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University of Toronto Scarborough

NROB60 Study Package: Week 5 Homira Osman Week 5: Lecture June 3, 2008 Lecture Topics Lecture V Readings: The Resting Neuronal Membrane (Pages 52-61) Lecture Summaries [L5] - The Cast of Chemicals o At rest, the cytosol along the inside surface of the membrane has a negative electrical charge compared to the outside Referred to as the Resting Membrane Potential When a cell with excitable membrane is not generating impulses, it is said to be at rest In the resting neuron, the cytosol along the inside surface of the membrane has a negative electrical charge compared to the outside This difference in electrical charge across the membrane is called the resting membrane potential o Also called resting potential o Action Potential is the brief reversal of this condition where for one instant, the inside surface becomes positively charged compared to the outside The axonal membrane has properties that enable it to conduct a special type of signal the nerve impulse, or action potential - that overcomes these biological constraints Action potentials do not diminish over distance They are signals of fixed size and duration Information is encoded in the frequency of action potentials of individual neurons, as well as in the distribution and number of neurons firing action potentials in a given nerve Cells adaptable of generating and conducting action potentials, which include both nerve and muscle cells, are said to have excitable membrane The action in action potentials occurs at the cell membrane o How does this occur? o Understand the Three Main Players Salty Fluids on either side of the membrane The Membrane itself The Proteins that span the membrane - Cytosol & Extracellular Fluid - Water o Polar Covalently bonds o Effective solvent for other charged or polar molecules - Ions o Net electrical charge o Dissolve in water because the charged portions of the water molecule have a stronger attraction for the ions than they have for each other o A sphere of water molecules surrounds each ion (spheres of hydration) o Insulate the ions from each other o Monovalent vs divalent + o Cations (net -ve charge) o Anions (net ve charge) NROB60 Study Package: Week 5 Homira Osman - Why Are Ions Important? - Ions are the major charge carriers involved in the conduction of electricity in biological systems (including neurons) - The ions of particular importance for cellular neurophysiology are: o The monovalent cation Na + (sodium) + o The monovalent cation K (potassium) 2+ o The divalent cation Ca (calcium)- o The monovalent anion Cl (chloride) - The Resting Membrane Potential Difference (Membrane Potential) - Electrical gradient between extracellular fluid and intracellular fluid due mostly to Potassium - Phospholipid Bilayer: A Barrier - Phospholipid bilayer is not permeable to ions - Each ion has a different chemical concentration within the cell than outside - Each ion has an electrical charge - The Phospholipid Membrane QUESTION: How are electrical signals generated? ANSWER: Through ION MOVEMENT ACROSS THE CELL MEMBRANE HOW? Through a change in either the potassium concentration gradient or by changing the ion permeabilities QUESTION: How can a change in ion permeability occur? ANSWER: Insert protein channel into the membrane that is specific to a particular ion Open a channel that is specific to a particular ionNROB60 Study Package: Week 5
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