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University of Toronto St. George
Human Biology

NROB60 Study Package Week 5 Homira OsmanWeek 5 LectureJune 3 2008Lecture TopicsLecture VReadings The Resting Neuronal Membrane Pages 5261Lecture Summaries L5The Cast of Chemicalso At rest the cytosol along the inside surface of the membrane has a negative electrical charge compared to the outsideReferred to as the Resting Membrane PotentialWhen a cell with excitable membrane is not generating impulses it is said to be at restIn the resting neuron the cytosol along the inside surface of the membrane has a negative electrical charge compared to the outsideThis difference in electrical charge across the membrane is called the resting membrane potentialo 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 outsideThe axonal membrane has properties that enable it to conduct a special type of signalthe nerve impulse or action potential that overcomes these biological constraints Action potentials do not diminish over distanceThey are signals of fixed size and durationInformation 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 PlayersSalty Fluids on either side of the membraneThe Membrane itselfThe Proteins that span the membraneCytosolExtracellular Fluid Water o Polar Covalently bonds o Effective solvent for other charged or polar moleculesIons 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 OsmanWhy Are Ions Important Ions are the major charge carriers involved in the conduction of electricity in biological systems including neuronsThe ions of particular importance for cellular neurophysiology are o The monovalent cation Na sodium o The monovalent cation K potassium 2o The divalent cation Ca calcium o The monovalent anion Cl chlorideThe Resting Membrane Potential Difference Membrane PotentialElectrical gradient between extracellular fluid and intracellular fluid due mostly to Potassium Phospholipid Bilayer A Barrier Phospholipid bilayer is not permeable to ionsEach ion has a different chemical concentration within the cell than outside Each ion has an electrical chargeThe Phospholipid Membrane QUESTIONHow are electrical signals generated ANSWERThrough ION MOVEMENT ACROSS THE CELL MEMBRANE HOW Through a change in either the potassium concentration gradient or by changing the ion permeabilities QUESTIONHow can a change in ion permeability occur ANSWERInsert protein channel into the membrane that is specific to a particular ion Open a channel that is specific to a particular ion
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