Handout Module A 2 Goals for today s lecture Neuroscience ES OT 342 RESTING MEMBRANE POTENTIAL Handout A 2 Bear 4th Ed Chapter 3 1 2 3 To describe basic movements of ions across a semi permeable membrane To describe basic features of ion channels To describe resting membrane properties of nerve cells Recorded Video Background to Lecture A 2 BIOPHYSICAL CONSIDERATIONS 66 REVIEW BEFORE LECTURE 2 Bear p 64 Neurons like many other cell types maintain a Vm Because of their unique structural and molecular specializations neurons are able to utilize their Vm for intracellular a Molecular chemical Motion b Electrical Motion see Bear Fig 3 8 to 3 12 i Cations ii Anions A membrane permits the passage of some substances but not others The ease with which a molecule can move through a membrane is called the The ease with which a charged molecule ion can move through a membrane is its 1 Handout Module A 2 In Class Lecture RMP In class lecture A 2A Ionic movement across a semi permeable membrane Thus to understand the concept of membrane potentials as they pertain to the nerve cell one needs to understand Membrane properties of the neuron A B The distribution and movement of ions across the nerve cell membrane A Membrane properties see Bear Fig 3 3 Current flow into and out of the cell is controlled by ion channels embedded in the cell membrane There are two types of ion channels in membranes and These channels have three important properties see Bear Figs 3 6 and 3 7 l They ions 2 They recognize and select among ions 3 They and in response to specific electrical mechanical or chemical signals i Non gated channels ii Gated channels Three major signals can gate ion channels A B C or voltage gated channels transmitter or ligand gated channels mechanically gated channels Note Under the influence of allosteric regulators gated channels can enter one of three functional states closed and activatable resting open active closed and non activatable refractory 2 Ionic Distribution No single ion species is distributed equally on the two sides of a nerve cell B membrane see Bear Fig 3 15 with special emphasis on Table describing ionic concentrations INTRACELL Conc EXTRACELL Conc Ratio mM mM Equil Pot mV Handout Module A 2 K Na Ca Cl A membrane s selectivity for permeant ions is determined by the relative proportions of various types of ion channels ESTABLISHING THE RESTING MEMBRANE POTENTIAL In class lecture A 2B i Glial Cells NOTE Movement of K continues to increase until it reaches a value that is equal and opposite to the effect of the concentration gradient At this value of membrane potential Vm the K concentrations inside and outside the cell are in equilibrium Ek In a cell permeable only to K ions the resting membrane potential Vr is therefore the K equilibrium potential Ek or 80 mV In a cell that has only K channels in its membrane metabolic energy is required to maintain the ionic concentration gradients 3 Handout Module A 2 ii Equilibrium Potential In 1888 the German physical chemist Walter Nernst derived the following equation known as the equation for equilibrium potential E read Bear Box 3 2 iii RMP Neurons The observed values of resting membrane potentials in neurons mV suggest that neurons at rest have significant numbers of open channels non gated channels that are selective to ions other that K 4 Handout Module A 2 Nerve cells at rest are permeable to What does this mean regarding ionic channels How can two concentration gradients for Na K be maintained across the cell membrane and how do these two concentration gradients interact to determine the resting membrane potential At a resting membrane potential of 65 mV Na will be 125 MV away from equilibrium and a strong electro chemical force will drive Na through the open Na channels Note Eventually Vm reaches a resting potential at which the outward movement of K just balances the inward movement of Na This balance point 65 mV is more positive than EK 80 mV but still far from ENa 60 mV Thus if the resting membrane is only slightly permeable to Na VR shifts slightly away from EK toward ENa 5 Handout Module A 2 iv Sodium Potassium Pump Bear P 71 72 and Fig 3 16 For the cell to have a steady resting membrane potential the charge separation across the membrane must be constant The influx of positive charge must be balanced by efflux of positive charge Note The Na K pump is an integral membrane protein Protein phosphorylation changes the conformation of the complex which leads to the removal of Na ions from the inside of the cell to the outside in exchange for extracellular K ions The Na K pump expends as much as of the total amount of ATP utilized by the brain As a general rule when Vm is determined by two or more species of ions the influence of each species is determined both by its concentrations inside and outside the cell and by the permeability of the membrane to the ion This relationship is given quantitatively by the equation read Bear Box 3 3 Note When permeability to one ion is exceptionally high the Goldman equation reduces to the Nernst equation for that ion For example if PK PCl PNa as in glial cells in reality PCl 0 and PNa 0 in glial cells the equation simply becomes the Nernst Equation READ Bear P 75 section also see Fig 3 20 Potassium Spatial Buffering by Astrocytes The importance of Regulating the External Potassium Concentrations and 6 Handout Module A 2 Prior to next class consider the effect of changing x 2 and x 0 5 the values provided in the Section B Table of this handout for example the extracellular concentration of K on RMP Use the Nernst equation to calculate the Ek under both conditions What trends do you observe So what have we learnt so far View Recorded Lecture Video A 2 Summary 7 Handout Module A 2 Neuroscience ES OT 342 Review Questions Module A 2 Resting Membrane Potential 1 If the permeability of a resting non active nerve cell to potassium ions is increased while the permeability of the cell to sodium ions remains constant unchanged from that normally at rest The resting membrane potential will be unaffected move toward zero depolarize become more positive move away from zero hyperpolarize become more negative 2 Which of the following statement most accurately summarizes the resting membrane potential RMP it exists only when no ions are moving across the nerve membrane it has as its primary cause potassium efflux it has as its primary cause sodium efflux a and b b and c 3 The resting membrane potential of