: Know the concentrations of Na+ and K+ ions inside and outside of cells Describe how electrical events in excitable cells are measured Describe the properties of cell membranes as applied to the lectures Explain in simple terms, potential difference, current and capacitance in relation to cell membranes 1. Electrochemical equilibria and the membrane potential Understand how the membrane separates and stores charges Understand how Na+ and K+ ion channels contribute in generating the membrane potential (MP) Understand the significance of the differential permeability of the membrane to Na+ and K+ Describe the establishment of electrochemical gradients (sodium pump, voltage dependent ion channels, membrane capacitance) 2. Action potential, propagation and nerve conduction Describe the properties of the action potential (AP) Explain the voltage and conductance changes during the AP capacitance) Explain what is meant by ‘threshold’ and ‘refractory periods’ Explain how the AP is conducted and the importance of myelin DD/Month/YYYY Professor/Dr: Topic title: [email protected]