Chapter 8 Slides PDF

Excitability and concentration gradients Excitability: the ability of a cell to send and receive electrical signals across the plasma membrane. Concentration gradient: difference in concentration of a substance between two...

Excitability and concentration gradients Excitability: the ability of a cell to send and receive electrical signals across the plasma membrane. Concentration gradient: difference in concentration of a substance between two compartments. Typical ion concentrations inside Visual representation of and outside a neuron. concentration gradients Ion [Ion]inside [Ion]outside Potassium 140mM 5mM Sodium 15mM 150mM Chloride 10mM 120mM Calcium 0.008mM 5mM 1 How to calculate concentration gradients GENERAL [Na+]INSIDE [Na+]INSIDE EXAMPLE EQUATION INCREASES DECREASES [Na+]OUT = 150 mM [Na+]OUT = 150 mM [Na+]OUT = 150 mM [Na+]IN = 15 mM [Na+]IN = 50 mM [Na+]IN = 5 mM Gradient = Na+ gradient Na+ gradient Na+ gradient [Ion]OUT – [Ion]IN =150 mM – 15 =150 mM – 50 =150 mM – 5 mM mM mM =145 mM =135 mM =100 mM Na+ gradient Na+ gradient INCREASES DECREASES Moving DOWN a concentration gradient = moving from HIGH to LOW concentration Moving UP a concentration gradient = moving from LOW to HIGH concentration 2 Ion channels transport ions across a membrane VOLTAGE- LIGAND- KEY POINTS ABOUT ION CHANNELS: GATED GATED 1. Ion channels only allow passive transport Only allow ions to move DOWN their concentration gradient Flux increases with as concentration gradient increases Active transport occurs via transporters and pumps 2. Channels are selective for specific Gated by Gated by ions change in chemical charge messenger E.g. Na channels only allow Na + + 3 KEY POINT: relative concentrations during homeostasis In a healthy person, the relative concentration of any one ion is CONSTANT.  The millions of molecules moving across a concentration gradient represent only a TINY fraction of the molecules present in the interstitial fluid and cytoplasm.  The concentration of sodium (for example) will ALWAYS be 10 times greater outside the cell than inside. 4 Ions are charged particles ELECTROSTATIC FORCES: If ions are distributed unevenly across a membrane, then there is uneven distribution of charge across that membrane. If ions are permeable and being transported across the membrane, then the charge distribution across the membrane is also changing. 5 Membrane potential across the plasma membrane MEMBRANE POTENTIAL = a form of potential energy created by a difference in charge between two environments When ions move across the membrane, they change the electrochemical gradient. This allows cells to use the stored energy (membrane potential) for critical cellular processes. 6 Membrane potential can be experimentally measured 7 The concept of electrochemical equilibrium E.g. Na+ is repulsed by the accumulation of positive charge inside the cell BALANCE of forces inward versus outward E.g. Na+ flows from high concentration to low 8 The Nernst equation calculates the electrochemical equilibrium point THE NERNST EQUATION DEFINITIONS Eion = the equilibrium potential for an ion 61.5 mV = a constant that assumes Eion = 61.5 mV / z log10[ion]out / the cell is at 37°C [ion]in log10[ion]out / [ion]in refers to the concentrations of the ion inside and outside of the cell z = charge of the ion 9  Some common ion equilibrium potentials IMPORTANT NOTES: If concentration gradient = 0, equilibrium potential = 0 Ion [Ion]inside [Ion]outside Eion Potassium 140mM 5mM -89.0mV Equilibrium potential is calculated Sodium 15mM 150mM +61.5mV separately for each ion Chloride 10mM 120mM -66.0mV Ion concentrations are regulated homeostatically, so equilibrium potentials generally do not change 10  Equilibrium potential is not the same as membrane potential Membrane potential also depends on ION PERMEABILITY across the membrane. Equations to calculate MEMBRANE PERMEABILITY Parallel Conductance Equation: V [Ion]insi [Ion]outsi Eion Pion de de Potassiu 140mM 5mM - 1 m 89.0m V Goldman-Hodgkin-Katz Equation: Sodium 15mM 150mM +61.5 0.04 mV Chloride 10mM 120mM - 0.45 66.0m 11 V Movement