BISC 421 1st Edition Lecture 3 Outline of Current LectureI. The Membrane and Action PotentialsCurrent LectureThe Membrane and Action Potentials Types of Neuronal Electrical Signals •There are various types of neuronal electrical signals •These are methods for measuring themMembrane Potentialy The plasma membranes of excitable cells have a membrane potential.Ń Electrical voltage difference across the membrane.Ń Due to differences in sodium and potassium ion concentration in and out of the cell.x Potassium is higher inside cell, sodium is higher outside.y Inside of the cell has a higher concentration of negative phosphate ions and proteins.•There is a voltage diference across the membrane called a membrane potential•K+ is higher on the inside (therefore will tend to fow outward) and Na+ is higher on the outside(therefore will tend to fow inward)Some Basic Electrical Conceptsy Voltage (V)Ń Represents the potential to do work.Ń Can be positive or negative.x Volts, usually milli- (10-3) voltsy Current (I)Ń The fow of positively or negatively charged ions.Ń Results in a movement of charge from one side of the membraneto another.x Amps, usually micro- (10-6) or nano- (10-9) ampsy Resistance (R)Ń Something that impedes the fow of ions.x Ohms.Ń The opposite of resistance is conductance (g).y Ohms lawŃ V=IR or I = V/R = Vg•Self explanatory-‐ make sure you know these conceptsSome basic termsy PolarizedŃ Difference in net charge on either side of membrane.y HyperpolarizedŃ More negatively charged (downward deflection of the voltage trace)y DepolarizedŃ More positively charged (upward deflection of the voltage trace)•Hyperpolarized states make the cell less excitable•Depolarized cells are more excitableMembrane PermeabilityThe phospholipid bilayer is impermeable to most things except sometimes water and gases•This is why we need methods for getng ions across the membrane (aka channels)Ions cross the membrane via transporters and channels•There are various methods for ions to cross the membrane-‐ they use transporters and ion channels•Transporters allow ions to move against concentration gradient whereas channels allow ions to flow down their concentration gradientsMembrane Potentialy The electrical gradientacross the plasma membrane is usually ~60-80 mV.Ń Inside negative tooutside.x Membrane width is 3.5 nm.x 0.07 V / 3.5 x10 -7 cm =200,000V/cm.Ń HV lines are ~200,000 V/km.y Simplifiedmodel.Since the resting potential of the membrane is naturally negative, there is a tendency for Na+ to fow down concentration gradient into the cell to make it more positivey What happens if the membrane is now permeable to only K+?Ń By inclusion of K+-channels in membrane.Ń K+ LRQV ZLOO PRYH ³down´ WKH concentration gradient.x Leaves Cl- ions behind.Ń Creates a net charge difference.Ń Eventually an equilibrium is reached between the concentration gradient and the electrical gradient.x No net movement will occur.Ń Electrochemical equilibrium•Use Nernst equation to calculate the equilibrium potential for a single ion•If only permeable to K+ ions, K+ will move out of the cell down its concentration gradientMembrane PotentialCalculating the equilibrium potential (E ion) using the NernstEquation.R = gas constantT =absolute temperature, KF =Faraday constantZ =ion valenceAt room temperature of20°C.Slide 10-‐Membrane Potential•Know this equationElectrochemical equilibrium•This is how you can figure out what the equilibrium of an ion isMembrane potential influences ion fuxesThe membrane potential efects whether ions will move in or out of the
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