Neurons and Neuronal Communication Lecture Notes PDF

Institute of Psychiatry, Psychology & Neuroscience 2023-2024 Dr Philip R Holland 4BBY1030 Cell Biology & Neuroscience Neurons and Neuronal communication: Establishing electrochemical potentials and axon potentials. How are electrical potentials measured? Extracellular Recording (electrode outside ce...

Institute of Psychiatry, Psychology & Neuroscience 2023-2024 Dr Philip R Holland 4BBY1030 Cell Biology & Neuroscience Neurons and Neuronal communication: Establishing electrochemical potentials and axon potentials. How are electrical potentials measured? Extracellular Recording (electrode outside cell) DD/Month/YYYY Professor/Dr: Intracellular Recording (electrode inside cell) Topic title: Patch Clamping (electrode sealed to cell surface) How are electrical potentials measured? Brain slice/cell recording DD/Month/YYYY Professor/Dr: In-vivo recording 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: Basic Principles DD/Month/YYYY Professor/Dr: Topic title: Hodgkin & Huxley 1939 How are electrical potentials measured? 100 mV Intracellular Recording (electrode inside cell) DD/Month/YYYY Professor/Dr: 0.1 mV Extracellular Recording (electrode outside cell) Topic title: Basic Principles: Patch Clamp Recording DD/Month/YYYY Professor/Dr: Topic title: Electrochemical Gradients The electrochemical gradient across a membrane is established because: 1. Specialised pumps move ions against their concentration gradients (energy dependent). 2. The membrane is relatively impermeable, so movement of ions is restricted to specialised channels in the membrane. 3. The membrane acts to separate and store ionic charge differentials between the outside and inside of the cell. 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: The Resting Membrane Potential The resting membrane potential (Vm) is typically around -70 mV. It is principally determined by Na+ and K+ 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: How much work is done at the membrane? + The amount of work depends on the size of the concentration gradient. Concentration gradient = [C]out / [C]in for +ve ions. = [C]in / [C]out for -ve ions. Once we know the concentration gradient you can then calculate the membrane voltage (potential) due to a specific ion via the Nernst equation. Nernst equation: E = 58 (mV) x log [C]out / [C]in. 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: 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+ + + + + +++ __ _ _ ____ Electrostatic force organic anion Cl- K+ 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: Permeability & Conductance K+ K+ K+ DD/Month/YYYY Professor/Dr: K+ K+ Topic title: K+ Permeability & Conductance K+ K+ K+ X K+ DD/Month/YYYY Professor/Dr: K+ Topic title: K+ X K+ K+ 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 Ignoring permeability Considering permeability Vm = 58 x log [4 + 140] [140 + 10] Vm = 58 x log [50x4] + 140 [50x140] + 10] = -1.0 mV DD/Month/YYYY Professor/Dr: = -76 mV Topic title: Practical Uses: Optogenetics Blue light Open (cations) flow in Open (Cl-) flow in depolarize DD/Month/YYYY Professor/Dr: Yellow light Topic title: hyperpolarize 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. 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: Functional States of Ion Channels Open (active) Closed (resting) Not all channels will have all 3 states V-gated Na channels have all 3 V-gated K channels have no inactivation state DD/Month/YYYY Professor/Dr: Topic title: Inactive (refractory) The Action Potential 5 4 3 1 2 1. Resting membrane potential 2. Depolarizing stimuli 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 8 9 6. Potassium ions (K+) moves out of the neuron 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: The Action Potential +30mV Vm gNa 0mV Depolarisation 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+ ++ + ++ + + + + + + + + + + _ _ ___ _ ___ _ + ___ _ _ ___ _ +++ X DD/Month/YYYY Professor/Dr: Topic title: Conduction in Non-Myelinated & Myelinated Axons Non-myelinated Myelinated DD/Month/YYYY Professor/Dr: Topic title: [email protected]