of ions at resting membrane potential At resting membrane potential: 12 NET FLUX = 0 The relationship between resting membrane potential and equilibrium potentials Permeability of an Resting membrane potential moves ion TOWARDS that ion’s equilibrium INCREASES potential Permeability of an Resting membrane potential moves ion AWAY FROM that ion’s equilibrium DECREASES potential 13 Summary so far… Ions have a concentration gradient across the plasma membrane. Ions will move down their concentration gradient through ion channels. Ions, because they are charged particles, are subject to electrostatic forces that influence their movements. Membrane potential is created by the uneven distribution of ions across the membrane. Membrane potential changes when ion concentration gradient change, or when changes in permeability change the flux (flow) of ions across the membrane 14 The neuron is an excitable cell NEURON: detects a stimulus and transduces it into an electrical signal. DENDRITES: projections that SOMA: cell body; contains extrude from the cell body; nucleus, organelles and site of signal input majority of cytoplasm AXON HILLOCK: “trigger zone”; where action potential AXON: long process extruding is generated from soma; where action potential travels AXON TERMINAL: distal portion of the axon; site of signal output 15 Neurons relay messages through synapses PRE-SYNAPTIC CELL: neuron located before a synapse; sends the signal SYNAPTIC CLEFT: gap between two cells filled with interstitial fluid POST-SYNAPTIC CELL: neuron located after a synapse; receives the signal 16 Graded potentials occur at the dendrite GRADED POTENTIAL: Transient change from resting membrane potential Decreases in intensity over time and distance DENDRITE: Contains mainly ligand-gated channels 17 Changes in membrane potential Membrane potential moves back to resting value Membrane potential Membrane potential moves from rest to a more moves from rest to a more 18 positive value negative value Graded potentials in dendrites are called synaptic potentials Types of synaptic potentials: DEPOLARIZING REPOLARIZING 19 Graded potentials are decremental Changes in membrane potential are RESTRICTED to local area where ions are moving The further away from the point of origin, the smaller the change in membrane potential 20 Summation of graded potentials suprathreshol d subthreshold 21 Phases of an action potential AXON: contains voltage-gated channels that respond to local electrical changes 22 The depolarization phase is characterized by rapid sodium entry Absolute refractory 23 The repolarization phase is accelerated by potassium efflux REPOLARIZATION Voltage-gated potassium channels reach peak PHASE permeability Still part of absolute refractory period Potassium channels still open HYPERPOLARIZATI Inactivation gate on sodium channels opens ON PHASE Relative refractory period – cell is hyperpolarized, so requires greater stimulus to reach threshold AFTER- Potassium permeability reduced HYPERPOLARIZATIO Membrane potential goes back to resting value N PHASE 24  Action potential propagation Action potential Axon is “recharged” at segments each segment by fresh flow of Na+. 25 Factors affecting action potential propagation AXON DIAMETER MYELINATION Internode Wider axons = Faster action potential Node of propagation Ranvier Produced by Schwann cells and oligodendrocytes Acts as electrical insulation 26 Chemical and electrical synapses CHEMICAL ELECTRICAL Specialized gap junction Current spreads passively across gap junction Allows action potential from one cell to move rapidly into another Avoids delay inherent in chemical synapses Types of neurotransmitters: amino acid Cannot be modulated derivatives, peptides, proteins, amines, gases, other chemical ligands E.g. cardiac cells 27 Post-synaptic responses at chemical synapses are fast or slow FAST responses: Ligand-gated ion channels open immediately upon neurotransmitter binding SLOW responses: Neurotransmitter binds to a G- protein coupled receptor that initiates a signaling cascade Downstream effectors eventually trigger the opening of ion channels 28