Chapter 4 and 7 Nervous System Electrical Gradients and Res ng Membrane Poten al Organiza on of the Nervous System Func onal Classes of Chemical Messengers Synap c secreted by neurons which then di uses a very short distance to bind receptors on a post synap c cell Paracrine secreted by one cell and di use to a nearby target cell Hormones secreted by endocrine cells which di uses into the blood stream for transport to target cells in the body Cells of the Nervous System Mechanisms of Membrane Transport Driving Forces Ac ng on Molecules Chemical Electrical Electrical Driving Forces Solu ons in the body contain posi vely charged ions ca ons and nega vely charged ions anions These charged ions are oJen distributed unequally between the inside of the cell and the outside of the cell Electrical Driving Forces Because posi ve and nega ve charges are distributed unequally a separa on of charge exists and the excess charges are clustered close to the membrane aOracted to the opposite charges on the other side This creates a source of energy called an electrical poten al in the form of poten al energy Electrical Driving Forces The membrane poten al works in a similar way to a baOery The separa on of charge is a poten al energy for current to ow caused by ion movement across the membrane The magnitude of the electrical driving force of an ion is determined by the size of the membrane poten al and the quan ty of charge carried by the ion Equilibrium Poten als and the Nernst Poten al The total force ac ng on the ions is a result of two driving forces An electrical force re ec ng the ions tendency to be pushed in one direc on or the other by the membrane poten al A chemical force re ec ng the ions tendency to move down its concentra on gradient The combina on of these two forces is called electrochemical driving force Balance of chemical and electrical driving forces Equilibrium Of charge and ion concentra on Equilibrium Poten als and the Nernst Poten al As a result scien sts calculate an ion s equilibrium poten al a hypothe cal value for the membrane poten al at which the electrical driving force is equal to the chemical driving force producing an electrochemical driving force of zero Nernst Equa on Equilibrium poten als and the Nernst equa on E membrane poten al z valence of ion charge F Faraday constant for electrical forces T absolute temperature in Kelvin R universal gas constant Ion concentra on of ion inside or outside the cell Equilibrium poten als and the Nernst equa on Plugging in constants and replacing natural log ln with log10 log we get Calculate the equilibrium potential for potassium K if Outside concentration 4 mM Inside concentration 140mM Ek 61 mV 1 log 4 mM out 140 mM in 94 mV 37 oC Na K ATPase Pump Se ng up electrochemical gradients inside a cell Largest single ATP user in the cell Cri cal in the nervous system and the survival of the body Ac ve Transport by Na K Pump The pump which possesses two potassium binding sites and three sodium binding sites uses ATP directly to transport sodium ions out of the cell and potassium ions into the cell against their concentra on gradients The Na K ATPase outside inside The Na K ATPase outside inside ATP The Na K ATPase outside inside ADP The Na K ATPase outside inside The Na K ATPase outside inside The Na K ATPase outside inside The Na K ATPase outside inside Millimolar concentrations of intracellular and extracellular solutes Na 145 mM K 4 mM EXTRACELLULAR INTRACELLULAR Na 15 mM K 140 mM Pumps and Leaks in a Cell Na K Pump establishes a high concentra on of sodium outside the cell and a high concentra on of potassium inside the cell But there are also ion channels in the membrane that leak ions allowing them to move back with their concentra on gradient Selec ve membrane permeability determines membrane poten al Genera ng membrane poten al Genera ng membrane poten al Genera ng membrane poten al Genera ng membrane poten al Genera ng membrane poten al
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